Anionic single-tail surfactant sodium dodecyl sulfate (SDS) and a molecule with multiple amido and amine groups (Lys-12-Lys) were used as building blocks to fabricate oligomeric surfactants through intermolecular interactions. Their interactions and the resultant complex and aggregate structures were investigated by turbidity titration, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, freeze-fracture transmission electron microscopy, 1H NMR, and 1D NOE techniques. At pH 11.0, the interaction between SDS and Lys-12-Lys is exothermic and mainly resulted from hydrogen bonding among the amido and amine groups of Lys-12-Lys and the sulfate group of SDS and hydrophobic interaction between the hydrocarbon chains of SDS and Lys-12-Lys. At pH 3.0, each Lys-12-Lys carries four positive charges and two hydrogen bonding sites. Then SDS and Lys-12-Lys form complexes Lys-12-Lys(SDS)6 and Lys-12-Lys(SDS)4 through the head groups by electrostatic attraction and hydrogen bonds assisted by hydrophobic interaction. Moreover, the complexes pack more tightly in their aggregates with the increase of the molar ratio. Especially the Lys-12-Lys(SDS)4 and Lys-12-Lys(SDS)6 complexes behave like oligomeric surfactants taking Lys-12-Lys as a spacer group, exhibiting a series of aggregates transitions with the increase of concentration, i.e., larger vesicles, smaller spherical micelles, and long threadlike micelles. Therefore, oligomeric surfactants Lys-12-Lys(SDS)4 and Lys-12-Lys(SDS)6 have been successfully fabricated by using a single chain surfactant and an oligomeric connecting molecule through noncovalent association.

The aggregation behavior of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) with chelating molecule ethylenediaminetetraacetic acid (EDTA) and the effects of calcium bromide (CaBr2) on the structure and morphology of the aggregates in the mixture have been investigated by surface tension, isothermal titration microcalorimetry, electrical conductivity, ζ potential, dynamic light scattering, cryogenic transmission electron microscopy, freeze–fracture transmission electron microscopy, and 1H NMR techniques. It was found that the electrostatic attraction between the carboxyl groups of EDTA and the headgroups of 12-6-12 leads to the formation of oligomeric-like surfactant EDTA(12-6-12)2 at an EDTA/12-6-12 molar ratio of 0.50. The critical aggregation concentration of the EDTA(12-6-12)2 complexes is much lower than that of 12-6-12, and the complexes form loose, large network-like premicellar aggregates and then transfer into small micelles with an increase in concentration. Moreover, the addition of CaBr2 induces the transition from the loose aggregates and micelles to vesicles owing to the coordination interaction between the calcium ion and EDTA and the electrostatic interaction between EDTA and 12-6-12. The work reveals that as a bridging molecule between the calcium ion and the gemini surfactant, the chelating molecule greatly promotes the assembly of the gemini surfactant and strengthens the molecular packing in the presence of calcium ions.

The surface structure of the trimeric surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) on mica and the interactions between two such DTAD-coated surfaces were determined using atomic force microscopy and a surface force balance. In an aqueous solution of 3 mM, 5 times the critical aggregation concentration (CAC), the surfaces are coated with wormlike micelles or hemimicelles and larger (∼80 nm) bilayer vesicles. Repulsive normal interactions between the surfaces indicate a net surface charge and a solution concentration of ions close to that expected from the CAC. Moreover, this surface coating is strongly lubricating up to some tens of atmospheres, attributed to the hydration–lubrication mechanism acting at the exposed, highly hydrated surfactant headgroups. Upon replacement of the DTAD solution with surfactant-free water, the surface structures have changed on the DTAD monolayers, which then jump into adhesive contact on approach, both in water and following addition of 0.1 M NaNO3. This trimeric surfactant monolayer, which is highly hydrophobic, is found to be positively charged, which is evident from the attraction between the DTAD monolayer and negatively charged bare mica across water. These monolayers are stable over days even under a salt solution. The stability is attributed to the several stabilization pathways available to DTAD on the mica surface.

Effects of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12–6–12) on the micellization of two triblock copolymers of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), F127 (EO97PO69EO97) and P123 (EO20PO70EO20), have been studied in aqueous solution by differential scanning calorimetry (DSC), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), and NMR techniques. Compared with traditional single-chain ionic surfactants, 12–6–12 has a stronger ability of lowering the CMT of the copolymers, which should be attributed to the stronger aggregation ability and lower critical micelle concentration of 12–6–12. The critical micelle temperature (CMT) of the two copolymers decreases as the 12–6–12 concentration increases and the ability of 12–6–12 in lowering the CMT of F127 is slightly stronger than that of P123. Moreover, a combination of ITC and DLS has shown that 12–6–12 binds to the copolymers at the temperatures from 16 to 40 °C. At the temperatures below the CMT of the copolymers, 12–6–12 micelles bind on single copolymer chains and induce the copolymers to initiate aggregation at very low 12–6–12 concentration. At the temperatures above the CMT of the copolymers, the interaction of 12–6–12 with both monomeric and micellar copolymers leads to the formation of the mixed copolymer/12–6–12 micelles, then the mixed micelles break into smaller mixed micelles, and finally free 12–6–12 micelles form with the increase of the 12–6–12 concentration.

This work studied the interactions of an oppositely charged surfactant mixture of oleyl bis(2-hydroxyethyl)methyl ammonium bromide (OHAB) and sodium dodecyl sulfate (SDS) with 1,2-di-(9Z-octadecenoyl)-sn-glycero-3-phosphocholine (DOPC) vesicles as well as the penetration of the OHAB/SDS mixture through model skin, aimed at understanding the relationship between the ability of different surfactant aggregates in solubilizing phospholipid vesicles and their potential in irritating skin. By changing the molar fraction of OHAB (XOHAB), five kinds of aggregates are constructed: OHAB and SDS separately form cationic and anionic small micelles, whereas the OHAB/SDS mixtures form cationic and anionic vesicles at XOHAB = 0.30 and 0.70, respectively, and weakly charged vesicles at XOHAB = 0.50. The mixtures have much lower critical micellar concentrations (CMCs) and much larger aggregates than either OHAB or SDS alone, and the CMC and the size of the OHAB/SDS vesicles decrease with the increase in XOHAB. The phase diagrams indicate that the OHAB/SDS mixtures show much stronger ability in solubilizing the DOPC vesicles than individual OHAB and SDS and decrease in the order of XOHAB = 0.30 > 0.50 > 0.70 ≫ 1.00 > 0. However, the ability of the surfactants in penetrating the model skin decreases reversely, and the penetration of the surfactants are significantly reduced by mixing. These results indicate that the surfactant mixture with a larger aggregate size and a smaller CMC value displays much stronger ability in solubilizing the DOPC vesicles but much weaker ability in penetrating the skin.

A star-shaped oligomeric-like surfactant with variable oligomeric degrees has been formed with a four-arm carboxylate salt (4EOCOONa) and cationic single chain surfactant dodecyl trimethylammonium bromide (DTAB). The aggregation of the 4EOCOONa/(DTAB)n complexes has been investigated by surface tension, electrical conductivity, isothermal titration microcalorimetry, ζ potential, dynamic light scattering, 1H NMR spectroscopy, and steady-state fluorescence measurements. The calorimetric result shows that 4EOCOONa interacts strongly with DTAB and each 4EOCOONa molecule binds with six DTAB molecules, wherein four DTAB molecules electrostatically bind to one 4EOCOONa molecule and additional two DTAB molecules further bind to the 4EOCOONa/(DTAB)n complex by hydrophobic interaction. The critical micelle concentration (CMC) of the 4EOCOONa/(DTAB)n complexes is remarkably lower than the CMC of DTAB, similar to synthesized star-shaped oligomeric surfactants. The micelle properties of the DTAB/4EOCOONa mixtures depend on the component changes of the 4EOCOONa/(DTAB)n complexes. By increasing the DTAB/4EOCOONa molar ratio and/or concentration, the DTAB/4EOCOONa mixtures gradually form the complexes of 4EOCOO(DTA)13–, 4EOCOO(DTA)22–, 4EOCOO(DTA)3–, 4EOCOO(DTA)4, and 4EOCOO(DTA)62+, and the corresponding aggregates are small anionic micelles with loose molecular packing, and nearly nonionic or positively charged small micelles with more compact packing. Moreover, the positive charge of the small micelles increases with the increase of the concentration and the DTAB/4EOCOONa molar ratio. Therefore, constructing oligomeric-like surfactants by adding appropriate organic salts into conventional surfactants is a convenient method to achieve desired properties of surfactant aggregates.

The interactions between a star-shaped hexameric cationic quaternary ammonium surfactant PAHB and calf thymus DNA and induced DNA condensation were investigated by ζ-potential, dynamic light scattering, atomic force microscopy, isothermal titration calorimetry, ethidium bromide exclusion assay, circular dichroism, and cytotoxicity assay. With the addition of PAHB, long extended DNA molecules exhibit successive conformational transitions from elongated coil to a partially condensed cluster-like aggregate, to a globules-on-a-string structure, and then to a fully condensed globule until the saturation point of interaction between PAHB and DNA, which is slightly above their charge neutralization point. The efficient condensation is mainly produced by the strong attractive electrostatic interaction between the multiple positively charged headgroups of PAHB and negatively charged phosphate groups of DNA, and the hydrophobic interaction among the multiple alkyl chains of PAHB. Moreover the transition of the DNA conformation is also affected by the transitions of PAHB molecular conformation from star-shaped to claw-like and pyramid-like. Although the DNA conformation is significantly changed by PAHB, the DNA secondary structure does not display obvious variations, and the PAHB/DNA mixture does not show cytotoxicity when DNA is partially condensed. These results indicate that star-shaped oligomeric cationic surfactant is a potential condensing agent for gene transfection.Keywords: cationic star-shaped hexameric surfactant; conformation transition; cytotoxicity; DNA condensation; DNA/surfactant interaction;

Conjugated polymers have great potential applications in bioimaging. However, the aggregation of conjugated polymers driven by electrostatic and hydrophobic interactions in aqueous media results in the reduction of photoluminescence quantum efficiency. In present work we synthesized a carboxylate gemini surfactant [sodium 2,6-didodecyl-4-hydroxy-2,6-diaza-1,7-heptanedicarboxylate (SDHC)] to adjust the aggregation behaviors and fluorescence properties of conjugated polymers [anionic poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene) (MPS-PPV) and cationic poly(3-alkoxy-4-methylthiophene) (PMNT)]. This gemini surfactant shows very low critical micellar concentration (CMC) in aqueous solution and forms vesicles above CMC. In neutral and acidic conditions, MPS-PPV combines with the SDHC vesicles mainly via hydrophobic interactions and forms the aggregates in which the photoluminescence quantum efficiency of MPS-PPV is highly enhanced from 0.1% to 27%. As to PMNT, the molecules are located in the bilayer of SDHC vesicles through both electrostatic and hydrophobic interactions, and this structure prevents the production and release of reactive oxygen species. Moreover, SDHC is nontoxic and can effectively decrease the dark- and photocytotoxicity of MPS-PPV and PMNT, laying a good foundation for their bioimaging application. The living cell imaging indicates that the surfactant/conjugated polymer aggregates can stain the MCF-7 cells with main location in the lysosome. This work provides insight into how to improve the fluorescence properties and bioimaging applications of conjugated polymers by surfactants.Keywords: bioimaging; conjugated polymer; cytotoxicity; fluorescence property; gemini surfactant;

Cationic quaternary ammonium gemini surfactants CnH2n+1(CH3)2N+CH2CHCHCH2(CH3)2N+CnH2n+12Br– (CnC4Cn, n = 12, 8, 6) with alkyl spacers, CnH2n+1(CH3)2N+CH2CHOHCHOHCH2(CH3)2N+CnH2n+12Br– (CnC4(OH)2Cn, n = 12, 8, 6, 4) with two hydroxyl groups in alkyl spacers, and cationic ammonium single-chain surfactants CnH2n+1(CH3)2N+Br– (CnTAB, n = 12, 8, 6) have been chosen to fabricate oppositely charged surfactant mixtures with anionic sulfonate gemini surfactant C12H25N(CH2CH2CH2SO3–)CH2CH2CH2(CH3)2N(CH2CH2CH2SO3–)C12H252Na (C12C3C12(SO3)2). Surface tension, electrical conductivity, and isothermal titration microcalorimetry (ITC) were used to study their surface properties, aggregation behaviors, and intermolecular interactions. The mixtures of C12C3C12(SO3)2/CnC4(OH)2Cn (n = 12, 8) and C12C3C12(SO3)2/C12C4C12 show anomalous larger critical micelle concentration (CMC) than C12C3C12(SO3)2, while the mixtures of C12C3C12(SO3)2/CnC4(OH)2Cn (n = 6, 4), C12C3C12(SO3)2/CnC4(OH)2Cn (n = 6, 4), and C12C3C12(SO3)2/CnTAB (n = 12, 8, 6) exhibit much lower CMC than C12C3C12(SO3)2. The results indicate that strong hydrophobic interactions between the alkyl chains assisted by strong electrostatic attractions between the headgroups and hydrogen bonds between the spacers lead to the formation of less surface active premicellar aggregates in bulk solution, resulting in the increase of CMC. If these interactions are weakened or inhibited, less surface active premicellar aggregates are no longer formed in the mixtures, and thus the CMC values are reduced. The work reveals that the combination of two surfactants with great self-assembling ability separately may have strong intermolecular binding interactions; however, their mixtures do not always generate superior synergism properties. Only moderate intermolecular interaction can generate the strongest synergism in CMC reduction.

The aggregation behaviors of the mixtures of cationic gemini surfactant 1,4-bis(dodecyl-N,N-dimethylammonium bromide)-2,3-butanediol (C12C4(OH)2C12Br2) and anionic amino acid surfactant N-dodecanoylglutamic acid (C12Glu) in aqueous solution of pH = 10.0 have been studied. The mixture forms spherical micelles, vesicles, and wormlike micelles at 25 °C by changing mixing ratios and/or total surfactant concentration. Then these aggregates undergo a series of transitions upon increasing the temperature. Smaller spherical micelles transfer into larger vesicles, vesicles transfer into solid spherical aggregates and then into larger irregular aggregates, and entangled wormlike micelles transfer into branched wormlike micelles. Moreover, the larger irregular aggregates and branched micelles finally lead to precipitation and clouding phenomenon, respectively. All these transitions are thermally reversible, and the transition temperatures can be tuned by varying the mixing ratios and/or total concentration. These temperature-dependent aggregate transitions can be elucidated on the basis of the temperature-induced variations in the dehydration, electrostatic interaction, and hydrogen bonds of the headgroup area and in the hydrophobic interaction between the hydrocarbon chains. The results suggest that the surfactants carrying multiple binding sites will greatly improve the regulation ability and temperature sensitivity.

Coacervation of cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12–6–12) with pH-sensitive N-benzoylglutamic acid (H2Bzglu) has been investigated by potentiometric pH-titration, turbidity titration, dynamic light scattering (DLS), isothermal titration calorimetry (ITC), TEM, 1H NMR, and light microscopy. Phase boundaries of the 12–6–12/H2Bzglu mixture were obtained over the pH range from 2 to 9 and in the H2Bzglu concentration range from 30.0 to 50.0 mM at pH 4.5. When the H2Bzglu concentration is beyond 30.0 mM, the 12–6–12/H2Bzglu mixed solution undergoes the phase transitions from soluble aggregate, to precipitate, coacervate, and soluble aggregate again as pH increases. The results indicate that coacervation occurs at extremely low 12–6–12 concentration and lasts over a wide surfactant range, and can be enhanced or suppressed by changing pH, 12–6–12/H2Bzglu molar ratio and H2Bzglu concentration. The coacervates present a disorderly connected lay structure. Coacervation only takes place at pH 4–5, where the aggregates are nearly charge neutralized, and a minimum H2Bzglu concentration of 30.0 mM is required for coacervation. In this pH range, H2Bzglu mainly exist as HBzglu–. The investigations on intermolecular interactions indicate that the aggregation of 12–6–12 is greatly promoted by the strong electrostatic and hydrophobic interactions with the HBzglu– molecules, and the interaction also promotes the formation of dimers, trimers, and tetramers of HBzglu– through hydrogen bonds. The double chains of 12–6–12 and the HBzglu– oligomers can play the bridging roles connecting aggregates. These factors endow the mixed system with a very high efficiency in generating coacervation.

The phase behavior and rheological properties of a multi-component system, made of a zwitterionic surfactant cocoamidopropyl betaine (CAPB), an anionic surfactant sodium lauryl sulfate (SLSS), and mixed salts (tetrasodium pyrophosphate, sodium acid pyrophosphate, sacharrin, and sodium fluoride) in sorbitol/H2O mixed solvent at different mass fraction of SLSS (XSLSS) were systematically investigated by steady and dynamic rheology, dynamic light scattering, and diffusion ordered spectroscopy (DOSY). When fixing the salt concentration and the mass ratio of sorbitol in mixed solvent (R), the zero-shear viscosity increases first and then decreases showing a maximum with increasing XSLSS, resulting from the formation and entanglement of wormlike micelles. Especially when XSLSS is between 0.33 and 0.80, the mixture is dominated by entangled wormlike micelles coexisting with small micelles and separated wormlike micelles, and shows high viscoelasticity. The maximum of the zero-shear viscosity is ca. 5 orders of magnitude larger than that of sorbitol/H2O mixed solvent or the CAPB/SLSS aqueous solution. The characteristic structural parameters for the micellar solutions at different XSLSS are also estimated from further analysis of the rheological results, and indicate the stronger network structures of the wormlike micelles are formed in our systems compared with the wormlike micelles formed by a traditional zwitterionic/anionic surfactant aqueous solutions. The great improvements of rheological properties are attributed to the strong screening effects of the mixed salts and the strong solvophobic effect of sorbitol on the electrostatic and hydrophobic interaction between the CAPB and SLSS molecules. The present work has improved our understanding about the aggregation behavior of zwitterionic/anionic mixed surfactants with salts in less polar solvent/H2O mixture, which would be of widely practical importance to optimize the formulation of products for personal care and household cleaning.

A water-soluble 2,6-helic[6]arene derivative was conveniently synthesized, and it showed strong binding abilities towards quaternary phosphonium salts in aqueous solution. Moreover, an acid/base controlled switchable complexation process was also described.

This work studied gemini-like surfactants formed from anionic surfactant sodium dodecyl sulfate (SDS) and cationic charged bola-type diamines with hydrophilic or hydrophobic spacers of different lengths using surface tension, small angle neutron scattering, isothermal titration microcalorimetry and cryogenic transmission electron microscopy. The critical micelle concentrations (CMC) and the surface tension at CMC (γCMC) for all the diamine/SDS mixtures are markedly lower than that of SDS. The shorter diamines reduce γCMC to a greater extent regardless of the hydrophilicity/hydrophobicity of the diamines. Meanwhile, either the hydrophobic diamine with a longer spacer or the hydrophilic diamine with a shorter spacer is more beneficial to decrease CMC and leads to the transition from spherical micelles into rodlike or wormlike micelles. This is principally because of the formation of gemini-like surfactants by the electrostatic binding between SDS and the diamines, where the electrostatic repulsion between the adjacent headgroups of SDS becomes much weaker due to the electrostatic binding of oppositely charged diamine with SDS, and the longer hydrophobic spacer may also bend into the hydrophobic domain of micelles to promote micellar growth. However, the hydrophilic spacers are more compatible with the headgroup region, leading to micelles with a larger curvature. This work contributes to the understanding of the relationship between the properties of constructed gemini-like surfactants and the natures of connecting molecules, and provides guidance to efficiently improve the performance of surfactants.

This work involved the construction of pH-responsive self-assembly systems from a pH-sensitive four-arm carboxylate acid (4EOCOOH) and either the cationic single chain surfactant dodecyl trimethyl ammonium bromide (DTAB) or the cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12). It was found that the constructed oligomeric-like structures from the mixtures of 4EOCOOH with DTAB or 12-6-12 greatly enhance the aggregation ability of the mixtures, thus improving the pH-responsivity. In particular, surfactant concentrations significantly affect the pH-responsivity at a fixed 4EOCOOH concentration. At higher surfactant concentrations, the pH-responsivity is suppressed, while at lower surfactant concentrations, the mixed aggregates gradually change from micelles to unstable large spherical aggregates or vesicles, and then to stable spherical aggregates, with decreasing pH. Moreover, the surfactant/4EOCOOH systems have different solubilization abilities for three hydrophobic drugs. For quercetin and baicalein, the systems support much better solubilization at lower pH values, while for indomethacin, the systems show better solubilization at higher pH values. In particular, compared with DTAB, 12-6-12 is more efficient in constructing pH-responsive systems, and the 12-6-12/4EOCOOH mixture shows better ability for solubilizing hydrophobic drugs. This work will be helpful in the design of high-efficiency, pH-responsive surfactant systems for solubilizing hydrophobic drugs by simply mixing pH-sensitive molecules with surfactants.

The interaction of sodium dodecyl sulfate (SDS) with poly(4-vinylpyridine N-oxide) (PVPNO) at different pH values and ionic strengths has been studied by isothermal titration microcalorimetry, electrical conductivity and turbidity measurements. The solution pH significantly effects the formation of the PVPNO/SDS complex. At pH 1.5, the polymer PVPNO is a strong polycation, and the binding is dominated by electrostatic 1:1 charge neutralization with the anionic surfactant. At pH 6 and 8, the electrostatic attraction between SDS and PVPNO is weak, and the hydrophobic interaction becomes stronger. The effect of salt concentration on the interaction of SDS and PVPNO depends on the competition between the increase of interaction and the screening of interaction. This study based on the thermodynamic process gained deep insight into the effects of pH value and ionic strength on the interaction mechanism of surfactant with polyelectrolyte. We also constructed a simple model for the interaction of PVPNO and SDS system in different solution regions.

The impact of mixed salts and sorbitol on the viscoelastic properties of a multi-component system, made of a zwitterionic surfactant cocoamidopropyl betaine (CAPB), an anionic surfactant sodium lauryl sulfate (SLSS) and mixed salts (tetrasodium pyrophosphate, sodium acid pyrophosphate, saccharin and sodium fluoride) in sorbitol/H2O mixed solvent are systematically investigated by steady state and dynamic rheology. As reported previously, the viscosity of the mixed system passes through a maximum with increase in the SLSS mass fraction (XSLSS) at a fixed total surfactant concentration, salt concentration (Csalt) and mass ratio of sorbitol in mixed solvent (R). The shape of the XSLSS-dependent viscosity curve does not change regardless of Csalt and R, but adding salts or sorbitol has different effects on the rheological properties of this system. The former due to a high screening effect plays an important role in the elongation and entanglement of the wormlike micelles, facilitating the enhancement of rheological properties and the formation of Maxwell fluids. The latter has a dual effect on the rheological properties and phase behavior of the mixtures. A certain amount of sorbitol can promote the formation entangled wormlike micelles, while the effect is reversed if the sorbitol content is too large. The electrostatic and hydrophobic interaction between CAPB and SLSS are the prerequisite for the aggregate formation and transition. Meanwhile, the aggregation behaviors are strongly influenced by the balance between low dielectric constant, strong solvophobic interaction and steric effect of sorbitol with the ability to form hydrogen bonds which favors the growth of micelles, and appearance of aqueous two-phase systems with smaller amounts of wormlike micelles in CAPB-rich regions which oppose enhancement of rheological properties. Our findings provide a new insight and approach to control and adjust the phase behavior of such a complicated applied multi-component system.

Despite efficacious antimicrobial activity, cationic oligomeric surfactants show strong skin irritation potential due to their larger cationic charge numbers and multiple hydrophobic chains. This work reports that the incorporation of α-, β-, and γ-CDs with different cavity sizes can effectively improve the mildness of cationic ammonium trimeric surfactant DTAD with a star-shaped spacer while maintaining its high antibacterial activity. On the basis of the different cavity sizes of CDs and the asymmetry in the spacer of DTAD, the CD/DTAD mixtures form α-CD@DTAD, 2α-CD@DTAD, β-CD@DTAD, and γ-CD@DTAD complexes. Compared to DTAD, these CD/DTAD complexes show much stronger self-assembly ability with much lower critical aggregation concentrations (CAC) and form more diverse aggregates with reduced zeta potential. Just above their CACs, the CD/DTAD complexes form vesicles or solid spherical aggregates of ∼50 nm and then transform into small micelles of ∼10 nm as the concentration increases. The strong self-assembly ability and the multiple sites of hydrogen bonds of the CD/DTAD complexes endow them with high antibacterial activity against E. coli, showing a very low minimum inhibitory concentration (2.22–2.48 μM) comparable to that of DTAD. In particular, the addition of CDs significantly reduces the abilities of DTAD in solubilizing zein (a skin model protein) and in binding with zein, and the mildness decreases in the order of 2α-CD@DTAD > β-CD@DTAD > γ-CD@DTAD > α-CD@DTAD. This tendency depends on their different self-assembling structures, and the formation of vesicles is approved to be in favor of the improvement of the mildness.Keywords: aggregation behavior; antimicrobial activities; cationic trimeric surfactant; cyclodextrin; mildness

This work reports that cationic micelles formed by cationic trimeric, tetrameric, and hexameric surfactants bearing amide moieties in spacers can efficiently kill Gram-negative E. coli with a very low minimum inhibitory concentration (1.70–0.93 μM), and do not cause obvious toxicity to mammalian cells at the concentrations used. With the increase of the oligomerization degree, the antibacterial activity of the oligomeric surfactants increases, i.e., hexameric surfactant > tetrameric surfactant > trimeric surfactant. Isothermal titration microcalorimetry, scanning electron microscopy, and zeta potential results reveal that the cationic micelles interact with the cell membrane of E. coli through two processes. First, the integrity of outer membrane of E. coli is disrupted by the electrostatic interaction of the cationic ammonium groups of the surfactants with anionic groups of E. coli, resulting in loss of the barrier function of the outer membrane. The inner membrane then is disintegrated by the hydrophobic interaction of the surfactant hydrocarbon chains with the hydrophobic domains of the inner membrane, leading to the cytoplast leakage. The formation of micelles of these cationic oligomeric surfactants at very low concentration enables more efficient interaction with bacterial cell membrane, which endows the oligomeric surfactants with high antibacterial activity.Keywords: action mechanism; antimicrobial activity; cationic micelle; hexameric surfactant; tetrameric surfactant; trimeric surfactant

The influence of perfume molecules on the self-assembly of the anionic surfactant sodium dodecyl sulfate (SDS) and their localization in SDS micelles have been investigated by ζ potential, small angle X-ray scattering (SAXS), one- and two-dimensional NMR and isothermal titration microcalorimetry (ITC). A broad range of perfume molecules varying in octanol/water partition coefficients P are employed. The results indicate that the surface charge, size and aggregation number of the SDS micelles strongly depend on the hydrophobicity/hydrophilicity degree of perfume molecules. Three distinct regions along the logP values are identified. Hydrophilic perfumes (logP < 2.0) partially incorporate into the SDS micelles and do not lead to micelle swelling, whereas hydrophobic perfumes (logP > 3.5) are solubilized close to the end of the hydrophobic chains in the SDS micelles and enlarge the micelles with higher ζ potential and a larger aggregation number. The incorporated fraction and micelle properties show increasing tendency for the perfumes in the intermediate logP region (2.0 < logP < 3.5). Besides, the molecular conformation of perfume molecules also affects these properties. The perfumes with a linear chain structure or an aromatic group can penetrate into the palisade layer and closely pack with the SDS molecules. Furthermore, the thermodynamic parameters obtained from ITC show that the binding of the perfumes in the intermediate logP region is more spontaneous than those in the other two logP regions, and the micellization of SDS with the perfumes is driven by entropy.

The superior physicochemical properties of gemini and oligomeric surfactants look promising in a variety of different applications. However, tedious covalent synthesis and complicated purification limit the development of these novel surfactants. Recently, it has been demonstrated as feasible to use noncovalent interactions to construct surfactant systems with the characteristics of gemini or oligomeric surfactants. This short review discusses the strategies of constructing gemini-like or oligomeric-like surfactants through noncovalent interactions by choosing proper building blocks, single-chain surfactants and gemini surfactants along with connecting molecules containing double or multiple binding sites. The current progress in this field has been summarized. This very simple and efficient way to obtain gemini and oligomeric surfactants may make a practical impact on the surfactant industry.

Surface tension and aggregation behavior in an aqueous solution of the mixture of cationic surfactant oleyl bis(2-hydroxyethyl)methylammonium bromide (OHAB) and anionic surfactant sodium dodecyl sulfate (SDS) have been studied by surface tension, conductivity, turbidity, zeta potential, isothermal titration microcalorimetry (ITC), cryogenic transmission electron microscopy (Cryo-TEM), and dynamic light scattering. The mixture shows pretty low critical micellar concentration and surface tension, and successively forms globular micelles, unilamellar vesicles, multilamellar vesicles, rod-like micelles, and globular micelles again by increasing the molar fraction of OHAB from 0 to 1.00. The cooperation of hydrophobic interaction between the alkyl chains, electrostatic attraction between the headgroups as well as hydrogen bonds between the hydroxyethyl groups leads to the abundant aggregation behaviors. Furthermore, the solubilization of zein by the OHAB/SDS aggregates and their interactions were studied by ITC, total organic carbon analysis (TOC), and Cryo-TEM. Compared with pure OHAB or pure SDS solution, the amount of zein solubilized by the OHAB/SDS mixture is significantly reduced. It means that the mixtures have much stronger abilities in solubilizing zein. This result has also been proved by the observed enthalpy changes for the interaction of OHAB/SDS mixture with zein. Mixing oppositely charged OHAB and SDS reduces the net charge of mixed aggregates, and thus, the electrostatic attraction between the aggregates and zein is weakened. Meanwhile, the large size of the aggregates may increase the steric repulsion to the zein backbone. This work reveals that surfactant mixtures with larger aggregates and smaller CMCs solubilize less zein, suggesting how to construct a highly efficient and nonirritant surfactant system for practical use.

Interactions of multivalent metal counterions with anionic sulfonate gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)-propane (C12C3C12(SO3)2) and the induced aggregate transitions in aqueous solution have been studied. Divalent metal ions Ca2+, Mg2+, Cu2+, Zn2+, Mn2+, Co2+, and Ni2+ and trivalent metal ions Al3+, Fe3+, and Cr3+ were chosen. The results indicate that the critical micelle concentration (CMC) of C12C3C12(SO3)2 is greatly reduced by the ions, and the aggregate morphologies of C12C3C12(SO3)2 are adjusted by changing the nature and molar ratio of the metal ions. These metal ions can be classified into four groups because the ions in each group have very similar interaction mechanisms with C12C3C12(SO3)2: (I) Cu2+ and Zn2+; (II) Ca2+, Mn2+ and Mg2+; (III) Ni2+ and Co2+; and (IV) Cr3+, Al3+ and Fe3+. Cu2+, Mg2+, Ni2+, and Al3+ then were selected as representatives for each group to further study their interaction with C12C3C12(SO3)2. C12C3C12(SO3)2 interacts with the multivalent metal ions by electrostatic interaction and coordination interaction. C12C3C12(SO3)2 forms prolate micelles and plate-like micelles with Cu2+, vesicles and wormlike micelles with Al3+ or Ni2+, and viscous three-dimensional network structure with Mg2+. Moreover, precipitation does not take place in aqueous solution even at a high ion/surfactant ratio. The related mechanisms have been discussed. The present work provides guidance on how to apply the anionic surfactant into the solutions containing the multivalent metal ions, and those aggregates may have potential usage in separating heavy metal ions from aqueous solutions.

Only specific base pairs on DNA can bind with each other through hydrogen bonds, which is called the Watson–Crick (W/C) pairing rule. However, without the constraint of DNA chains, the nucleobases in bulk aqueous solution usually do not follow the W/C pairing rule anymore because of the strong competitive effect of water and the multi-interaction edges of nucleobases. The present work applied surfactant aggregates noncovalently functionalized by nucleotide to enhance the recognition between nucleobases without DNA chains in aqueous solution, and it revealed the effects of their self-assembling ability and morphologies on the recognition. The cationic ammonium monomeric, dimeric, and trimeric surfactants DTAB, 12–3–12, and 12–3–12–3–12 were chosen. The surfactants with guanine-5′-monophosphate-disodium (GMP) form micelles, vesicles, and fingerprint-like and plate-like aggregates bearing the hydrogen-bonding sites of GMP, respectively. The binding parameters of these aggregates with adenine (A), uracil (U), guanine (G), and cytosine(C) indicate that the surfactants can promote W/C recognitions in aqueous solution when they form vesicles (GMP/DTAB) or plate-like aggregates (GMP/12–3–12) with proper molecular packing compactness, which not only provide hydrophobic environments but also shield non-W/C recognition edges. However, the GMP/12–3–12 micelles with loose molecular packing, the GMP/12–3–12 fingerprint-like aggregates where the hydrogen bond sites of GMP are occupied by itself, and the GMP/12–3–12–3–12 vesicles with too strong self-assembling ability cannot promote W/C recognition. This work provides insight into how to design self-assemblies with the performance of enhanced molecule recognition.Keywords: aggregate transition; noncovalent bonds; oligomeric amphiphile; self-assembling; Watson−Crick pairing;

A peptide gemini surfactant, 12-G(NH2)LG(NH2)-12, has been constructed with two dodecyl chains separately attached to the two terminals of a glutamic acid–lysine–glutamic acid peptide and the aggregation behavior of the surfactant was studied in aqueous solution. The 12-G(NH2)LG(NH2)-12 molecules form fiber-like precipitates around pH 7.0, and the precipitation range is widened on increasing the concentration. At pHs 3.0 and 11.0, 12-G(NH2)LG(NH2)-12 forms soluble aggregates because each molecule carries two positively charged amino groups at the two ends of the peptide spacer at pH 3.0, while each molecule carries one negatively charged carboxyl group in the middle of the peptide spacer at pH 11.0. 12-G(NH2)LG(NH2)-12 displays a similar concentration-dependent process at these two pHs: forming small micelles above the critical micelle concentration and transferring to fibers at pH 3.0 or twisted ribbons at pH 11.0 above the second critical concentration. The fibers formed at pH 3.0 tend to aggregate into bundles with twisted structure. Both the twisted fibers at pH 3.0 and the twisted ribbons at pH 11.0 contain β-sheet structure formed by the peptide spacer.

The aggregation behavior of anionic single-chain surfactant sodium lauryl ether sulfate containing three ether groups (SLE3S) with positively bicharged organic salt 1,2-bis(2-benzylammoniumethoxy)ethane dichloride (BEO) has been investigated in aqueous solution, and the effects of the BEO/SLE3S aggregate transitions on the fluorescent properties of anionic conjugated polyelectrolyte MPS-PPV with a larger molecular weight and cationic conjugated oligoelectrolyte DAB have been evaluated. Without BEO, SLE3S does not affect the fluorescent properties of MPS-PPV and only affects the fluorescent properties of DAB at a higher SLE3S concentration. With the addition of BEO, SLE3S and BEO form gemini-like surfactant (SLE3S)2-BEO. When the BEO/SLE3S molar ratio is fixed at 0.25, with increasing the BEO/SLE3S concentration, the BEO/SLE3S mixture forms large, loosely arranged aggregates and then transforms to closely packed spherical aggregates and finally to long thread-like micelles. The photoluminescence (PL) intensity of MPS-PPV varies with the morphologies of the BEO/SLE3S aggregates, while the PL intensity of DAB is almost independent of the aggregate morphologies. The results demonstrate that gemini-like surfactants formed through intermolecular interactions can effectively adjust the fluorescent properties of conjugated polyelectrolytes.

Construction of gemini-like surfactants using the cationic single-chain surfactant cetyltrimethylammonium bromide C16H33N(CH3)3Br2 (CTAB) and the anionic dicarboxylic acid sodium salt NaOOC(CH2)n-2COONa (CnNa2, n = 4, 6, 8, 10, 12) by way of non-covalent interactions has been investigated by surface tension measurements, hydrogen-1 nuclear magnetic resonance (1H NMR) spectroscopy and isothermal titration microcalorimetry (ITC). The critical micelle concentrations (cmc) of the CTAB/CnNa2 mixtures are obviously lower than that of CTAB and strongly depend on the mixing ratio. Moreover, the cmc values of the CTAB/CnNa2 mixtures decrease gradually with an increasing methylene chain length of CnNa2, indicating hydrophobic interaction between the hydrocarbon chains of CTAB and CnNa2 facilitates micellization of the mixtures. In particular, the ITC curves and 1H NMR spectra indicate that the binding ratio of CTAB to CnNa2, except C4Na2, is around 2:1, i.e., (CTAB)2CnNa2. Additionally, CTAB/CnNa2 mixtures are soluble in a whole molar ratio and concentration ranges have been studied, even at the electrical neutralization point. Therefore, these results reveal that highly soluble gemini-like surfactants are conveniently constructed with oppositely-charged cationic single-chain surfactants and dicarboxylic acid sodiums. In an attempt at improving the performance of surfactants this work provides guidance for choosing additives that form gemini-like surfactants via an uncomplicated synthesis.

Coacervation in an aqueous solution of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (C12C6C12Br2) with sodium benzoate (NaBz) has been investigated at 25 °C by turbidity titration, light microscopy, dynamic light scattering, cryogenic temperature transmission electron microscopy (Cryo-TEM), scanning electron microscopy (SEM), isothermal titration calorimetry, ζ potential and 1H NMR measurements. There is a critical NaBz concentration of 0.10 M, only above which coacervation can take place. However, if the NaBz concentration is too large, coacervation also becomes difficult. Coacervation takes place at a very low concentration of C12C6C12Br2 and exists in a very wide concentration region of C12C6C12Br2. The phase behavior in the NaBz concentration from 0.15 to 0.50 M includes spherical micelles, threadlike micelles, coacervation, and precipitation. With increasing NaBz concentration, the phase boundaries of coacervation shift to higher C12C6C12Br2 concentration. Moreover, the C12C6C12Br2–NaBz aggregates in the coacervate are found to be close to charge neutralized. The Cryo-TEM and SEM images of the coacervate shows a layer–layer stacking structure consisting of a three-dimensional network formed by the assembly of threadlike micelles. Long, dense and almost uncharged threadlike micelles are the precursors of coacervation in the system.

Interactions of trianionic curcumin (Cur3−) with a series of cationic surfactants, monomeric surfactant dodecyl trimethylammonium bromide (DTAB), dimeric surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12–6–12) and trimeric surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD), have been investigated in aqueous solution of pH 13.0. Surface tension and spectral measurements indicate that the cationic surfactants display a similar surfactant concentration dependent interaction process with Cur3−, involving three interaction stages. At first the three cationic surfactants electrostatically bind on Cur3− to form the surfactant–Cur3− complex. Then the bound and unbound cationic surfactants with Cur3− aggregate into surfactant–Cur3− mixed micelles through hydrophobic interactions above the critical micelle concentration of the surfactants (CMCC) in the presence of Cur3−. Finally excess unbound surfactants self-assemble into micelles like those without Cur3−. For all the three surfactants, the addition of Cur3− only decreases the critical micelle concentration of 12–6–12 but does not affect the critical micelle concentration of DTAB and DTAD. As the oligomeric degree of surfactants increases, the intermolecular interaction of the cationic surfactants with Cur3− increases and the surfactant amount needed for Cur3− encapsulation decreases. Compared with 12–6–12, either the weaker interaction of DTAB with Cur3− or stronger interaction of DTAD with Cur3− limits the stability or solubility of Cur3− in surfactant micelles. Therefore, gemini surfactant 12–6–12 is the best choice to effectively suppress Cur3− degradation at very low concentrations. Isothermal titration microcalorimetry, surface tension and 1H NMR results reveal that 12–6–12 and Cur3− form a (12–6–12)2–Cur3− complex and start to form micelles at extremely decreased concentrations, where either 12–6–12 or Cur3− works as a bridge linkage and the resultant structure exhibits the characteristics of oligomeric surfactants.

Cationic quaternary ammonium gemini surfactants CnH2n+1(CH3)2N+CH2CHCHCH2(CH3)2N+CnH2n+12Br– (CnC4Cn, n = 12, 8, 6) with alkyl spacers, CnH2n+1(CH3)2N+CH2CHOHCHOHCH2(CH3)2N+CnH2n+12Br– (CnC4(OH)2Cn, n = 12, 8, 6, 4) with two hydroxyl groups in alkyl spacers, and cationic ammonium single-chain surfactants CnH2n+1(CH3)2N+Br– (CnTAB, n = 12, 8, 6) have been chosen to fabricate oppositely charged surfactant mixtures with anionic sulfonate gemini surfactant C12H25N(CH2CH2CH2SO3–)CH2CH2CH2(CH3)2N(CH2CH2CH2SO3–)C12H252Na (C12C3C12(SO3)2). Surface tension, electrical conductivity, and isothermal titration microcalorimetry (ITC) were used to study their surface properties, aggregation behaviors, and intermolecular interactions. The mixtures of C12C3C12(SO3)2/CnC4(OH)2Cn (n = 12, 8) and C12C3C12(SO3)2/C12C4C12 show anomalous larger critical micelle concentration (CMC) than C12C3C12(SO3)2, while the mixtures of C12C3C12(SO3)2/CnC4(OH)2Cn (n = 6, 4), C12C3C12(SO3)2/CnC4(OH)2Cn (n = 6, 4), and C12C3C12(SO3)2/CnTAB (n = 12, 8, 6) exhibit much lower CMC than C12C3C12(SO3)2. The results indicate that strong hydrophobic interactions between the alkyl chains assisted by strong electrostatic attractions between the headgroups and hydrogen bonds between the spacers lead to the formation of less surface active premicellar aggregates in bulk solution, resulting in the increase of CMC. If these interactions are weakened or inhibited, less surface active premicellar aggregates are no longer formed in the mixtures, and thus the CMC values are reduced. The work reveals that the combination of two surfactants with great self-assembling ability separately may have strong intermolecular binding interactions; however, their mixtures do not always generate superior synergism properties. Only moderate intermolecular interaction can generate the strongest synergism in CMC reduction.

The self-assembly of a 1% hydrophobically modified and 30% hydrolyzed polyacrylamide (C12PAM) with cationic star-shaped oligomeric surfactants has been investigated by isothermal titration microcalorimetry, turbidimetry, ζ potential, scanning electron microscopy, and 1H NMR techniques. The oligomeric surfactants are composed of quaternary dodecyldimethylammonium ions with three or six hydrophobic chains connected by a polyamine spacer at the headgroup level, abbreviated as DTAD and PAHB, respectively. DTAD/C12PAM and PAHB/C12PAM mixed systems undergo the same aggregate transitions with increases in surfactant concentration from soluble networklike aggregates to precipitated denser and more cross-linked structures and then to soluble spherical aggregates. The networklike aggregates are generated at very low surfactant concentration. However, at the corresponding surfactant concentration without C12PAM, DTAD cannot form aggregates and PAHB forms only networklike aggregates with a very loose structure. The strong electrostatic and hydrophobic interaction of DTAD and PAHB with C12PAM and the hydrophobic interaction between the alkyl chains of DTAD and PAHB themselves evidently promote the formation of networklike aggregates. As the surfactant concentration increases, cationic surfactants become excessive. The molecular configuration is changed by the stronger hydrophobic association among the DTAD and PAHB molecules and the enhanced electrostatic repulsion between the mixed aggregates. Thus, the networklike aggregates transfer to spherical aggregates.

Effects of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12–6–12) on the micellization of two triblock copolymers of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), F127 (EO97PO69EO97) and P123 (EO20PO70EO20), have been studied in aqueous solution by differential scanning calorimetry (DSC), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), and NMR techniques. Compared with traditional single-chain ionic surfactants, 12–6–12 has a stronger ability of lowering the CMT of the copolymers, which should be attributed to the stronger aggregation ability and lower critical micelle concentration of 12–6–12. The critical micelle temperature (CMT) of the two copolymers decreases as the 12–6–12 concentration increases and the ability of 12–6–12 in lowering the CMT of F127 is slightly stronger than that of P123. Moreover, a combination of ITC and DLS has shown that 12–6–12 binds to the copolymers at the temperatures from 16 to 40 °C. At the temperatures below the CMT of the copolymers, 12–6–12 micelles bind on single copolymer chains and induce the copolymers to initiate aggregation at very low 12–6–12 concentration. At the temperatures above the CMT of the copolymers, the interaction of 12–6–12 with both monomeric and micellar copolymers leads to the formation of the mixed copolymer/12–6–12 micelles, then the mixed micelles break into smaller mixed micelles, and finally free 12–6–12 micelles form with the increase of the 12–6–12 concentration.

Dysfunctional interaction of amyloid-β (Aβ) with excess metal ions is proved to be related to the etiology of Alzheimer’s disease (AD). Using metal-binding compounds to reverse metal-triggered Aβ aggregation has become one of the potential therapies for AD. In this study, the ability of a carboxylic acid gemini surfactant (SDUC), a widely used metal chelator (EDTA), and an antifungal drug clioquinol (CQ) in reversing the Cu2+-triggered Aβ(1–40) fibers have been systematically studied by using turbidity essay, BCA essay, atomic force microscopy, transmission electron microscopy, and isothermal titration microcalorimetry. The results show that the binding affinity of Cu2+ with CQ, SDUC, and EDTA is in the order of CQ > EDTA > SDUC, while the disaggregation ability to Cu2+-triggered Aβ(1–40) fibers is in the order of CQ > SDUC > EDTA. Therefore, the disaggregation ability of chelators to the Aβ(1–40) fibers does not only depend on the binding affinity of the chelators with Cu2+. Strong self-assembly ability of SDUC and π–π interaction of the conjugate group of CQ also contributes toward the disaggregation of the Cu2+-triggered Aβ(1–40) fibers and result in the formation of mixed small aggregates.

A novel synthetic approach was developed for creating versatile hollow Au nanostructures by stepwise reductions of AuIII upon the use of cationic gemini surfactant hexamethylene-1,6-bis(dodecyl dimethylammonium bromide) (C12C6C12Br2) as a template agent. It was observed that the AuI ions obtained from the reduction of AuIII by ascorbic acid can assist the gemini surfactant to form vesicles, capsule-like, and tube-like aggregates that subsequently act as soft templates for hollow Au nanostructures upon further reduction of AuI to Au0 by NaBH4. It was demonstrated that the combination of C12C6C12Br2 and AuI plays a key role in regulating the structure of the hollow precursors not only because C12C6C12Br2 has a stronger aggregation ability in comparison with its single chain counterpart but also because the electrostatic repulsion between head groups of C12C6C12Br2 is greatly weakened after AuIII is converted to AuI, which is in favor of the construction of vesicles, capsule-like, and tube-like aggregates. Compared with solid Au nanospheres, the resultant hollow nanostructures exhibit enhanced electrocatalytic activities in methanol oxidation, following the order of elongated nanocapsule > nanocapsule > nanosphere. Benefiting from balanced interactions between the gemini surfactant and AuI, this soft-template method may present a facile and versatile approach for the controlled synthesis of Au nanostructures potentially useful for fuel cells and other Au nanodevices.Keywords: electrocatalytic oxidation; gemini surfactant; hollow Au nanostructures; stepwise reduction; template;

Snowflake-like two-dimensional highly branched gold nanostructures (2DHBNs) have been constructed in the presence of hexamethylene-1,6-bis (dodecyl dimethylammonium bromide) (C12C6C12Br2) through the reduction of HAuCl4 by ascorbic acid. High resolution transmission electron microscopy results indicate that the 2DHBNs are uniform in crystallographic orientation, where the branched nanostructures grow along the <211> direction and present multiple twin planes stacked along the <111> direction. An ex situ kinetic study, followed by TEM observations, shows that the 2DHBNs are evolved from triangular nanoprisms through overgrowth along the <211> direction. The gemini surfactant C12C6C12Br2 is found to be the most essential factor for the formation of the 2DHBNs. C12C6C12Br2 may selectively absorb onto the (111) planes, which promotes the construction of 2D nanostructures. Moreover, C12C6C12Br2 can act as an excellent kinetic modifier to control the growth of the nanostructures, which also affects the generation of branched nanostructures. The as-prepared 2DHBNs exhibit an efficient surface-enhanced Raman scattering (SERS) property which has potential application in constructing sensitive SERS substrates.

Aggregation behaviors in mixtures of an anionic gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)–propane (C12C3C12(SO3)2) and a cationic single-chain surfactant cetyltrimethylammonium bromide (CTAB) have been investigated in aqueous solutions at pH 9.5 by turbidity, rheology, isothermal titration microcalorimetry (ITC), cryogenic transmission electron microscopy, and dynamic light scattering. Reversible aggregate transitions from spherical micelles to wormlike micelles, vesicles, and back to wormlike micelles and spherical micelles are successfully realized through fine regulation over the mixing ratio of surfactants, i.e., the anionic/cationic charge ratio. The five aggregate regions display distinguished phase boundaries so that the aggregate regions can be well controlled. From thermodynamic aspect, the ITC curves clearly reflect all the aggregate transitions and the related interaction mechanism. The self-assembling ability of the C12C3C12(SO3)2/CTAB mixtures are significantly improved compared with both individual surfactants. Micelle growth from spherical to long wormlike micelles takes place at a relative low total concentration, i.e., 2.0 mM. The wormlike micelle solution at 10 mM or higher shows high viscosity and shear thinning property. Moreover, the C12C3C12(SO3)2/CTAB mixtures do not precipitate even at 1:1 charge ratio and relative high concentration. It suggests that applying gemini surfactant should be an effective approach to improve the solubility of anionic/cationic surfactant mixtures and in turn may promote applications of the surfactant mixtures.

Coacervation of cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (12–6–12) with pH-sensitive N-benzoylglutamic acid (H2Bzglu) has been investigated by potentiometric pH-titration, turbidity titration, dynamic light scattering (DLS), isothermal titration calorimetry (ITC), TEM, 1H NMR, and light microscopy. Phase boundaries of the 12–6–12/H2Bzglu mixture were obtained over the pH range from 2 to 9 and in the H2Bzglu concentration range from 30.0 to 50.0 mM at pH 4.5. When the H2Bzglu concentration is beyond 30.0 mM, the 12–6–12/H2Bzglu mixed solution undergoes the phase transitions from soluble aggregate, to precipitate, coacervate, and soluble aggregate again as pH increases. The results indicate that coacervation occurs at extremely low 12–6–12 concentration and lasts over a wide surfactant range, and can be enhanced or suppressed by changing pH, 12–6–12/H2Bzglu molar ratio and H2Bzglu concentration. The coacervates present a disorderly connected lay structure. Coacervation only takes place at pH 4–5, where the aggregates are nearly charge neutralized, and a minimum H2Bzglu concentration of 30.0 mM is required for coacervation. In this pH range, H2Bzglu mainly exist as HBzglu–. The investigations on intermolecular interactions indicate that the aggregation of 12–6–12 is greatly promoted by the strong electrostatic and hydrophobic interactions with the HBzglu– molecules, and the interaction also promotes the formation of dimers, trimers, and tetramers of HBzglu– through hydrogen bonds. The double chains of 12–6–12 and the HBzglu– oligomers can play the bridging roles connecting aggregates. These factors endow the mixed system with a very high efficiency in generating coacervation.

Anionic single-tail surfactant sodium dodecyl sulfate (SDS) and a molecule with multiple amido and amine groups (Lys-12-Lys) were used as building blocks to fabricate oligomeric surfactants through intermolecular interactions. Their interactions and the resultant complex and aggregate structures were investigated by turbidity titration, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, freeze-fracture transmission electron microscopy, 1H NMR, and 1D NOE techniques. At pH 11.0, the interaction between SDS and Lys-12-Lys is exothermic and mainly resulted from hydrogen bonding among the amido and amine groups of Lys-12-Lys and the sulfate group of SDS and hydrophobic interaction between the hydrocarbon chains of SDS and Lys-12-Lys. At pH 3.0, each Lys-12-Lys carries four positive charges and two hydrogen bonding sites. Then SDS and Lys-12-Lys form complexes Lys-12-Lys(SDS)6 and Lys-12-Lys(SDS)4 through the head groups by electrostatic attraction and hydrogen bonds assisted by hydrophobic interaction. Moreover, the complexes pack more tightly in their aggregates with the increase of the molar ratio. Especially the Lys-12-Lys(SDS)4 and Lys-12-Lys(SDS)6 complexes behave like oligomeric surfactants taking Lys-12-Lys as a spacer group, exhibiting a series of aggregates transitions with the increase of concentration, i.e., larger vesicles, smaller spherical micelles, and long threadlike micelles. Therefore, oligomeric surfactants Lys-12-Lys(SDS)4 and Lys-12-Lys(SDS)6 have been successfully fabricated by using a single chain surfactant and an oligomeric connecting molecule through noncovalent association.

The coassembly of poly(ethylene glycol)-b-poly(glutamate sodium) copolymer (PEG113-PGlu100) with cationic gemini surfactants alkanediyl-α,ω-bis-(dodecyldimethylammonium bromide) [C12H25(CH3)2N(CH2)SN(CH3)2C12H25]Br2 (designated as C12CSC12Br2, S = 3, 6, and 12) have been studied by isothermal titration microcalorimetry, cryogenic transmission electron microscopy, circular dichroism, small-angle X-ray scattering, zeta potential, and size measurement. It has been shown that the electrostatic interaction of C12CSC12Br2 with the anionic carboxylate groups of PEG113-PGlu100 leads to complexation, and the C12CSC12Br2/PEG113-PGlu100 complexes are soluble even at the electroneutral point. The complexes display the feature of superamphiphiles and assemble into ordered nanosheets with a sandwich-like packing. The gemini molecules which were already bound with PGlu chains associate through hydrophobic interaction and constitute the middle part of the nanosheets, whereas the top and bottom of the nanosheets are hydrophilic PEG chains. The size and morphology of the nanosheets are affected by the spacer length of the gemini surfactants. The average sizes of the aggregates at the electroneutral point are 81, 68, and 90 nm for C12C3C12Br2/PEG113-PGlu100, C12C6C12Br2/PEG113-PGlu100, and C12C12C12Br2/PEG113-PGlu100, respectively. Both C12C3C12Br2/PEG113-PGlu100 and C12C12C12Br2/PEG113-PGlu100 mainly generate hexagonal nanosheets, while the C12C6C12Br2/PEG113-PGlu100 system only induces round nanosheets.

Modulation of the fibrillogenesis of amyloid peptide Aβ(1–40) with two Aβ-based peptide amphiphiles has been studied. Both peptide amphiphiles contain two alkyl chains but in different positions. The two alkyl chains of 2C12–Aβ(11–17) are attached to the same terminus of Aβ(11–17), while those of C12–Aβ(11–17)–C12 are separately attached to opposite termini of Aβ(11–17). Thioflavin T fluorescence spectroscopy shows that all the peptide amphiphiles promote the formation of the cross-β-sheet structure of Aβ(1–40) and the aggregation of Aβ(1–40), while 2C12–Aβ(11–17) does this more efficiently. The atom force microscopy images indicate that the modulations of these two peptide amphiphiles on the Aβ(1–40) aggregation experience two distinct pathways. 2C12–Aβ(11–17) leads to amorphous aggregates, whereas C12–Aβ(11–17)–C12 generates short rodlike fibrils. However, Fourier transform infrared spectroscopy suggests that the amorphous aggregates and rodlike fibrils display similar secondary structures. This work suggests that the aggregation ability and the aggregate structures of the peptide amphiphiles significantly affect their interactions with Aβ(1–40) and lead to different morphologies of the Aβ(1–40) aggregates.

Salt effects on the aggregation behavior of tripolar zwitterionic surfactants in aqueous solutions have been investigated using surface tension, dynamic light scattering (DLS), freeze-fracture transmission electron microscopy (FF-TEM), and 1H NMR. The tripolar zwitterionic surfactants with different inter-charge spacers are [C14H29(CH3)2N+CsN+(CH3)2CH2CH2CH2SO3−]Br− (C14CsTri, Cs = –(CH2)2–, –(CH2)6–, –(CH2)10–, and p-xylyl). It is found that the critical micelle concentration (CMC) values of the corresponding traditional zwitterionic surfactant C14H29(CH3)2N+CH2CH2CH2SO3− (TPS) are almost constant with the increase of the NaBr concentration. However, the CMC values of C14CsTri decrease sharply at a lower NaBr concentration and then level off at a higher NaBr concentration. Moreover, the decreasing extents of the CMC values for C14C2Tri, C14C6Tri, and C14CpxTri are very close, but more significant than that for C14C10Tri, suggesting that the self-assembly ability of the tripolar zwitterionic surfactants with a longer inter-charge spacer is less sensitive to NaBr. The DLS and FF-TEM results reveal that C14C2Tri, C14C6Tri, and C14CpxTri form micelles without NaBr and that the size slightly increases with the increase of NaBr concentration, whereas micelles and vesicles coexist for C14C10Tri and TPS without NaBr and then transfer to micelles upon the addition of NaBr. The salt-induced morphological transition for C14C10Tri is further studied using 1H NMR. The addition of NaBr reduces both the electrostatic repulsion between the same charged ammoniums and the electrostatic attraction between the oppositely charged ammonium and sulfonate. Thus, the longer inter-charge spacer of C14C10Tri tends to be more bended and the sulfonate group becomes available to contact the ammonium, which promotes micellization.

Effects of calcium ions on the solubility and aggregation behavior of an anionic sulfonate gemini surfactant 1,3-bis(N-dodecyl-N-propylsulfonate sodium)-propane (12-3-12(SO3)2) have been studied in aqueous solution. Compared with single-chain surfactant sodium dodecylsulfate, 12-3-12(SO3)2 shows much better performance to the hardness tolerance with calcium ions. Moreover aggregates of the Ca2+/12-3-12(SO3)2 complexes in clear solutions influence the morphologies of the precipitates. At 12-3-12(SO3)2 concentrations lower than 1.5 mM, the small spherical micelles of Ca2+/12-3-12(SO3)2 in clear solutions generate precipitates of solid particles owing to complexation of surfactant monomers with Ca2+. At 12-3-12(SO3)2 concentrations higher than 1.5 mM, the Ca2+/12-3-12(SO3)2 complexes transform into large compact spherical aggregates and then into long wormlike micelles. These large aggregates are well dispersed in aqueous solutions and efficiently complex calcium ions. In particular, long wormlike micelles are entangled with each other at 100.0 mM CaCl2 and 100.0 mM 12-3-12(SO3)2 exhibiting viscoelastic properties. In addition, the stacking of long wormlike micelles produces precipitates with ordered fibrillar structures. This work reveals that such anionic sulfonate gemini surfactants are better candidates than single-chain surfactants in applications with high hardness levels, and the ordered aggregate structures may have potential applications in materials science.Graphical abstractHighlights► The anionic gemini surfactant 12-3-12(SO3)2 shows excellent tolerance to Ca2+ in water. ► Aggregates in clear solutions lead to precipitates with similar morphologies. ► Wormlike micelles are observed in concentrated Ca2+/12-3-12(SO3)2 clear solutions. ► Stacking of long wormlike micelles produces fiber based precipitate structures. ► The larger aggregates are well dispersed in aqueous solution and also complex Ca2+.

Effects of a “gemini-type” organic salt 1,2-bis(2-benzylammoniumethoxy) ethane dichloride (BEO) on the aggregation behavior of sodium dodecylsulfate (SDS) have been investigated by turbidity, surface tension, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, 1H NMR spectroscopy, and differential scanning microcalorimetry. The aggregation behavior of the SDS/BEO mixed aqueous solution shows strong concentration and ratio dependence. For the SDS/BEO solution with a molar ratio of 5:1, large loose irregular aggregates, vesicles, and long thread-like micelles are formed in succession with the increase of the total SDS and BEO concentration. Because BEO has two positive charges, the SDS/BEO solution may consist of the (SDS)2–BEO gemini-type complex, the SDS–BEO complex and extra SDS. The aggregation ability and surface activity of the SDS/BEO mixture exhibit the characteristics of gemini-type surfactants. Along with the results of DSC and 1H NMR, the (SDS)2–BEO gemini-type structure is confirmed to exist in the system. This work provides an approach to construct the surfactant systems with the characteristics of gemini surfactants through intermolecular interaction between a two-charged organic salt and oppositely charged single-chain surfactants.

The association behaviors of single-chain surfactant dodecyltrimethylammonium bromide (DTAB) with double hydrophilic block co-polymers poly(ethylene glycol)-b-poly(sodium glutamate) (PEG113–PGlu50 or PEG113–PGlu100) were investigated using isothermal titration microcalorimetry, cryogenic transmission electron microscopy, circular dichroism, ζ potential, and particle size measurements. The electrostatic interaction between DTAB and the oppositely charged carboxylate groups of PEG–PGlu induces the formation of super-amphiphiles, which further self-assemble into ordered aggregates. Dependent upon the charge ratios between DTAB and the glutamic acid residue of the co-polymer, the mixture solutions can change from transparent to opalescent without precipitation. Dependent upon the chain length of the PGlu block, the mixture of DTAB and PEG–PGlu diblocks can form two different aggregates at their corresponding electroneutral point. Spherical and rod-like aggregates are formed in the PEG113–PGlu50/DTAB mixture, while the vesicular aggregates are observed in the PEG113–PGlu100/DTAB mixture solution. Because the PEG113–PGlu100/DTAB super-amphiphile has more hydrophobic components than that of the PEG113–PGlu50/DTAB super-amphiphile, the former prefers forming the ordered aggregates with higher curvature, such as spherical and rod aggregates, but the latter prefers forming vesicular aggregates with lower curvature.

Two peptide–amphiphiles (PAs), 2C12–Lys–Aβ(12–17) and C12–Aβ(11–17)–C12, were constructed with two alkyl chains attached to a key fragment of amyloid β-peptide (Aβ(11–17)) at different positions. The two alkyl chains of 2C12–Lys–Aβ(12–17) were attached to the same terminus of Aβ(12–17), while the two alkyl chains of C12–Aβ(11–17)–C12 were separately attached to each terminus of Aβ(11–17). The self-assembly behavior of both the PAs in aqueous solutions was studied at 25 °C and at pHs 3.0, 4.5, 8.5, and 11.0, focusing on the effects of the attached positions of hydrophobic chains to Aβ(11–17) and the net charge quantity of the Aβ(11–17) headgroup. Cryogenic transmission electron microscopy and atomic force microscopy show that 2C12–Lys–Aβ(12–17) self-assembles into long stable fibrils over the entire pH range, while C12–Aβ(11–17)–C12 forms short twisted ribbons and lamellae by adjusting pHs. The above fibrils, ribbons, and lamellae are generated by the lateral association of nanofibrils. Circular dichroism spectroscopy suggests the formation of β-sheet structure with twist and disorder to different extents in the aggregates of both the PAs. Some of the C12–Aβ(11–17)–C12 molecules adopt turn conformation with the weakly charged peptide sequence, and the Fourier transform infrared spectroscopy indicates that the turn content increases with the pH increase. This work provides additional basis for the manipulations of the PA’s nanostructures and will lead to the development of tunable nanostructure materials.

Controllable aggregate transitions were realized by mixing two kinds of cationic surfactants, hexylene-1,6-bis(dodecyldimethylammonium bromide) (C12C6C12Br2) and didodecyldimethylammonium bromide (DDAB). It was found that two parameters are the main factors determining the aggregation behavior of the mixed system, the total concentration of DDAB and C12C6C12Br2 (CT), and the mole fraction of DDAB in the mixtures of DDAB and C12C6C12Br2 (XDDAB). How these two parameters act on the aggregate transitions was studied in detail by various measurements including surface tension, turbidity, electrical conductivity, ζ potential, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and 1H NMR. When CT was constant, spontaneous vesicle-to-micelle transitions were found with decreasing XDDAB at high CT. When XDDAB was constant, aggregate transitions were generated by gradually increasing CT, depending on different XDDAB ranges. At XDDAB < 0.6, small spherical aggregates formed first and then transferred to vesicles, and finally the vesicles transitioned to micelles. At XDDAB ≥ 0.6, the progressive increase in CT led to aggregate transitions on the order of the arising of vesicles, the continuous growth of vesicles, the disruption of vesicles into micelles, and the final coexistence of vesicles and micelles. The hydrophobic interaction and electrostatic repulsion between DDAB and C12C6C12Br2 together with the related degree of ionization and hydration of the surfactants were gradually adjusted by changing the ratio and the total concentration of these two surfactants, which should be responsible for the complicated aggregation behavior.

The interaction between a peptide amphiphile C12–Aβ(11–17) and an anionic dye TPE with aggregation-induced emission (AIE) feature at different pH has been studied. C12–Aβ(11–17) was constructed by attaching a key fragment of amyloid β-peptide (Aβ(11–17)) to a dodecanoic acid through an amide bond. It was found that highly fluorescent nanofibrils can be constructed through the self-assembly of C12–Aβ(11–17) with TPE. Moreover, formation, structure and fluorescence intensity of the nanofibrils can be modulated by changing pH and the concentration of C12–Aβ(11–17). In the solution of 0.05 mM C12–Aβ(11–17) and 0.05 mM TPE, as the pH decreases, the fluorescence intensity of the mixed solution is invariable at pH > 7.0, then fast increases to the maximum while 4.0 < pH < 7.0, and finally fast decreases at pH < 4.0. Only at pH 4.0–7.0, fluorescent nanofibrils of less than 5 μm length with the β-sheet secondary structure are formed upon the addition of TPE molecules. In 1.0 mM C12–Aβ(11–17) solution with 0.05 mM TPE, the fluorescence intensity of the mixed solution keeps increasing as the pH decreases in the pH range of 3.4–11.0, and the mixture forms strongly fluorescent nanotapes of more than 50 μm length with an ordered β-sheet structure at pH 3.0 or relatively weaker fluorescent nanoribbons with a random coil structure at pH 10.0.

Gemini surfactants are constructed by two hydrophobic chains and two polar/ionic head groups covalently connected by a spacer group at the level of the head groups. Gemini surfactants possess unique structural variations and display special aggregate transitions. Their aggregation ability and aggregate structures can be more effectively adjusted through changing their molecular structures compared with the corresponding monomeric surfactants. Moreover, gemini surfactants exhibit special and useful properties while interacting with polymers and biomacromolecules. Their strong self-aggregation ability can be applied to effectively influence the aggregation behavior of both polymers and biomacromolecules. This short review is focused on the performances of gemini surfactants in aqueous solutions investigated in the last few years, and summarizes the effects of molecular structures on aggregation behavior of gemini surfactants in aqueous solution as well as the interaction of gemini surfactants with polymers and biomacromolecules respectively.

Branched alkylbenzenesulfonate gemini surfactants: sodium 6,6′-(propane-1,3-diylbis(oxy))bis(3-(2-propylpentyl)benzenesulfonate) (C8BC3C8B), sodium 6,6′-(propane-1,3-diylbis(oxy))bis(3-(3,5,5-trimethylhexyl)benzenesulfonate) (C9BC3C9B), and sodium 6,6′-(propane-1,3-diylbis(oxy))bis(3-(2,4,4-trimethylpentan-2-yl)benzenesulfonate) (T-C8BC3C8B) have been synthesized. Their interfacial activity and aggregation behavior in aqueous solution were studied by surface/interface tension measurement, electrical conductivity, isothermal titration microcalorimetry, 1H NMR spectroscopy, dynamic light scattering, steady-state fluorescence and cryogenic transmission electron microscopy. The critical aggregation concentration (CAC) and the minimum average surface area/molecule (Amin) decrease with the decrease of the branching factor, i.e., in the order of T-C8BC3C8B, C8BC3C8B and C9BC3C9B. Moreover, alkyl side chain branches generate much more significant increases in CAC and Amin for the gemini surfactants than single-chain surfactants. However, the branching factor does not change the air/water surface tension at CAC regularly. Instead, the air/water surface tension decreases with the increase of the carbon number of the hydrocarbon chains. In addition, it is noted that the branched gemini surfactants display high efficiency in reducing toluene/water interfacial tension. Interestingly, the increase in the branching factor leads to much more endothermic enthalpy of aggregation. All these three surfactants form spherical vesicles in aqueous solution and may present a parallel-displaced structure with a directional arrangement of the benzene ring in the vesicles.Graphical abstractThree branched alkylbenzenesulfonate gemini surfactants were synthesized, and spherical vesicles were observed in the surfactant solutions. The increase in the branching factor leads to much more endothermic enthalpy of aggregation.Highlights► Three branched alkylbenzenesulfonate gemini surfactants have been synthesized. ► Alkyl side branches generate significant increases in CAC and Amin. ► The branching factor does not change the air/water surface tension regularly. ► The surfactants display high efficiency in reducing toluene/water interfacial tension. ► These branched gemini surfactants form spherical vesicles in aqueous solution.

We report a finding that not only the micelles but also the premicellar aggregates of a star-like tetrameric quaternary ammonium surfactant PATC can disassemble and clear mature β-amyloid Aβ(1−40) fibrils in aqueous solution. Different from other surfactants, PATC self-assembles into network-like aggregates below its critical micelle concentration (CMC). The strong self-assembly ability of PATC even below its CMC enables PATC to disaggregate the Aβ(1−40) fibrils far below the charge neutralization point of the Aβ(1−40) with PATC. There may be two key features of the fibril disassembly induced by the surfactant. First, the positively charged surfactant molecules bind with the negatively charged Aβ(1−40) fibrils through electrostatic interaction. Second, the self-assembly of the surfactant molecules bound onto the Aβ(1−40) fibrils disaggregate the fibrils, and the surfactant molecules form mixed aggregates with the Aβ(1−40) molecules. The result reveals a structural approach of constructing efficient disassembly agents to mature β-amyloid fibrils.

A star-shaped hexameric quaternary ammonium surfactant (PAHB), bearing six hydrophobic chains and six charged hydrophilic headgroups connected by an amide-type spacer group, was synthesized. The self-assembly behavior of the surfactant in aqueous solution was studied by surface tension, electrical conductivity, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and NMR techniques. The results reveal that there are two critical aggregate concentrations during the process of aggregation, namely C1 and C2. The aggregate transitions are proved to be caused by the changes of the surfactant configuration through hydrophobic interaction among the hydrocarbon chains. Below C1, PAHB may present a star-shaped molecular configuration due to intramolecular electrostatic repulsion among the charged headgroups, and large aggregates with network-like structure are observed. Between C1 and C2, the hydrophobic interaction among the hydrophobic chains may become stronger to make the hydrophobic chains of the PAHB molecules curve back and pack more closely, and then the network-like aggregates transfer to large spherical aggregates of ∼100 nm. Beyond C2, the hydrophobic interaction may become strong enough to cause the PAHB molecular configuration to turn into a pyramid-like shape, resulting in the transition of the spherical large aggregates to spherical micelles of ∼10 nm. Interestingly, the PAHB displays high emulsification ability to linear fatty alkyls even at very low concentration.

Two star-like trimeric cationic surfactants with amide groups in spacers, tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) and tri(dodecyldimethylammonioacetoxy)tris(2-aminoethyl)amine trichloride (DDAD), have been synthesized, and the aggregation behavior of the surfactants in aqueous solution has been investigated by surface tension, electrical conductivity, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and NMR techniques. Typically, both the surfactants form vesicles just above critical aggregation concentration (CAC), and then the vesicles transfer to micelles gradually with an increase of the surfactant concentration. It is approved that the conformation of the surfactant molecules changes in this transition process. Just above the CAC, the hydrophobic chains of the surfactant molecules pack more loosely because of the rigid spacer and intramolecular electrostatic repulsion in the three-charged headgroup. With the increase of the surfactant concentration, hydrophobic interaction becomes strong enough to pack the hydrophobic tails tightly and turn the molecular conformation into a pyramid-like shape, thus leading to the vesicle to micelle transition.

A star-shaped tetrameric quaternary ammonium surfactant PATC, which has four hydrophobic chains and charged hydrophilic headgroups connected by amide-type spacer group, has been synthesized in this work. Surface tension, electrical conductivity, ITC, DLS, and NMR have been used to investigate the relationship between its chemical structure and its aggregation properties. Interestingly, a large size distribution around 75 nm is observed below the critical micelle concentration (cmc) of PATC, and the large size distribution starts to decrease beyond the cmc and finally transfers to a small size distribution. It is proved that the large size premicellar aggregates may display network-like structure, and the size decrease beyond the cmc is the transition of the network-like aggregates to micelles. The possible reason is that intramolecular electrostatic repulsion among the charged headgroups below the cmc leads to a star-shaped molecular configuration, which may form the network-like aggregates through intermolecular hydrophobic interaction between hydrocarbon chains, while the hydrophobic effect becomes strong enough to turn the molecular configuration into pyramid-like shape beyond the cmc, which make the transition of network-like aggregates to micelles available.

Isothermal titration microcalorimetry, turbidity, and steady-state fluorescence measurements have been used to study interactions of cationic ammonium gemini surfactant (C12C6C12Br2) and single-chain surfactant dodecyltrimethylammonium bromide (DTAB) with anionic polyelectrolytes poly(sodium styrenesulfonates) (NaPSS) and poly(sodium acrylates) (NaPAA) with different molar masses. Without any surfactants, NaPSS with lower molar mass has already self-aggregated into aggregates, whereas NaPAA has no aggregation at any molar mass. All of the polyelectrolytes show a remarkable interaction with the cationic surfactants. Compared with DTAB, C12C6C12Br2 can bind to NaPSS and NaPAA at a very low concentration and has stronger interactions with NaPSS and NaPAA. The flexible NaPAA shows moderately endothermic enthalpies while interacting with the surfactants, but the interaction of the stiff NaPSS with the surfactants exhibits highly exothermic enthalpies. Moreover, the interaction of the stiff NaPSS with the surfactants strongly depends on the polyelectrolyte molar mass, but the polyelectrolyte molar mass almost does not affect the interaction of the flexible NaPAA with the surfactants. Especially, the effect of the polyelectrolyte molar mass becomes more significant when the polyelectrolytes interact with gemini surfactant than with single-chain surfactant. It is revealed that the effects of polyelectrolyte molar mass, chain flexibility, and surfactant architecture on surfactant/polyelectrolyte interactions confine each other.

Three types of magnetic iron oxide nanoparticles with various kinds of surface modifications were synthesized, and the interactions between the nanoparticles and two types of high abundant plasma proteins were investigated by isothermal titration calorimetry and dynamic light scattering (DLS) methods. It was found that these interactions were strongly dependent on the surface properties of the nanoparticles. Enthalpy−entropy analysis suggested that poly(ethylene glycol) (PEG) modification on the particle surface could effectively reduce the interactions between the magnetic nanoparticles and the plasma proteins. DLS investigations further implied that electrostatic attractions could either increase or decrease the colloidal stability of the nanoparticles, depending on the particle surface properties, which will give rise to different in vivo biodistributions for the intravenously injected nanoparticles, according to literature reports. Proper surface modifications, upon the use of PEG in combination with various types of small molecules for reducing surface charges, were found to be effective for eliminating the strong interactions between nanoparticles and proteins, which is of the utmost importance for developing iron oxide magnetic nanoparticles with long blood circulation time for in vivo applications.

All salts studied effectively reduce critical micelle concentration (CMC) values of the cationic gemini surfactants. The ability to promote the surfactant aggregation decreases in the order of C6H5COONa > p-C6H4(COONa)2 > Na2SO4> NaCl. Moreover, only C6H5COONa distinctly reduces both the CMC values and the surface tension at CMC. For 12-4-12 solution, the penetration of C6H5COO− anions and charge neutralization induce a morphology change from micelles to vesicles, whereas the other salts only slightly increase the sizes of micelles. The 12-4(OH)2-12 solution changes from the micelle/vesicle coexistence to vesicles with the addition of C6H5COONa, whereas the other salts transfer the 12-4(OH)2-12 solution from the micelle/vesicle coexistence to micelles. As compared with 12-4-12, the two hydroxyls in the spacer of 12-4(OH)2-12 promote the micellization of 12-4(OH)2-12 and reduce the amounts of C6H5COONa required to induce the micelle-to-vesicle transition.

The effects of anionic surfactants on the aggregation-induced emission (AIE) feature of cationic M-silole molecules have been studied. The electrostatic binding of M-silole with the surfactants greatly promotes the aggregation of the mixtures. The M-silole/surfactant aggregates at 1:1 charge ratio exhibit the maximum fluorescence intensity. Excess surfactant molecules will distribute the M-silole molecules into different micelles and weaken the fluorescence. The fluorescence intensity of the mixed M-silole/surfactant aggregates can be effectively modulated by choosing different surfactants. The gemini surfactants display a much stronger ability of enhancing fluorescence intensity than do the single-chain surfactants. Especially, the gemini surfactant with benzene rings shows the best performance in enhancing fluorescence of M-silole due to both the strongest aggregation ability and the π−π interaction with M-silole.

Cationic surfactant/anionic surfactant/β-CD ternary aqueous systems provide a platform for the coexistence of the host−guest (β-CD/surfactant) equilibrium and the biased aggregation (monomeric/aggregated surfactants) equilibrium. We report here that the interplay between the two equilibria dominates the systems as follows. (1) The biased aggregation equilibrium imposes an apparent selectivity on the host−guest equilibrium, namely, β-CD has to always selectively bind the major surfactant (molar fraction > 0.5) even if binding constants of β-CD to the pair of surfactants are quite similar. (2) In return, the host−guest equilibrium amplifies the bias of the aggregation equilibrium, that is, the selective binding partly removes the major surfactant from the aggregates and leaves the aggregate composition approaching the electroneutral mixing stoichiometry. (3) This composition variation enhances electrostatic attractions between oppositely charged surfactant head groups, thus resulting in less-curved aggregates. In particular, the present apparent host−guest selectivity is of remarkably high values, and the selectivity stems from the bias of the aggregation equilibrium rather than the difference in binding constants. Moreover, β-CD is defined as a “stoichiometry booster” for the whole class of cationic/anionic surfactant systems, which provides an additional degree of freedom to directly adjust aggregate compositions of the systems. The stoichiometry boosting of the compositions can in turn affect or even determine microstructures and macroproperties of the systems.

The interfacial property of cationic ammonium gemini surfactants with the structure of [C12H25(CH3)2N(CH2)SN(CH3)2C12H25]Br2 (S = 3, 4, 6, 10, and 12), referred to as C12CSC12Br2, has been studied at the n-heptane/water interface as a function of the surfactant concentration. The interfacial tension vs surfactant concentration curves show an apparent minimum for the homologues with S ≥ 6. This behavior may be resulted from conformational and location changes of the spacer adsorbed at the interface due to its flexibility as well as the hydrophobic environment provided by the sublayer composed of gemini surfactant aggregates. In addition, the interfacial tension at the interface between n-heptane and the aqueous solution of C12C6C12Br2 with polyacrylamide (PAM) and hydrophobically modified polyacrylamide (HMPAM) has been studied to aid the understanding on the adsorption mechanism proposed for the abnormal interfacial behavior of C12CSC12Br2 surfactants. The interfacial tension is reduced significantly by the C12C6C12Br2/PAM mixture at very low concentration of C12C6C12Br2. For C12C6C12Br2/HMPAM, no minimum is shown in the interfacial tension curve and poorer efficiency in reducing interfacial tension is observed.

The accumulation of a peptide of 38−43 amino acids, in the form of fibrillar plaques, was one of the essential reasons for Alzheimer’s disease (AD). Discovering an agent that is able to disassemble and clear disease-associated Aβ peptide fibrils from the brains of AD patients would have critical implications not only in understanding the dynamic process of peptide aggregation but also in the development of therapeutic strategies for AD. This study reported a new finding that cationic gemini surfactant C12C6C12Br2 micelles can effectively disassemble the Aβ(1−40) fibrils in vitro. Systematic comparisons with other surfactants using ThT fluorescence, AFM, and FTIR techniques suggested that the disassembly effectiveness of gemini surfactant micelles arises from their special molecular structure (i.e., positively bicharged head and twin hydrophobic chains). To track the disassembly process, systematic cryoTEM characterization was also done, which suggested a three-stage disassembly process: (i) Spherical micelles are first absorbed onto the Aβ fibrils because of attractive electrostatic interaction. (ii) Elongated fibrils then disintegrate into short pieces and form nanoscopic aggregates via synergistic hydrophobic and electrostatic interactions. (iii) Finally, complete disaggregation of fibrils and dynamic reassembly result in the formation of peptide/surfactant complexes.

Three series of zwitterionic surfactants with different ionic headgroups, various hydrophobic chain length, and flexible or rigid inter-charge spacers have been synthesized and their aggregation behaviors in aqueous solution have been studied. Series I and III represent [CnH2n+1(CH3)2N+(CH2)6N+(CH3)2CH2CH2CH2SO3−]Br− (CnC6Tri, n = 12, 14, and 16) and CnH2n+1(CH3)2N+CH2CH2CH2SO3− (n = 12, 14, and 16), respectively. Series II are [C14H29(CH3)2N+CsN+(CH3)2CH2CH2CH2SO3−]Br– (C14CsTri, Cs = –(CH2)2–, –(CH2)6–, –(CH2)8–, –(CH2)10–, p–xylyl), where Cs stands for the inter-charge spacer connecting two quaternary ammonium groups. The critical micelle concentrations (CMC) of the surfactants with two cationic ammonium groups and one anionic sulfonate group (Series I and II) are much larger than those of the corresponding surfactants with one cationic ammonium group and one anionic sulfonate group (Series III). These surfactants exhibit obviously different aggregate morphologies. For Series III, only micelles are observed. For Series I and II, if the surfactants have short or rigid inter-charge spacer (C14C2Tri and C14CpxTri), the aggregates also appear as micelles and have more exothermic micellization enthalpy change. However, even for Series I and II, if the surfactants have longer and flexible inter-charge spacer and longer hydrocarbon chains (C14C6Tri, C16C6Tri, C14C8Tri and C14C10Tri), vesicles are observed due to the reduced hydrophilic moiety size and the strong hydrophobic interaction, whereas vesicles cannot be formed if the surfactants have longer and more flexible inter-charge spacer but without enough longer hydrocarbon chain (C12C6Tri).

Molecular dynamics simulations have been performed on the monolayers of dodecyltrimethylammonium bromide and gemini surfactants 12−S−12 with S = 3, 6, and 12 at the n-heptane/water interfaces. The normal density profiles of the interface show that the distributions of surfactants at the liquid/liquid interface are significantly broader than those at air/water interfaces from comparisons with neutron reflection experiments and previous simulations. The spacers of 12−3−12 and 12−6−12 do not migrate much from the interface, while that of 12−12−12 tends to bend into the oil phase. The conformation of the surfactants shows that the spacers are more flexible than the tails. The characteristic angles of the surfactant well depict the geometry of the surfactants at the interface. The connected N+s of 12−3−12 and 12−6−12 have a prominent peak in the radial distribution functions, while those of 12−12−12 have nearly the same peak with those not connected. It is also found by three-dimensional spatial distribution functions that water molecules and bromide ions prefer to be shared between the positively charged methyl or methylene groups.

A peptide−amphiphile (C12−Aβ(11−17)) was constructed with a key fragment of amyloid β-peptide (Aβ(11−17)) attached to dodecanoic acid through an amide bond. The self-assembly behavior of C12−Aβ(11−17) in aqueous solution is studied at 25 °C and at pH 3.0 and 10.0. Aβ(11−17) cannot form ordered self-assemblies. But C12−Aβ(11−17) exhibits a very strong ability to form ordered nanofibrils, and the specific fine structure of the nanofibrils can be modulated simply by adjusting the concentration or pH. The critical micelle concentration of C12−Aβ(11−17) was determined as 0.063 and 0.11 mM at pH 3.0 and 10.0, respectively, indicating a stronger assembling ability of C12−Aβ(11−17) at acidic pH. In 0.47 mM C12−Aβ(11−17) solution at pH 3.0, rodlike fibrils with a diameter of ∼5 nm and varying length of hundreds of nanometers are observed. When the C12−Aβ(11−17) concentration increases to 1.87 mM at pH 3.0, the above rodlike fibrils pack in parallel and form tapelike fibrils through lateral association. In 1.87 mM C12−Aβ(11−17) solution at pH 10.0, twisted fibrils with regular periodicity of ∼200 nm are formed by the twisting of ∼20 nm wide and ∼11 nm thick nanoribbons. The hydrophobic moiety is necessary in fibril formation, whereas the β-sheet secondary structure of the peptide moiety plays an essential role in the twisting morphology. This work helps to understand the possible mechanism in amyloid fibrillogenesis and provides an approach to inscribe biological signals in self-assemblies with potential application in biomaterial fabrication.

Controllable aggregate transitions are achieved in this work by adding due amounts of β-cyclodextrin (β-CD) to mixed cationic/anionic surfactant aqueous solutions. In contrast to its “aggregate breaking” effect in single surfactant systems, aggregate growth is observed in nonstoichiometrical mixed cationic/anionic surfactant systems upon addition of β-CD. The aggregate growth typically undergoes a micellar elongation and a following micelle-to-vesicle transition, which in turn greatly influences the viscosity and absorbance of the solutions. A possible mechanism of this β-CD-induced aggregate growth is proposed. In mixed cationic/anionic surfactant systems, the surfactants strongly tend to reach electroneutral equilibrium in aggregates. In the present case, added β-CD is found to greatly facilitate the equilibrium by transferring the “major” component (whose molar fraction > 0.5) of a cationic/anionic surfactant mixture from the aggregates to β-CD cavities. Consequently, the surfactants in the aggregates approach electroneutral mixing, in favor of low-curved aggregates such as vesicles. This work shows that β-CD provides an additional degree of freedom to control microstructures and macroproperties for the whole class of mixed cationic/anionic surfactant systems.

Coacervation of cationic gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (C12C6C12Br2) with 10% hydrolyzed polyacrylamide (PAM) has been observed and investigated by turbidity titration, isothermal titration calorimetry, dynamic light scattering and microscopy. Without any assistant additive, the coacervation takes place at very low surfactant concentration, and exists in a broad surfactant concentration range. The morphology of the coacervate sponge phase varies in pore size as a function of C12C6C12Br2 concentration. The polymer/surfactant aggregates grow from soluble complexes with sizes smaller than 20 nm to micrometer during coacervation, and break up into soluble complexes of about 40 nm after coacervate redissolution.

The adsorbed layers of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (ΦC10TAB) and N,N  ,N′N′,N′N′-tetramethyl-N  ,N′N′-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide [(ΦC10)2C6] at the air/water interface have been studied by neutron reflection. The coverage of the surfactants was obtained over the concentration range from critical micelle concentration (CMC) to CMC/100. The area per ΦC10TAB molecule changes from 50±350±3 to 390±60 Å2390±60 Å2 over this concentration range and the area per (ΦC10)2C6 molecule changes from 139±3139±3 to 288±10 Å2288±10 Å2. The overall thicknesses (single uniform layer) of the surfactant layers at CMC are about 19 and 16 Å for ΦC10TAB and (ΦC10)2C6 respectively. The distributions of the C10 chains show that the chains of both surfactants are tilted away from surface normal, with the tilt increasing in the outer part of the layer. The distribution of C10 chains in (ΦC10)2C6 is narrower than that in ΦC10TAB, indicating that the alkyl chains of (ΦC10)2C6 are more tilted. For both surfactants, the broad nitrophenoxy distribution may indicate significant positional disorder of the nitrophenoxy groups along the surface normal direction and their intermixing with alkyl chains in the adsorbed layer.The adsorption of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (ΦC10TAB) and N,N  ,N′N′,N′N′-tetramethyl-N  ,N′N′-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide [(ΦC10)2C6] at the air/water interface indicates significant tilt of the hydrophobic chains from the surface normal and substantial intermixing of alkyl chains with nitrophenoxy groups.

A series of anionic sulfonate gemini surfactants with the general structure of [(CnH2n+1)(C3H6SO−3) NCsN(C3H6SO−3)(CnH2n+1)]⋅2Na+ have been synthesized. While the spacer group Cs represents p-xylyl or (CH2)3, the surfactants are abbreviated as CnCpxCn(SO3)2 (n=8,10,12n=8,10,12) or C12C3C12(SO3)2(n=12)(n=12), respectively. A corresponding monomeric surfactant C12H25N(CH3)(C3H6SO−3)⋅Na+(C12NSO3) has also been prepared. The aggregation behavior of these surfactants has been studied at pH 9.2 and ionic strength of 30 mM. The gemini surfactants exhibit stronger aggregation tendencies and much less endothermic enthalpy changes of micellization (ΔHmicΔHmic) compared with the monomeric surfactant. The critical micelle concentrations (CMC) of the gemini surfactants decrease with the increase of the hydrophobic chain length from C8CpxC8(SO3)2 to C10CpxC10(SO3)2, but the CMC values of C10CpxC10(SO3)2 and C12CpxC12(SO3)2 are very close. The ΔHmicΔHmic values vary from endothermic for C8CpxC8(SO3)2 to almost zero for C12CpxC12(SO3)2. Besides, vesicles are observed above the CMC for all these surfactants. The water-mediated intermolecular hydrogen bonding between the tertiary nitrogen groups may assist C12NSO3 and C12C3C12(SO3)2 in their vesicle formation, while the π–π interaction between aromatic rings should be another additional driving force for the vesicle formation of CnCpxCn(SO3)2. Meanwhile, the hydrogen bonding, π–π interaction, and strong hydrophobic interaction provide the possibility of a multilayer formation for C12CpxC12(SO3)2 and C12C3C12(SO3)2 at the air/water interface, which is a possible reason for the extremely small minimum area occupied per surfactant molecule at the air/water interface for these two gemini surfactants.

Novel trimeric cationic surfactant tri(dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) has been synthesized, and its self-assembly morphology on a mineral surface has been studied. From its micelle solution, highly ordered bilayer patterns are obtained on a mica surface, whereas randomly distributed bilayer patches are formed on a silica substrate. The highly ordered bilayer patterns on mica are first caused by the matching of the special structure of DTAD headgroups with the negative charge sites on mica, which leads to the specific nucleation of DTAD on the mica surface via electrostatic interaction. Furthermore, hydrophobic interaction among the DTAD hydrocarbon chains results in the formation of the bilayer structure, and intermolecular hydrogen-bonding among the DTAD headgroups promotes the directional growth of such bilayer structures.

The aggregation of amyloid β-peptide (Aβ(1-40)) into fibrils is a key pathological process associated with Alzheimer’s disease. This work has investigated the micellization process of biosurfactant surfactin and its effect on the aggregation behavior of Aβ(1-40). The results show that surfactin has strong self-assembly ability to form micelles and the micelles tend to form larger aggregates. Surfactin adopts a β-turn conformation at low micelle concentration but a β-sheet conformation at high micelle concentration. The effect of surfactin on the Aβ(1-40) aggregation behavior exhibits a strong concentration-dependent fashion. Below the critical micelle concentration of surfactin, the electrostatic binding of surfactin monomers on Aβ(1-40) causes Aβ(1-40) molecules to unfold. Assisted by the hydrophobic interaction among surfactin monomers on the Aβ(1-40) chain, the conformation of Aβ(1-40) transfers to the β-sheet structure, which promotes the formation of fibrils. At low surfactin micelle concentration, besides the electrostatic force and hydrophobic interaction, hydrogen bonds formed between surfactin micelles and adjacent Aβ(1-40) peptide chains may promote the ordered organization of these Aβ(1-40) peptide chains, thus leading to the formation of β-sheets and fibrils to a great extent. At high surfactin micelle concentration, the separating of Aβ(1-40) chains by the excessive surfactin micelles and the aggregation of the complexes of Aβ(1-40) with surfactin micelles inhibit the formation of β-sheets and fibrils.

Compaction of DNA by cationic gemini surfactant hexamethylene-1,6-bis-(dodecyldimethylammoniumbromide) (C12C6C12Br2) and the subsequent decompaction of the DNA−C12C6C12Br2 complexes by β-cyclodextrin (β-CD) or sodium dodecyl sulfate (SDS) have been studied by using ζ potential and particle size measurements, atomic force microscopy (AFM), isothermal titration microcalorimetry (ITC), and circular dichroism. The results show that C12C6C12Br2 can induce the collapse of DNA into densely packed bead-like structures with smaller size in an all-or-none manner, accompanied by the increase of ζ potential from highly negative values to highly positive values. In the decompaction of the DNA−C12C6C12Br2 complexes, β-CD and SDS exhibit different behaviors. For β-CD, the experimental results suggest that it can remove the outlayer hydrophobically bound C12C6C12Br2 molecules from the DNA−C12C6C12Br2 complexes by inclusion interaction, and the excess β-CD may attach on the complexes by forming inclusion complexes with the hydrocarbon chains of the electrostatically bound C12C6C12Br2 that cannot be removed. The increase of steric hindrance due to the attachment of β-CD molecules results in the decompaction of the DNA condensates though the true release of DNA cannot be attained. However, for SDS, the experimental results suggest that it can realize the decompaction and release of DNA from its complexes with C12C6C12Br2 due to both ion-pairing and hydrophobic interaction between SDS and C12C6C12Br2.

A proline surfactant including two chiral carbons, sodium N-dodecanoyl-(4R)-hydroxy-L-prolinate (SDHP), has been synthesized, and its micellization behavior in aqueous solution has been investigated by 1H NMR spectroscopy. Two conformational isomers of SDHP, namely, Z and E, are discriminated in the NMR time scale, and critical micelle concentration is derived for each isomer separately. The transformation from E to Z is observed upon micellization, and the amount of Z isomer is approximately three times that of E isomer in the equilibrated system. Moreover, the variation in chemical shifts with the surfactant concentration reveals the shielding effect of the carboxyl group on the syn-side protons of the pyrrolidine ring, which implies that the pyrrolidine rings arrange in a side-to-side manner and lie parallel to the plane of the carboxyl bonds in the neighboring molecules. The difference in the directions of the carbonyl group between Z and E isomers essentially determines their different micellization abilities and molecular arrangements in the micellization process.

The aggregation behavior of mixed systems of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) or sodium bis(4-phenylbutyl) sulfosuccinate (SBPBS) with nonionic surfactant pentaethylene glycol mono-n-dodecyl ether (C12E5) have been studied by means of steady-state fluorescence, electrical conductivity, dynamic light scattering, transmission electron microscopy, electrophoretic light scattering and pyrene solubilization measurements. The critical concentrations for aggregation, micropolarity, mobility, solubilization capacity and morphology of aggregates are characterized. Two critical concentrations for aggregation are observed in the mixed surfactants, which may correspond to the formation of different kinds of aggregates. Moreover, it is more favorable for AOT–C12E5 to form mixed vesicles compared to SBPBS–C12E5 at higher mole fraction of C12E5. In addition, it is revealed that SBPBS–C12E5 mixture has larger solubilization capacity for pyrene than AOT–C12E5 system.

The aggregate states of partially fluorinated gemini surfactant [(CF3)2CF(CF2)2(CH2)10N(CH3)2]2(CH2)6Br2 (CF5C10–C6–C10CF5) on silica surface were investigated with atomic force microscopy (AFM) and water contact angle (CA) measurement by analyzing the effects of bulk concentration and adsorption time on stack state. On surfactant-adsorbed silica surfaces, there was a flat surface layer interspersed with some scattering surfactant aggregates. In the case of short adsorption times, the aggregates would be hemisphere. In the case of long adsorption times, the aggregates would be present in the form of bilayers. With the increase of bulk concentration, the adsorbed amount was enlarged and the surface layer became more compact. The formation of patchy bilayer aggregates indicated the saturation of the surface layer. Furthermore, organic solvent effects on the aggregate state of the surfactant on a silica surface were studied with four organic solvents, including n-hexane, dehydrated ethanol, 1,1,2-trichloro-1,2,2-trifluoroethane, and toluene. With the treatment of different organic solvents, the hemisphere aggregates on the surface layer can rearrange into spherical bilayer, rodlike monolayer, and branched rodlike monolayer aggregates, respectively. The polarity of solvents and affinity of organic solvents for surfactant molecules may have a great impact on the stack state of the fluorinated gemini surfactant molecules.

The interaction of a series of dissymmetric gemini surfactants, [CmH2m+1(CH3)2N(CH2)6N(CH3)2CnH2n+1]Br2 (designated as CmC6CnBr2, with constant m+n=24m+n=24, and m=12m=12, 14, 16, and 18) with DNA in 10 mM NaCl solution has been investigated by isothermal titration microcalorimetry (ITC). The curves for titration of the surfactants into DNA solution show noticeable differences from those into 10 mM NaCl solution without DNA. It is attributed to the interaction between DNA and surfactants. The critical aggregation concentration (CAC), the saturation concentration (C2C2), and the thermodynamic parameters for the aggregation and interaction processes were obtained from the calorimetric titration curves. The results show that the dissymmetry degree (m/nm/n) has a marked effect on the interaction of the CmC6CnBr2 surfactants with DNA. The CAC and C2C2 tend to become smaller with increased m/nm/n. The enthalpy change (ΔHaggΔHagg) and the Gibbs free energy change (ΔGaggΔGagg) for aggregation become more negative down the series, indicating that the hydrophobic interaction between the hydrophobic chains of the surfactant molecules increases and the aggregation process is more spontaneous with increased m/nm/n. The entropy changes of aggregation (ΔSaggΔSagg) are all positive and TΔSaggTΔSagg is much larger than |ΔHagg||ΔHagg|, revealing that the aggregation process is mainly entropy-driven. However, the calculated Gibbs free energy (ΔGDSΔGDS) for the interaction between the gemini surfactants and DNA becomes less negative with increased m/nm/n, which reveals that the interaction between the gemini surfactants and DNA tends to be weaker with increased m/nm/n. This is induced by the disruption of the chain–chain hydrophobic interaction between the surfactant molecules at higher m/nm/n, where the entropy change ΔSDSΔSDS for the interaction process tends to be an unfavorable factor. In addition, the DNA concentration also has a remarkable influence on the interaction.

Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C16TAX) with different counterions, F−, Cl−, Br−, NO−3, and ½SO2−4, have been studied. Critical micelle concentration (CMC), degree of micelle ionization (α)(α), and enthalpy of micellization (ΔHmic)(ΔHmic) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CMC and the α increase in the order SO2−4 < NO−3 < Br− < Cl− < F−, consistent with a decrease in binding of counterion, except for the divalent anion sulfate. ΔHmicΔHmic becomes less negative through the sequence NO−3 < Br− < Cl− < F− < SO2−4, and even becomes positive for the divalent sulfate. The special behavior of sulfate is associated with both its divalency and its degree of dehydration. Gibbs free energies of micellization (ΔGmic)(ΔGmic) and entropies of micellization (ΔSmic)(ΔSmic) have been calculated from the values of ΔHmicΔHmic, CMC, and α   and can be rationalized in terms of the Hofmeister series. The variations in ΔHmicΔHmic and ΔSmicΔSmic have been compared with those for the corresponding series of gemini surfactants. Electron spin resonance has been used to assess the micropolarity and the microviscosity of the micelles. The results show that the microenvironment of the spin probe in the C16TAX surfactant micelles depends strongly on the binding of the counterion.

The mixed micelles of cationic gemini surfactants C12CSC12Br2 (S=3S=3, 6, and 12) with the nonionic surfactant Triton X-100 (TX100) have been studied by steady-state fluorescence, time-resolved fluorescence quenching, electrophoretic light scattering, and electron spin resonance. Both the surfactant composition and the spacer length are found to influence the properties of mixed micelles markedly. The total aggregation number of alkyl chains per micelle (NT)(NT) goes through a minimum at XTX100=0.8XTX100=0.8. Meanwhile, the micropolarity of the mixed micelles decreases with increasing XTX100XTX100, while the microviscosity increases. The presence of minimum in NTNT is explained in terms of the competition of the reduction of electrostatic repulsion between headgroups of cationic gemini surfactant with the enhancement of steric repulsion between hydrophilic headgroups of TX100 caused by the addition of TX100. The variations of micropolarity and microviscosity indicate that the incorporation of TX100 to the gemini surfactants leads to a more compact and hydrophobic micellar structure. Moreover, for the C12C3C12Br2/TX100 mixed micelle containing C12C3C12Br2 with a shorter spacer, the more pronounced decrease of NTNT at XTX100XTX100 lower than 0.8 may be attributed to the larger steric repulsion between headgroups of TX100. Meanwhile, the increase of microviscosity and the decrease of micropolarity are more marked for the C12C12C12Br2/TX100 mixed micelle, owing to the looped conformation of the longer spacer of C12C12C12Br2.

A series of partially fluorinated cationic gemini surfactants and their corresponding monomeric surfactants have been studied by isothermal titration microcalorimetry. The critical micelle concentration (CMC) and enthalpy of micellization (ΔHmic)(ΔHmic) were obtained from calorimetric curves. The CMCs of the gemini surfactants are much lower than those of the corresponding monomeric surfactants and decrease with an increase in the number of fluorine atoms on the hydrophobic chain. The micellization of partially fluorinated cationic gemini surfactants is much more exothermic than that of the corresponding monomeric surfactants. Because of the incompatibility of hydrocarbon spacer and partially fluorinated chain, ΔHmicΔHmic values of the surfactants with a C6 spacer are more negative than those of the surfactants with a C12 spacer. The variations in the architecture of the fluorocarbon chain segments may be the reason of the irregularities in the change of ΔHmicΔHmic for the gemini surfactants. Moreover, the contribution of the enthalpy generally increases with an increase in the number of fluorine atoms.

•Simple coacervation occurs in aqueous solution of single-chain surfactants and gemini surfactants with and without additives.•Complex coacervation occurs in aqueous solutions of single-chain surfactants and gemini surfactants with polymers with and without additives.•Gemini surfactants show more possibilities and unique feature in generating coacervation.Coacervation is a spontaneous process during which a colloidal dispersion separates into two immiscible liquid phases: a colloid-rich liquid phase in equilibrium with a diluted phase. Coacervation is usually divided into simple coacervation and complex coacervation according to the number of components. Surfactant-based coacervation normally contains traditional single-chain surfactants. With the development of surfactants, gemini surfactants with two amphiphilic moieties have been applied to form coacervation. This review summarizes the development of simple coacervation and complex coacervation in the systems of single-chain surfactants and gemini surfactants. Simple coacervation in surfactant solutions with additives or at elevated temperature and complex coacervation in surfactant/polymer mixtures by changing charge densities, molecular weight, ionic strength, pH, or temperature are reviewed. The comparison between gemini surfactants and corresponding monomeric single-chain surfactants reveals that the unique structures of gemini surfactants endow them with higher propensity to generate coacervation.

Gemini surfactants are constructed by two hydrophobic chains and two polar/ionic head groups covalently connected by a spacer group at the level of the head groups. Gemini surfactants possess unique structural variations and display special aggregate transitions. Their aggregation ability and aggregate structures can be more effectively adjusted through changing their molecular structures compared with the corresponding monomeric surfactants. Moreover, gemini surfactants exhibit special and useful properties while interacting with polymers and biomacromolecules. Their strong self-aggregation ability can be applied to effectively influence the aggregation behavior of both polymers and biomacromolecules. This short review is focused on the performances of gemini surfactants in aqueous solutions investigated in the last few years, and summarizes the effects of molecular structures on aggregation behavior of gemini surfactants in aqueous solution as well as the interaction of gemini surfactants with polymers and biomacromolecules respectively.

The suspending behaviors of multiple-wall carbon nanotubes (MWNTs), including pristine MWNTs (p-MWNTs) and acid-mixture-treated MWNTs (MWNTCOOH), stabilized by cationic single-chain surfactant, dodecyltrimethylammonium bromide (DTAB), and cationic gemini surfactant hexyl-α,β-bis(dodecyldimethylammonium bromide) (C12C6C12Br2) were studied systematically. The surfactant structure influences the suspendability of MWNTs dramatically as well as the surfactant adsorption behavior on the nanotubes. Although both the surfactants can disperse the MWNTs effectively, they actually show different stabilizing ability. DTAB is not capable of stabilizing these two MWNTs below critical micelle concentration (CMC). However, C12C6C12Br2 can suspend both the nanotubes effectively even well below its CMC. Moreover, the adsorption of these two surfactants reaches equilibrium at twice the CMC with the original MWNT concentration of 2 mg/mL, 2 mM for C12C6C12Br2, and 30 mM for DTAB. After the adsorption equilibrium, the maximum amounts of the two suspended MWNTs in C12C6C12Br2 solution are about twice as much as those in DTAB solution. The strong hydrophobic interaction among the C12C6C12Br2 molecules and between the C12C6C12Br2 molecules and the nanotubes as well as the high charge capacity of C12C6C12Br2 lead to its much stronger adsorption ability on the MWNTs and result in its superior stabilizing ability for the MWNTs in aqueous phase. The gemini surfactant provides a possibility to effectively stabilize the MWNTs in aqueous solutions even at very low surfactant concentration well below its CMC.