Dispersions of poly(methyl methacrylate) (PMMA) latexes were prepared in a low dielectric, nonpolar solvent (dodecane) both with and without the oil-soluble electrolyte, tetradodecylammonium-tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. For dispersions with a high concentration of background electrolyte, the latexes become colloidally unstable and sediment in a short period of time (<1 h). This is completely reversible upon dilution. Instability of the dispersions is due to an apparent attraction between the colloids, directly observed using optical tweezers by bringing optically trapped particles into close proximity. Simple explanations generally used by colloid scientists to explain loss of stability (charge screening or stabilizer collapse) are insufficient to explain this observation. This unexpected interaction seems, therefore, to be a consequence of the materials that can be dispersed in low dielectric media and is expected to have ramifications for studying colloids in such solvents.

A random copolymer, poly(methyl methacrylate-co-2-dimethylaminoethyl methacrylate) (poly(MMA-co-DMAEMA)) is shown to form nanoscale aggregates (NAs) (∼20 nm) at copolymer concentrations ≥10% w/w, directly from the preformed surfactant-stabilized latex (∼120 nm) in aqueous solution. The copolymer is prepared by conventional emulsion polymerization. Introducing a small mole fraction of DMAEMA (∼10%) allows the copolymer hydrophilicity to be adjusted by the pH and external temperature, generating NAs with tuneable sizes and a defined weight-average aggregation number, as observed by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). These NAs are different from the so-called mesoglobular systems and are insensitive to temperature at fixed pH. The relatively broad chemical composition distribution of the copolymer and lumpy (or blocky but not diblock) incorporation of DMAEMA mean that the NAs cannot be simply thought of as conventional polymer micelles. In the acidic pH regime, the amphiphilic copolymer exhibits a defined critical assembly concentration (CAC) and a minimum air–water surface tension of 45.2 mN m–1. This copolymer represents a convenient route to self-assembled NAs, which form directly in aqueous dispersions after pH and temperature triggers, rather than the typically applied (and time-consuming) water-induced micellization approach for common polymer micelles.

For equivalent micellar volume fraction (ϕ), systems containing anisotropic micelles are generally more viscous than those comprising spherical micelles. Many surfactants used in water-in-CO2 (w/c) microemulsions are fluorinated analogues of sodium bis(2-ethylhexyl) sulfosuccinate (AOT): here it is proposed that mixtures of CO2-philic surfactants with hydrotropes and cosurfactants may generate elongated micelles in w/c systems at high-pressures (e.g., 100–400 bar). A range of novel w/c microemulsions, stabilized by new custom-synthesized CO2-phillic, partially fluorinated surfactants, were formulated with hydrotropes and cosurfactant. The effects of water content (w = [water]/[surfactant]), surfactant structure, and hydrotrope tail length were all investigated. Dispersed water domains were probed using high pressure small-angle neutron scattering (HP-SANS), which provided evidence for elongated reversed micelles in supercritical CO2. These new micelles have significantly lower fluorination levels than previously reported (6–29 wt % cf. 14–52 wt %), and furthermore, they support higher water dispersion levels than other related systems (w = 15 cf. w = 5). The intrinsic viscosities of these w/c microemulsions were estimated based on micelle aspect ratio; from this value a relative viscosity value can be estimated through combination with the micellar volume fraction (ϕ). Combining these new results with those for all other reported systems, it has been possible to “map” predicted viscosity increases in CO2 arising from elongated reversed micelles, as a function of surfactant fluorination and micellar aspect ratio.

Surface tensiometry and small-angle neutron scattering have been used to characterize a new class of low-surface energy surfactants (LSESs), “hedgehog” surfactants. These surfactants are based on highly branched hydrocarbon (HC) chains as replacements for environmentally hazardous fluorocarbon surfactants and polymers. Tensiometric analyses indicate that a subtle structural modification in the tails and headgroup results in significant effects on limiting surface tensions γcmc at the critical micelle concentration: a higher level of branching and an increased counterion size promote an effective reduction of surface tension to low values for HC surfactants (γcmc ∼ 24 mN m–1). These LSESs present a new class of potentially very important materials, which form lamellar aggregates in aqueous solutions independent of dilution.

The interaction between deuterium-labeled Aerosol OT surfactant (AOT-d34) and sterically stabilized poly(methyl methacrylate) (PMMA) latex particles dispersed in nonpolar solvents has been studied using contrast-variation small-angle neutron scattering (CV-SANS). The electrophoretic mobilities (μ) of the latexes have been measured by phase-analysis light scattering, indicating that μ is negative. Two analogues of the stabilizers for the particles have been studied as free polymers in the absence of PMMA latexes: poly(12-hydroxystearic acid) (PHSA) polyester and poly(methyl methacrylate)-graft-poly(12-hydroxystearic acid) (PMMA-graft-PHSA) stabilizer copolymer. The scattering from both PHSA in dodecane and PMMA-graft-PHSA in toluene is consistent with extended polymer chains in good solvents. In dodecane, PMMA-graft-PHSA forms polymer micelles, and SANS is consistent with ellipsoidal aggregates formed of around 50 polymer chains. CV-SANS measurements were performed by measuring SANS from systems of PHSA, PMMA-graft-PHSA, and PMMA latexes with 10 and 100 mM surfactant solutions of AOT-d34 in both polymer/particle and AOT contrast-matched solvent. No excess scattering above the polymer or surfactant was found for PHSA in dodecane or PMMA-graft-PHSA in dodecane and toluene. This indicates that AOT does not significantly interact with the free polymers. Excess scattering was observed for systems with AOT-d34 and PMMA latexes dispersed in particle contrast-matched dodecane, consistent with the penetration of AOT into the PMMA latexes. This indicates that AOT does not interact preferentially with the stabilizing layers but, rather, is present throughout the colloids. Previous research (Langmuir 2010, 26, 6967–6976) suggests that AOT surfactant is located in the latex PHSA-stabilizer layer, but all the results in this study are consistent with AOT poorly interacting with alkyl-stabilizer polymers.

HypothesisReports of random copolymers capable of solubilising hydrophobic oils are rare. This is primarily because random copolymers are unlikely to self-assemble into suitable aggregates (or micelles) in water. A random copolymer with a “blocky” (or lumpy) microstructure may have potential to solubilise hydrophobic oils in water. This type of polymer would have advantages over block copolymers which are more laborious and costly to synthesise.ExperimentsThe solubilising capacity of a blocky random copolymer, namely poly(methyl methacrylate-co-2-dimethylaminoethyl methacrylate) (PMMA-co-PDMAEMA) is assessed by UV–visible spectroscopy and compared with common reference surfactants. The relative solubilising performance of random copolymers (across a narrow range of DMAEMA mol % fraction) for aromatic and aliphatic oils was also studied. The morphology of the aggregates was monitored as a function of the solubilisation capacity by small-angle neutron scattering (SANS) and dynamic-light scattering (DLS).FindingsSimilarly to well-defined block copolymers, these random copolymers have a specific preference for solubilising aromatic over aliphatic oils. Increasing hydrophobicity of the copolymer enhances the solubilisation capacity. SANS has highlighted that aggregates become swollen and more uniform/spherical with increasing concentration of aromatic solubilisate, and that the aromatic solubilisate partitions throughout the random copolymer aggregates.Download high-res image (187KB)Download full-size image

•Advances in stabilisation of aqueous foams by surfactants, proteins and particles•Discussion of modern methods for studying foams.•Natural foams provide protection, moisture and incubation during reproduction.•Overview of foams used for fire-fighting, mineral flotation and other advances.•Review of how production of undesirable foams is inhibited by defoaming agents.This article discusses different natural and man-made foams, with particular emphasis on the different modes of formation and stability. Natural foams, such as those produced on the sea or by numerous creatures for nests, are generally stabilised by dissolved organic carbon (DOC) molecules or proteins. In addition to this, foam nests are stabilised by multifunctional mixtures of surfactants and proteins called ranaspumins, which act together to give the required physical and biochemical stability. With regards to industrial foams, the article focuses on how various features of foams are exploited for different industrial applications. Stability of foams will be discussed, with the main focus on how the chemical nature and structure of surfactants, proteins and particles act together to produce long-lived stable foams. Additionally, foam destabilisation is considered, from the perspective of elucidation of the mechanisms of instability determined spectroscopically or by scattering methods.Foam image taken by author, Spittle bug photo reprinted with permission from David Iliff. License: CC-BY-SA 3, Túngara frog photo is free to use from the public domain (https://creativecommons.org/publicdomain/mark/1.0/), Fire man photo reprinted with permission from Angus Fire LTD.Download high-res image (225KB)Download full-size image

Presented here are the results for a novel class of hydrocarbon surfactants, termed trimethylsilyl hedgehogs (TMS-hedgehogs), due to the presence of silicon in the tails. By comparing the surface properties of these hybrid hedgehogs to purely hydrocarbon equivalents, links between performance and the structure are made. Namely, by controlling the molecular volume of the surfactant fragments, improvements can be made in surface coverage, generating lower surface energy monolayers. Small-angle neutron scattering (SANS) data have been collected showing that these novel surfactants aggregate to form ellipsoidal micelles which grow with increasing concentration. This study highlights the sensitive relationship between surface tension and the surfactant chain, for designing new super-efficient surfactants close to the limit of the lowest surface tensions possible.

HypothesisOwing to attractive interactions between negatively charged graphene oxide (GO) and a paramagnetic cationic polyelectrolyte (polyallydimethylammonium chloride with a FeCl4− counterion (Fe-polyDADMAC) it should be possible to generate magnetic materials. The benefit of using charge-based adsorption is that the need to form covalently linked magnetic materials is offset, which is expected to significantly reduce the time, energy and cost to make such responsive materials. These systems could have a wide use and application in water treatment.ExperimentsNon-covalent magnetic materials were formed through the mixing of Fe-pDADMAC and GO. A systematic study was conducted by varying polymer concentration at a fixed GO concentration. UV–Vis was used to confirm and quantify polymer adsorption onto GO sheets. The potential uses of the systems for water purification were demonstrated.FindingsFe-polyDADMAC adsorbs to the surface of GO and induces flocculation. Low concentrations of the polymer (<9 mmol/L) favour flocculation, whereas higher concentrations (>20 mmol/L) induce restabilization. Difficult-to-recover gold nanoparticles can be separated from suspensions as well as the pollutant antibiotic tetracycline. Both harmful materials can be magnetically recovered from the dispersions. This system therefore has economical and practical applications in decontamination and water treatment.

Hypothesis: Sodium dioctylsulfosuccinate (Aerosol OT or NaAOT) is a well-studied charging agent for model poly(methyl methacrylate) (PMMA) latexes dispersed in nonpolar alkane solvents. Despite this, few controlled variations have been made to the molecular structure. A series of counterion-exchanged analogs of NaAOT with other alkali metals (lithium, potassium, rubidium, and cesium) were prepared, and it was expected that this should influence the stabilization of charge on PMMA latexes and the properties of the inverse micelles.Experiments:   The electrophoretic mobilities of PMMA latexes were measured for all the counterion-exchanged AOT analogs, and these values were used to calculate the electrokinetic or ζζ potentials. This enabled a comparison of the efficacy of the different surfactants as charging agents. Small-angle scattering measurements (using neutrons and X-rays) were performed to determine the structure of the inverse micelles, and electrical conductivity measurements were performed to determine the ionized fractions and Debye lengths.Findings: Sodium AOT is a much more effective charging agent than any of the other alkali metal AOTs. Despite this, the inverse micelle size and electrical conductivity of NaAOT are unremarkable. This shows a significant non-periodicity in the charging efficiency of these surfactants, and it emphasizes that charging particles in nonpolar solvents is a complex phenomenon.Figure optionsDownload full-size imageDownload high-quality image (148 K)Download as PowerPoint slide

Highlights•CMCs for inverse micelle formation can be measured using neutron scattering.•CMCs for Aerosol OT in aliphatic and aromatic solvents are essentially the same.•CMCs for sodium and tetrapropylammonium Aerosol OT are essentially the same.•Surfactants are too solvophobic for these variations in structure to influence the CMC.Critical micelle concentrations (CMCs) for the formation of inverse micelles have been determined for anionic surfactants in nonpolar, hydrocarbon solvents. Sodium dioctylsulfosuccinate (Aerosol OT or AOT) was chosen as the model surfactant, with systematic variations in both the solvent (benzene, cyclohexane, and dodecane) and the surfactant counterion (sodium and tetrapropylammonium). Recent work (Langmuir 29 (2013) 3352–3258) has shown that high-resolution small-angle neutron scattering (SANS) measurements can be used to directly determine the presence or absence of aggregates in solution. No variation in the value of the CMC was found within the resolution of the measurements for changing either solvent or counterion; some effects on the structure of inverse micelles were observed. This lack of a significant difference in the onset of inverse micellization with changes to the molecular species is surprising, and the implications on the solvophobic effect in nonpolar solvents are discussed.Graphical abstract

An oxygen-rich hydrocarbon (HC) amphiphile has been developed as an additive for supercritical CO2 (scCO2). The effects of this custom-designed amphiphile have been studied in water-in-CO2 (w/c) microemulsions stabilized by analogous fluorocarbon (FC) surfactants, nFG(EO)2, which are known to form spherical w/c microemulsion droplets. By applying contrast-variation small-angle neutron scattering (CV-SANS), evidence has been obtained for anisotropic structures in the mixed systems. The shape transition is attributed to the hydrocarbon additive, which modifies the curvature of the mixed surfactant films. This can be considered as a potential method to enhance physicochemical properties of scCO2 through elongation of w/c microemulsion droplets. More importantly, by studying self-assembly in these mixed systems, fundamental understanding can be developed on the packing of HC and FC amphiphiles at water/CO2 interfaces. This provides guidelines for the design of fluorine-free CO2 active surfactants, and therefore, practical industrial scale applications of scCO2 could be achieved.

•Introduce the 3 existing classes of magnetic surfactants; ionic, coordinating, covalently bound.•Consider a potential 4th class of magnetic surfactant.•Describe how these surfactants function as molecular magnets.•Report potential applications ranging biochemistry to water treatment.•Discuss the origins of magnetism in dilute aqueous solutions.Surfactants are ubiquitous, being important commodity chemicals with wide industrial applications, and essential components of living organisms. With stimuli-responsive surfactants, self-assembly and physicochemical properties of a wide variety of materials may be readily manipulated, both reversibly and irreversibly. Until recently, magnetically responsive surfactants had not been reported. This review reports the recent progress in magnetoresponsive surfactants, covering control of interface and bulk solution properties. The use of these magneto-surfactants as novel molecular magnets is reported, and potential applications are identified.

HypothesisThe interaction of Aerosol OT (AOT) surfactant with systems of model colloids in nonaqueous solvents (water-in-oil microemulsions, surfactant-stabilized silica organosols, and sterically-stabilized PMMA latexes) is expected to be system specific. Two limiting cases are expected: adsorption, with surfactant located at the particle surfaces, or absorption, with surfactant incorporated into the particle cores.ExperimentsTwo approaches have been used to determine how AOT is distributed in the colloidal systems. The stability of the colloids in different alkanes (heptane to hexadecane, including mixtures) has been studied to determine any effects on the colloid surfaces. Contrast-variation small-angle neutron scattering (SANS) measurements of the colloid cores and of AOT-colloid mixtures in colloid-matched solvent have also been performed. Normalization to account for the different scattering intensities and different particle radii have been used to enable a system-independent comparison.FindingsAOT in water-in-oil microemulsions and surfactant-stabilized silica organosols is determined to be adsorbed, whereas, surprisingly, AOT in sterically-stabilized PMMA latexes is found to be absorbed. Possible origins of these differences are discussed.Figure optionsDownload full-size imageDownload high-quality image (72 K)Download as PowerPoint slide

Sterically-stabilized poly(methyl methacrylate) (PMMA) latexes dispersed in nonpolar solvents are a classic, well-studied system in colloid science. This is because they can easily be synthesized with a narrow size distribution and because they interact essentially as hard spheres. These PMMA latexes can be charged using several methods (by adding surfactants, incorporating ionizable groups, or dispersing in autoionizable solvents), and due to the low relative permittivity of the solvents (εr ≈ 2 for alkanes to εr ≈ 8 for halogenated solvents), the charges have long-range interactions. The number of studies of these PMMA particles as charged species has increased over the past ten years, after few studies immediately following their discovery. A large number of variations in both the physical and chemical properties of the system (size, concentration, surfactant type, or solvent, as a few examples) have been studied by many groups. By considering the literature on these particles as a whole, it is possible to determine the variables that have an effect on the charge of particles. An understanding of the process of charge formation will add to understanding how to control charge in nonaqueous solvents as well as make it possible to develop improved technologically relevant applications for charged polymer nanoparticles.

This article analyzes how the individual structural elements of surfactant molecules affect surface properties, in particular, the point of reference defined by the limiting surface tension at the aqueous cmc, γcmc. Particular emphasis is given to how the chemical nature and structure of the hydrophobic tails influence γcmc. By comparing the three different classes of surfactants, fluorocarbon, silicone, and hydrocarbon, a generalized surface packing index is introduced which is independent of the chemical nature of the surfactants. This parameter ϕcmc represents the volume fraction of surfactant chain fragments in a surface film at the aqueous cmc. It is shown that ϕcmc is a useful index for understanding the limiting surface tension of surfactants and can be useful for designing new superefficient surfactants.

A series of eight sodium sulfonic acid surfactants with differently branched tails (four double-chain sulfosuccinates and four triple-chain sulfocarballylates) were studied as charging agents for sterically stabilized poly(methyl methacrylate) (PMMA) latexes in dodecane. Tail branching was found to have no significant effect on the electrophoretic mobility of the latexes, but the number of tails was found to influence the electrophoretic mobility. Triple-chain, sulfocarballylate surfactants were found to be more effective. Several possible origins of this observation were explored by comparing sodium dioctylsulfosuccinate (AOT1) and sodium trioctylsulfocarballylate (TC1) using identical approaches: the inverse micelle size, the propensity for ion dissociation, the electrical conductivity, the electrokinetic or ζ potential, and contrast-variation small-angle neutron scattering. The most likely origin of the increased ability of TC1 to charge PMMA latexes is a larger number of inverse micelles. These experiments demonstrate a small molecular variation that can be made to influence the ability of surfactants to charge particles in nonpolar solvents, and modifying molecular structure is a promising approach to developing more effective charging agents.

•Cylindrical morphologies of reverse microemulsions induced by small molecule hydrotropes.•Ellipsoidal droplets of high aspect ratio in reverse AOT microemulsions are induced by short chain alkanoate additives.•A cylinder to sphere transition in reverse AOT microemulsions is seen with increasing water content.HypothesisInitial studies (Hopkins Hatzopoulos et al. (2013)) have shown that ionic hydrotropic additives can drive a sphere-to-cylinder (ellipsoid) transition in water-in-oil (w/o) microemulsions stabilized by the anionic surfactant Aerosol-OT; however the origins of this behaviour remained unclear. Here systematic effects of chemical structure are explored with a new set of hydrotropes, in terms of an aromatic versus a saturated cyclic hydrophobic group, and linear chain length of alkyl carboxylates. It is proposed that hydrotrope-induced microemulsion sphere-to-cylinder (ellipsoid) transitions are linked to additive hydrophobicity, and so a correlation between the bulk aqueous phase critical aggregation concentration (cac) and perturbation of microemulsion structure is expected.ExperimentsWater-in-oil microemulsions were formulated as a function of water content w (= [water]/[AOT]) and concentration of different hydrotropes, being either cyclic (sodium benzoate or sodium cyclohexanoate), or linear chain systems (sodium hexanoate, sodium heptanoate and sodium octanoate). Phase behaviour studies were performed as a function of w, additive type and temperature at total surfactant concentration [ST] = 0.10 M and constant mole fraction x = 0.10 (x = [hydrotrope]/[ST]). Microemulsion domain structures were investigated by small-angle neutron scattering (SANS), and these data were fitted by structural models to yield information on the shapes (spheres, ellipsoids or cylinders) and sizes of the nanodroplets.FindingsUnder the conditions of study hydrotrope chemical structure has a significant effect on microemulsion structure: sodium cyclohexanoate does not induce the formation of cylindrical/ellipsoidal nanodroplets, whereas the aromatic analogue sodium benzoate does. Furthermore, the short chain sodium hexanoate does not cause anisotropic microemulsions, but the more hydrophobic longer chain heptanoate and octanoate analogues do induce sphere-to-ellipsoid transitions. This study shows that underlying microemulsion structures can be tuned by hydrotropes, and that the strength of the effect can be identified with hydrotrope hydrophobicity in terms of the bulk aqueous phase cac.Graphical abstract

Surface tensiometry and small-angle neutron scattering have been used to characterize a new class of low-surface energy surfactants (LSESs), “hedgehog” surfactants. These surfactants are based on highly branched hydrocarbon (HC) chains as replacements for environmentally hazardous fluorocarbon surfactants and polymers. Tensiometric analyses indicate that a subtle structural modification in the tails and headgroup results in significant effects on limiting surface tensions γcmc at the critical micelle concentration: a higher level of branching and an increased counterion size promote an effective reduction of surface tension to low values for HC surfactants (γcmc ∼ 24 mN m–1). These LSESs present a new class of potentially very important materials, which form lamellar aggregates in aqueous solutions independent of dilution.

The ability to induce morphological transitions in water-in-oil (w/o) and water-in-CO2 (w/c) microemulsions stabilized by a trichain anionic surfactant 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate (TC14) with simple hydrotrope additives has been investigated. High-pressure small-angle neutron scattering (SANS) has revealed the addition of a small mole fraction of hydrotrope can yield a significant elongation in the microemulsion water droplets. For w/o systems, the degree of droplet growth was shown to be dependent on the water content, the hydrotrope mole fraction, and chemical structure, whereas for w/c microemulsions a similar, but less significant, effect was seen. The expected CO2 viscosity increase from such systems has been calculated and compared to related literature using fluorocarbon chain surfactants. This represents the first report of hydrotrope-induced morphology changes in w/c microemulsions and is a significant step forward toward the formation of hydrocarbon worm-like micellar assemblies in this industrially relevant solvent.

The interaction between deuterium-labeled Aerosol OT surfactant (AOT-d34) and sterically stabilized poly(methyl methacrylate) (PMMA) latex particles dispersed in nonpolar solvents has been studied using contrast-variation small-angle neutron scattering (CV-SANS). The electrophoretic mobilities (μ) of the latexes have been measured by phase-analysis light scattering, indicating that μ is negative. Two analogues of the stabilizers for the particles have been studied as free polymers in the absence of PMMA latexes: poly(12-hydroxystearic acid) (PHSA) polyester and poly(methyl methacrylate)-graft-poly(12-hydroxystearic acid) (PMMA-graft-PHSA) stabilizer copolymer. The scattering from both PHSA in dodecane and PMMA-graft-PHSA in toluene is consistent with extended polymer chains in good solvents. In dodecane, PMMA-graft-PHSA forms polymer micelles, and SANS is consistent with ellipsoidal aggregates formed of around 50 polymer chains. CV-SANS measurements were performed by measuring SANS from systems of PHSA, PMMA-graft-PHSA, and PMMA latexes with 10 and 100 mM surfactant solutions of AOT-d34 in both polymer/particle and AOT contrast-matched solvent. No excess scattering above the polymer or surfactant was found for PHSA in dodecane or PMMA-graft-PHSA in dodecane and toluene. This indicates that AOT does not significantly interact with the free polymers. Excess scattering was observed for systems with AOT-d34 and PMMA latexes dispersed in particle contrast-matched dodecane, consistent with the penetration of AOT into the PMMA latexes. This indicates that AOT does not interact preferentially with the stabilizing layers but, rather, is present throughout the colloids. Previous research (Langmuir 2010, 26, 6967–6976) suggests that AOT surfactant is located in the latex PHSA-stabilizer layer, but all the results in this study are consistent with AOT poorly interacting with alkyl-stabilizer polymers.

Dicationic magnetic ionic liquids with heteroanionic anions allow for tunability of physicochemical properties while retaining magnetic susceptibility.

Recent progress in stimuli-responsive surfactants is reviewed, covering control of both interfaces and bulk solution properties. Particular attention is devoted to potential future directions and applications.

A systematic study of the physico-chemical properties of a series of new catanionic surfactants with ionic liquid properties is reported. Importantly, by avoiding environmentally unfriendly halide and imidazolium based moieties highly tunable surfactant ionic liquids have been prepared.Graphical abstractHighlights► New catanionic surfactants are synthesized with ionic liquid properties. ► Ionic liquid properties are tunable through careful consideration of molecular architecture. ► Solubility controlled by selection of ion pair. ► Ionic liquids are imidazolium- and halide-free and considered environmentally friendly. ► Aqueous self-aggregation of surfactant ionic liquids investigated through SANS.

The formation of ions in nonpolar solvents (with relative permittivity εr of approximately 2) is more difficult than in polar liquids; however, these charged species play an important role in many applications, such as electrophoretic displays. The low relative permittivities of these solvents mean that charges have to be separated by large distances to be stable (approximately 28 nm or 40 times that in water). The inverse micelles formed by surfactants in these solvents provide an environment to stabilize ions and charges. Common surfactants used are sodium dioctylsulfosuccinate (Aerosol OT or AOT), polyisobutylene succinimide, sorbitan oleate, and zirconyl 2-ethyl hexanoate. The behavior of charged inverse micelles has been studied on both the bulk and on the microscopic scale and can be used to determine the motion of the micelles, their structure, and the nature of the electrostatic double layer. Colloidal particles are only weakly charged in the absence of surfactant, but in the presence of surfactants, many types, including polymers, metal oxides, carbon blacks, and pigments, have been observed to become positively or negatively charged. Several mechanisms have been proposed as the origin of surface charge, including acid–base reactions between the colloid and the inverse micelle, preferential adsorption of charged inverse micelles, or dissolution of surface species. While most studies vary only the concentration of surfactant, systematic variation of the particle surface chemistry or the surfactant structure have provided insight into the origin of charging in nonpolar liquids. By carefully varying system parameters and working to understand the interactions between surfactants and colloidal surfaces, further advances will be made leading to better understanding of the origin of charge and to the development of more effective surfactants.

High-pressure small-angle neutron scattering (HP-SANS) studies were conducted to investigate nanostructures and interfacial properties of water-in-supercritical CO2 (W/CO2) microemulsions with double-fluorocarbon-tail anionic surfactants, having different fluorocarbon chain lengths and linking groups (glutarate or succinate). At constant pressure and temperature, the microemulsion aqueous cores were found to swell with an increase in water-to-surfactant ratio, W0, until their solubilizing capacities were reached. Surfactants with fluorocarbon chain lengths of n = 4, 6, and 8 formed spherical reversed micelles in supercritical CO2 even at W0 over the solubilizing powers as determined by phase behavior studies, suggesting formation of Winsor-IV W/CO2 microemulsions and then Winsor-II W/CO2 microemulsions. On the other hand, a short C2 chain fluorocarbon surfactant analogue displayed a transition from Winsor-IV microemulsions to lamellar liquid crystals at W0 = 25. Critical packing parameters and aggregation numbers were calculated by using area per headgroup, shell thickness, the core/shell radii determined from SANS data analysis: these parameters were used to help understand differences in aggregation behavior and solubilizing power in CO2. Increasing the microemulsion water loading led the critical packing parameter to decrease to ∼1.3 and the aggregation number to increase to >90. Although these parameters were comparable between glutarate and succinate surfactants with the same fluorocarbon chain, decreasing the fluorocarbon chain length n reduced the critical packing parameter. At the same time, reducing chain length to 2 reduced negative interfacial curvature, favoring planar structures, as demonstrated by generation of lamellar liquid crystal phases.

The concentration-dependent aggregation of two surfactants, anionic sodium dioctylsulfosuccinate (Aerosol OT or AOT) and nonionic pentaethylene glycol monododecyl ether (C12E5), has been studied in cyclohexane-D12 using small-angle neutron scattering (SANS). A clear monomer-to-aggregate transition has been observed for both surfactants, spherical inverse micelles for AOT and hank-like micelles for C12E5. This suggests that a critical micelle concentration exists for surfactants of these kinds in nonpolar solvents. The nature of the transition is different for the two surfactants. AOT aggregates are the same size and shape with decreasing concentration until a sharp critical micelle concentration, after which they cannot be detected. However, C12E5 aggregates gradually decrease in size. These differences demonstrate that the strength of the solvophobic effect can influence the formation of surfactant aggregates in nonaqueous solvents.

Magnetic microemulsions comprising of magnetic surfactants exhibit monodomain magnetic behaviour intermediate between magnetic nanoparticles and molecular magnets. Importantly, due to partitioning of surfactant molecules at the water–oil interface only surface anisotropy is observed. These new systems allow for in situ tunability through composition and the solubilization of hydrophobic additives.

Emulsions are mixtures of two or more immiscible fluids, stabilised by interfacial adsorption of surfactants or particles. As such emulsions are essential components in multifarious processes and products, such as foods, pharmaceutical and agrochemical formulations, paints, inks, lubricants, oils and oil recovery. Stability and structure of responsive colloids and emulsions can be controlled by changes in composition, pH, as well as by external stimuli temperature, pressure and light. This is the first report of easy to formulate magnetically responsive emulsions stabilized by a new class of magnetic surfactant stabilizers.

This work reveals new principles for designing new self-assembling additives as viscosity enhancers for supercritical fluid CO2. Employing the approaches outlined in this work a maximum increase in CO2 viscosity of 100% has been achieved, the highest ever reported for surfactant CO2 viscosifiers. A series of semi-fluorinated F–H hybrid surfactants were synthesised based on the pentadecafluoro-5-dodecyl (F7H4) sulfate anion, but featuring different metallic counterions: Li-F7H4, K-F7H4, Na-F7H4 and Rb-F7H4 (denoted M-F7H4). These M-F7H4 variants were designed to reveal the effects of hydrated cation radius (rhyd) on packing self-assembly in supercritical CO2 (scCO2). The CO2-philic M-F7H4 surfactants were investigated at the air–water interface by surface tensiometry, and in the bulk by small-angle neutron scattering (SANS), in both aqueous solutions and scCO2. Surface tensiometry of aqueous solutions showed that interfacial packing density and limiting head group area at the critical micelle concentration (Acmc), depends implicitly on the identity of M+, suggesting that micelles in water or scCO2 should have different geometric packing parameters (PlimC) as a function of M+. SANS measurements on M-F7H4 aqueous micellar solutions confirmed sphere → ellipsoid → vesicle transitions as a function of M+ with decreasing rhyd. Using high pressure SANS (HP-SANS), and changing solvent from water to scCO2 showed that all the M-F7H4 variants stabilise water-in-CO2 (w/c) microemulsion droplets on addition of water. In scCO2 Li-F7H4 forms prolate ellipsoidal droplets, Na-F7H4 forms long cylindrical droplets, K-F7H4 forms ellipsoidal droplets, whereas Rb-F7H4 stabilises spherical microemulsion droplets. The ability of M-F7H4 additives to enhance the viscosity of scCO2 was shown by high pressure falling cylinder viscometry. The HP-SANS and viscometry experiments were shown to be quantitatively consistent, in that aggregate aspect ratios (X) determined in structural studies account for the enhanced relative viscosities (ηmic/ηCO2).

For the first time a series of anionic surfactant ionic liquids (SAILs) has been synthesized based on organic surfactant anions and 1-butyl-3-methyl-imidazolium cations. These compounds are more environmentally friendly and chemically tunable as compared to other common ionic liquids. A detailed investigation of physicochemical properties highlights potential applications from battery design to reaction control, and studies into aqueous aggregation behavior, as well as structuring in pure ILs, point to possible uses in electrochemistry.

The relationships between molecular architecture, aggregation, and interfacial activity of a new class of CO2-philic hybrid surfactants are investigated. The new hybrid surfactant CF2/AOT4 [sodium (4H,4H,5H,5H,5H-pentafluoropentyl-3,5,5-trimethyl-1-hexyl)-2-sulfosuccinate] was synthesized, having one hydrocarbon chain and one separate fluorocarbon chain. This hybrid H–F chain structure strikes a fine balance of properties, on one hand minimizing the fluorine content, while on the other maintaining a sufficient level of CO2-philicity. The surfactant has been investigated by a range of techniques including high-pressure phase behavior, UV–visible spectroscopy, small-angle neutron scattering (SANS), and air–water (a/w) surface tension measurements. The results advance the understanding of structure–function relationships for generating CO2-philic surfactants and are therefore beneficial for expanding applications of CO2 to realize its potential using the most economic and efficient surfactants.

Hydrotropes are small molecule amphiphiles, having considerable industrial importance as agents for solubilization of hydrophobic substances in aqueous systems. The physico-chemical origin and mechanism of hydrotrope action has been a subject of academic debate and controversy for many years. One important issue is how close the solution physical chemistry of hydrotropes resembles that of common surfactants. This article seeks to improve the appreciation of this field by comparing thermodynamic, phase, spectroscopic and scattering studies of hydrotrope aqueous solutions. In addition, alkyl-hydrotropes are discussed, which represent a structural evolution from classic hydrotropes towards common surfactants, having solution properties more reminiscent of surfactants.

A new isothermal approach to the recovery of inorganic nanoparticles (NPs) is demonstrated. The NPs can be incorporated into a background microemulsion (ME) supporting fluid, and they can be recovered by addition of non-adsorbing polymer. A clean liquid–liquid (L–L) phase transition can be readily induced by addition of polymer to the MEs. Furthermore, the L–L transitions are also observed in the presence of added NPs, but now the nanoparticles concentrate in the lower co-existing ME phases. Once recovered, the NPs can be redispersed by adding extra ME as a solvent.

This article reviews approaches for modification of solvent properties of supercritical carbon dioxide (scCO2), with particular reference to self-assembly of oligomeric and polymeric solute additives. Of special interest are viscosity modifiers for scCO2 based on molecular self-assembly. Background on polymers and surfactants with CO2-compatible functionalities is covered, leading on to the attempts made so far to increase the scCO2 viscosity, which are described in detail. The significance of this field, and the implications a breakthrough could bring environmentally and economically will be addressed.

Water-in-oil microemulsions (w/o μEs) stabilized by the cationic surfactant cetyltrimethylammonium chloride (CTACl) have been used as reaction media to generate Au nanoparticles (Au-NPs). In addition the pure μEs have been used as media to disperse Au and Pd-NPs, which have been pre-synthesised in aqueous phases and stabilized by sodium 2-mercaptoethanesulfonate (MES) ligands, and also commercially available SiO2-NPs. A general method for recovery and separation of the nanoparticles from these mixed NP-μE systems has been demonstrated by tuning phase behavior of the background microemulsions. Addition of appropriate aliquots of water drives a clean liquid–liquid phase transition, resulting in two macroscopic layers, the NPs preferentially partition into an upper oil-rich phase and are separated from excess surfactant which resides in a lower aqueous portion. UV–vis and 1H NMR spectroscopy have been used to follow these separation processes and quantify the recovery and recycle efficiencies for the different NPs. For example, ∼90% of the microemulsion-prepared Au-NPs can be recovered; with even greater separation efficiencies attainable for pre-synthesised MES-stabilized Au-MES-NPs (∼98%) and Pd-MES-NPs (92%). For the silica NP-μE dispersions gravimetry indicates ∼84% recovery of the NPs. TEM images of all systems showed that NP shapes and size distributions were generally preserved after these phase transfer processes. This low-energy and cost-effective purification route appears to be a quite general approach for processing different inorganic NPs, having advantages of being isothermal, using only commercially available inexpensive components and requiring no additional organic solvents.Graphical abstractNanoparticles synthesised in water-in-oil microemulsions can be easily separated from the reaction medium by steep dilution with water.Research highlights► A low-energy, isothermal, easy to apply and cost-effective purification route for processing inorganic nanoparticles from microemulsions is described. ► A water-induced separation can be used to recover nanoparticles from the reaction medium “at the flick of a switch”. ► This water dilution method appears to be quite general.

A viable cost-effective approach employing mixtures of non-ionic surfactants Triton X-114/Triton X-100 (TX-114/TX-100), and subsequent cloud point extraction (CPE), has been utilized to concentrate and recycle inorganic nanoparticles (NPs) in aqueous media. Gold Au- and palladium Pd-NPs have been pre-synthesized in aqueous phases and stabilized by sodium 2-mercaptoethanesulfonate (MES) ligands, then dispersed in aqueous non-ionic surfactant mixtures. Heating the NP-micellar systems induced cloud point phase separations, resulting in concentration of the NPs in lower phases after the transition. For the Au-NPs UV/vis absorption has been used to quantify the recovery and recycle efficiency after five repeated CPE cycles. Transmission electron microscopy (TEM) was used to investigate NP size, shape, and stability. The results showed that NPs are preserved after the recovery processes, but highlight a potential limitation, in that further particle growth can occur in the condensed phases.Graphical abstractNanoparticles can be easily separated, recovered and recycled by cloud point extraction.Highlights► Inorganic nanoparticles (NPs) can be recovered from non-ionic micellar media by cloud point extraction (CPE). ► A cost-effective purification route for processing NPs requiring relatively cheap and commercially available surfactants. ► Mixing two non-ionic surfactants allows for temperature tunability and optimization of the NP–CPE recovery process. ► The NP–CPE process can be repeated demonstrating reproducibility and recyclability.

The article addresses an important, and still unresolved question in the field of CO2 science and technology: what is the minimum fluorine content necessary to obtain a CO2-philic surfactant? A previous publication (Langmuir2002, 18, 3014) suggested there should be an ideal fluorination level: for optimization of possible process applications in CO2, it is important to establish just how little F is needed to render a surfactant CO2-philic. Here, optimum chemical structures for water-in-CO2 (w/c) microemulsion stabilization are identified through a systematic study of CO2-philic surfactant design based on dichain sulfosuccinates. High pressure small-angle neutron scattering (HP-SANS) measurements of reversed micelle formation in CO2 show a clear relationship between F content and CO2 compatibility of any given surfactant. Interestingly, high F content surfactants, having lower limiting aqueous surface tensions, γcmc, also have better performance in CO2, as indicated by lower cloud point pressures, Ptrans. The results have important implications for the rational design of CO2-philic surfactants helping to identify the most economic and efficient compounds for emerging CO2 based fluid technologies.

Small-angle neutron scattering and surface tension have been used to characterize a class of surfactants (SURFs), including surfactant ionic liquids (SAILs). These SURFs and SAILs are based on organic surfactant anions (single-tail dodecyl sulfate, DS, double-chain aerosol-OT, AOT, and the trichain, TC) with substituted quaternary ammonium cations. This class of surfactants can be obtained by straightforward chemistry, being cheaper and more environmentally benign than standard cationic SAILs. A surprising aspect of the results is that, broadly speaking, the physicochemical properties of these SURFs and SAILs are dominated by the nature of the surfactant anion and that the chemical structure of the added cation plays only a secondary role.

The fluorinated double-tailed glutarate anionic surfactant, sodium 1,5-bis[(1H,1H,2H,2H-perfluorodecyl)oxy]-1,5-dioxopentane-2-sulfonate (8FG(EO)2), was found to stabilize water-in-supercritical CO2 microemulsions with high water-to-surfactant molar ratios (W0). Studies were carried out here to obtain detailed information on the phase stability and nanostructure of the microemulsions by using a high-pressure UV−vis dye probe and small-angle neutron scattering (SANS) measurements. The UV−vis spectra, with methyl orange as a reporter dye, indicated a maximum attainable W0 of 60 at 45 and 75 °C, and SANS profiles indicated regular droplet swelling with a linear relationship between the water core nanodroplet radius and W0. This represents the highest water solubilization reported to date for any water-in-CO2 microemulsion. Further analysis of the SANS data indicated critical packing parameters for 8FG(EO)2 at the microemulsion interface >1.34, representing approximately 1.1 times the value for common aerosol-OT in water-in-heptane microemulsions under equivalent conditions.

A new class of photoreactive surfactants (PRSs) is presented here, consisting of amphiphiles that can also act as reagents in photochemical reactions. An example PRS is cobalt 2-ethylhexanoate (Co(EH)2), which forms reverse micelles (RMs) in a hydrocarbon solvent, as well as mixed reversed micelles with the standard surfactant Aerosol-OT (AOT). Small-angle neutron scattering (SANS) data show that mixed AOT/PRS RMs have a spherical structure and size similar to that of pure AOT micelles. Excitation of the ligand-to-metal charge transfer (LMCT) band in the PRSs promotes electron transfer from PRS to associated metal counterions, leading to the generation of metal and metal–oxide nanoparticles inside the RMs. This work presents proof of concept for employing PRSs as precursors to obtain nearly monodisperse inorganic nanoparticles: here both Co3O4 and Bi nanoparticles have been synthesized at high metal concentration (10–2 M) by simply irradiating the RMs. These results point toward a new approach of photoreactive self-assembly, which represents a clean and straightforward route to the generation of nanomaterials.

New fluorinated microemulsions (F-MEs) formulated from partially fluorinated solvent–co-solvent mixtures and fluorinated anionic AOT-analogue surfactants are reported. These F-MEs permit incorporation of water into fluorinated solvents, up to a volume fraction ϕwater ≈ 0.13. Interestingly, phase behavior and structures, determined by small-angle neutron scattering (SANS), parallel many other classical AOT-type microemulsions, both in normal hydrocarbon and in supercritical-CO2 (scCO2) solvents. Using these new F-MEs as reaction media, F-capped silver nanoparticles (Ag-NPs) have been synthesized, representing the first reported preparation of F-NPs in fluorous phase microemulsions.

A simple method to disperse and recover gold nanoparticles (AuNPs) based on pH-induced heteroaggregation of mixed positive/negative aqueous microgels (MGs) is described. Interestingly, stability of these composite inorganic–organic systems can be readily and reversibly controlled by using pH, affording flocculated (pH 3) or re-dispersed (pH 10) Au-NPs as desired. Atomic absorption spectroscopy shows that an efficient and effective recovery of the AuNPs can be achieved. These systems provide novel triggerable and recyclable alternatives to classical recovery methods.

The interaction of a photodegradable surfactant (PS, 4-hexylphenylazosulfonate, C6PAS) with microgels (MGs) of poly(2-vinyl pyridine) (MGA) in the protonated state (pH 3) has been investigated. Electrophoretic mobility measurements confirm that negatively charged PS interacts with positively charged MGA to form mixed PS-MG complexes. This was sensed by a decrease in the effective PS-MGA charge and a switch in sign of electrophoretic mobility, from positive to negative, with increasing PS concentration. After the addition of extra positive microgels (MGB), the system undergoes coflocculation. Incident UV irradiation was used to photolyze the anionic PS, effectively eliminating the headgroups, thereby lowering the electrostatic interactions between PS and MGA microgel networks. Consequently, a reversal of MGA charge occurred, leading to electrostatic repulsions and causing the MGs to reswell and redisperse, with both MGA and MGB now being positively charged and hence stabilized against coflocculation. Extending this approach, negatively charged gold nanoparticles (AuMES) have been incorporated into the PS-MGA complexes. Atomic absorption spectroscopy (AAS) showed that 100% of the AuMES particles were recovered after coflocculation of (PS-MGA)-AuMES complexes with MGB. Furthermore, approximately 75% of the AuMES could be redispersed after UV irradiation to restabilize the dispersion. This system provides an interesting method for phase separation and gold nanoparticle recovery for reuse and recycling.

A new approach to thicken dense liquid CO2 is described using the principles of self-assembly of custom-made CO2 compatible fluorinated dichain surfactants. Solutions of surfactants in CO2 have been investigated by high-pressure phase behavior, small-angle neutron scattering (HP-SANS) and falling cylinder viscosity experiments. The results show that it is possible to control surfactant aggregation to generate long, thin reversed micellar rods in dense CO2, which at 10 wt % can lead to viscosity enhancements of up to 90% compared to pure CO2. This represents the first example of CO2 viscosity modifiers based on anisotropic reversed micelles.

Here is demonstrated a novel approach to reversible control over nanoparticle (NP) stability, permitting facile recovery for the reuse of inorganic NPs. For the first time, the separation of NPs is achieved by suspending the nanostructures in a background-supporting colloidal fluid, which itself shows a liquid−liquid critical-type phase transition at a temperature Tc instead of using a normal molecular solvent.

Reversed-micelle synthesis has been used to generate CTAB-stabilized gold (Au-NPs) and silver nanoparticles (Ag-NPs). By inducing a phase transition and subsequent separation of the background supporting microemulsion, it has been possible to extract and purify the NPs from the reaction medium. After addition of excess water, the NPs concentrate into an upper octane-rich phase, with impurities and reaction debris (in particular CTAB) partitioning into the water-rich lower phase. UV and 1H NMR showed that 82% of the original mass of Au-NPs can be purified from the excess CTAB and other salt impurities. The concentrated and purified NPs can be dried down, by solvent removal, and then redispersed in octane. Using the complementary techniques small-angle neutron and X-ray scattering (SANS and SAXS), the structures of microemulsions both with and without nanoparticles prior to separation, and in both upper and lower phases after separation, have been elucidated. The approach has also been applied to the synthesis and recovery of silver nanoparticles, but on a larger scale. This new approach compares favorably with existing methods as it uses no additional organic solvents, has a low-energy demand, and requires no specialist surfactants. The new advance here is that by using a colloidal system to prepare and support the nanoparticles as a structured solvent, a simple soft purification method becomes accessible, which is otherwise impossible with a normal molecular solvent.

A trichain anionic surfactant sodium 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulfonate (TC14) is shown to aggregate in three different types of solvent: water, heptane, and liquid CO2. Small-angle neutron scattering (SANS) has been used to characterize the surfactant aggregates in water, heptane, and dense CO2. Surface tension measurements, and analyses, show that the addition of a third branched chain to the surfactant structural template is critical for sufficiently lowering the surface energy, tipping the balance between a CO2-incompatible surfactant (AOT) and CO2-philic compounds that will aggregate to form micelles in dense CO2 (TC14). These results highlight TC14 as one of the most adaptable and useful surfactants discovered to date, being compatible with a wide range of solvent types from high dielectric polar solvent water to alkanes with low dielectrics and even being active in the uncooperative and challenging solvent environment of liquid CO2.

The effect of solvent mixtures on the phase behavior of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) stabilized water-in-oil microemulsions has been studied by using heptane/dodecane, decane/dodecane, octane/dodecane, and nonane/undecane blends. Small-angle neutron scattering was employed to explore the effect of changing the solvent composition on the microemulsion properties, especially near the cloud point (Tcloud) and the liquid−liquid critical separation (Tcrit). It is shown that droplet interactions can be strongly influenced by changing the solvent blend compostion, which has implications for the locations of observed phase boundaries. Of particular interest is the use of carefully selected solvent blends, which have the effect of lowering Tcrit by up to 6 °C from the value found in pure decane.

The phase behavior and structural properties of “protein limit” mixtures of small (radius 20−30 Å) water-in-oil microemulsion droplets (colloids) and large (radius 130−580 Å) nonadsorbing polymer chains have been investigated. Accepted theoretical scaling relations for describing correlations have been applied and do not account fully for the observations; solvency effects may account for the deviations. The polymer/colloid size ratio has been varied from around 4 to 19 by using three different molecular weights of polyisoprene. Small-angle neutron scattering (SANS) has been used to determine partial structure factors (PSF) through contrast variation. The structure factors describing colloid−colloid interactions for the three polymers at fixed polymer concentration are shown to exhibit the same scaling behavior as the phase boundaries, provided that samples are sufficiently far from the demixing phase transition. The structure factors show a dramatic increase at low wavevectors on approaching the phase boundary, and behavior in this region does not obey expected scaling relations. By calculating effective polymer Flory−Huggins parameters, the effect of apparent solvent properties on adding microemulsion are shown to be less dramatic for the higher molecular weight polymers. This study extends previous work carried out on microemulsion-polymer mixtures (Langmuir 2008, 24, 3053−3060).

The effect of improving “solvent quality” of pure liquid CO2 with a heptane cosolvent on the phase behavior and micellization of commercially available surfactants has been explored using high-pressure small-angle neutron scattering (HP-SANS). The nonionic C12E5 was found to be highly soluble in both pure CO2 and the solvent blends, but no aggregation was detected by HP-SANS for any of the compositions studied, even up to 12 vol % surfactant. On the other hand, improving CO2 solvent quality by adding heptane above 30 vol % promoted solubility and aggregate formation with normal sodium bis(ethylhexyl)sulfosuccinate (AOT). The solvent quality index Hildebrand solubility parameter, used to predict surfactant aggregation in pure hydrocarbon solvents (Langmuir, 2008, 24 (21), 12235−12240) has been tested here for CO2−heptane mixtures. The results show how solubility and efficiency of AOT, a commercially viable, well-known, and commonly used surfactant, can be boosted in alkane-containing CO2-rich fluids compared to pure CO2 alone.

Here it is shown that the chemical nature of outer organic surfactant layers, used to stabilize inorganic nanoparticles (NPs), is a key factor controlling solubility in a mixed liquid CO2—heptane (10% vol) solvent.

The effect of solvent on stability of water-in-oil microemulsions has been studied with AOT (sodium bis(2-ethylhexyl)sulfosuccinate) and different solvent mixtures of n  -heptane, toluene and dodecane. Dynamic light scattering DLS was used to monitor the apparent diffusion coefficient DADA and effective microemulsion droplet diameter on changing composition of the solvent. Interdroplet attractive interactions, as indicated by variations in DADA, can be tuned by formulation of appropriate solvent mixtures using heptane, toluene, and dodecane. In extreme cases, solvent mixtures can be used to induce phase transitions in the microemulsions. Aggregation and stability of model AOT-stabilized silica nanoparticles in different solvents were also investigated to explore further these solvent effects. For both systems the state of aggregation can be correlated with the effective molecular volume of the solvent Vmoleff mixture.Microemulsion phase separation with increasing dodecane mole fraction in solvent mixtures of heptane and dodecane at 298 K.

Aggregate structures of two model surfactants, AOT and C12E5 are studied in pure solvents D2O, dioxane-d8 (d-diox) and cyclohexane-d12 (C6D12) as well as in formulated D2O/d-diox and d-diox/C6D12 mixtures. As such these solvents and mixtures span a wide and continuous range of polarities. Small-angle neutron scattering (SANS) has been employed to follow an evolution of the preferred aggregate curvature, from normal micelles in high polarity solvents, through to reversed micelles in low polarity media. SANS has also been used to elucidate the micellar size, shape as well as to highlight intermicellar interactions. The results shed new light on the nature of aggregation structures in intermediate polarity solvents, and point to a region of solvent quality (as characterized by Hildebrand Solubility Parameter, Snyder polarity parameter or dielectric constant) in which aggregation is not favored. Finally these observed trends in aggregation as a function of solvent quality are successfully used to predict the self-assembly behavior of C12E5 in a different solvent, hexane-d14 (C6D14).

Gold particles have been formed in water-in-oil microemulsions doped with a photodestructible surfactant. UV light-induced nanoparticle flocculation has been achieved after photolysis of the photosurfactant, leading to a reduction in the steric stabilization provided by the surfactant layer.

Employing photodestructible surfactants in gelatin-based aqueous gels presents novel possibilities for controlling colloidal and aggregation properties of surfactant gelatin complexes. Light-triggered breakdown of the gelatin-bound photosurfactant aggregates causes dramatic changes in viscosity and aggregation.

This review covers recent advances with an intriguing class of functionalised light-sensitive surfactants. The main chemical classes are described, and the photo-responses in interfacial and aggregation systems are discussed.

Recent advances in design of surfactants and polymers for liquid and supercritical carbon dioxide, and partially fluorinated alkanes, are reviewed. The focus is on surfactant structure–performance relationships, development of non-fluorinated surfactants, and applications of CO2 continuous dispersions for nanoparticle and organic synthesis. Future prospects for these emerging technologies are discussed, including the need for economically viable and environmentally friendly stabilizers for CO2 dispersions.

Liquid or supercritical carbon dioxide has important environmental and economic advantages over petrochemical solvents currently used for industrial processes. However, low solubility in CO2, particularly of polar compounds, is a hurdle to its implementation as an acceptable alternative. These solubility problems have been overcome by employing specialised fluorinated surfactants to stabilise water nano-droplets as water-in-supercritical/liquid CO2 microemulsions. Such novel microemulsions can now facilitate innovative ‘green-and-clean’ applications of carbon dioxide technology.

Stability and aggregation structures of various economically viable surfactants for CO2 are reported. The compounds are either commercially available octylphenol nonionics (Triton X-100, X-100 reduced, and X-45) or custom-made analogues of aerosol-OT (J. Am. Chem. Soc. 123 (2001) 988). These were selected to reveal the influence of chain terminal group structure, namely highly methylated t-butyl units, on solubility and aggregation in CO2. In addition the mean ethylene oxide block length is varied for the Triton surfactants (X-100 ∼ EO10, X-45 ∼ EO8). High-pressure small-angle neutron scattering (SANS) experiments revealed the presence of aggregates, consistent with spheroidal reverse micelles. The nonionics show a temperature and pressure dependence on solubility. These results confirm the special affinity of highly methyl-branched tails for CO2. However, none of these systems were able to disperse significant amounts of water or brine; therefore hydrated reversed micelles or microemulsion droplets were not stabilized. Hence the utility of these cheap methyl-branched surfactants in CO2 is limited, and so groups of greater CO2-philicity are needed to achieve the goal of water–hydrocarbon surfactant–CO2 dispersions.

Dilute aqueous phase behavior of a novel tris(hydroxymethyl)acrylamidomethane (THAM)-derived telomer bearing a perfluorohexyl hydrophobic chain, F6THAM6, has been investigated. Fluorinated polyhydroxy surfactants of this kind find use in emerging biomedical applications. Neutron reflection (NR) and drop volume surface tension (DVT) methods have been used to determine the critical micelle concentration ( and surface adsorption parameters (at the cmc NR gives a molecular area acmc=67.4 and 62 and surface excess . The aggregation structures were determined by small-angle neutron scattering (SANS), indicating globular (polydisperse spheres) micelles of radius ∼30 Å are present. These findings are compared with literature on surfactants with related structures, to identify how the unusual molecular structure of F6THAM6 affects surfactant properties.

Effects of chemical structure on adsorption and aggregation behaviour have been investigated for sodium perfluorononanoate (C8F17COO− Na+ or Na PFN) and 9-H perfluorononanoate (H–C8F16COO− Na+ or H Na PFN). Replacing the terminal F for H in this way gives rise to a permanent dipole moment at the hydrophobic tip of the chain. The critical micelle concentrations, determined by surface tension and electrical conductivity, were 10 and 40 mmol dm−3 for the Na PFN and H Na PFN, respectively. Tensiometric and neutron reflection (NR) methods were employed to investigate adsorption at the air-water interface. Problems associated with the experimental measurements, and data interpretation, are discussed. In particular effects on surface tensions of trace levels of Ca2+, Mg2+ and Ba2+, as well as the sequestering agent ethylenediaminetetraacetic acid (EDTA), were investigated. The background levels of a principal contaminant, Ca2+, were directly measured by atomic absorption spectrophotometry, indicating a typical ratio of Ca2+:Na+ of 1:104 in these surfactant solutions. Hence, an appropriate level of added EDTA, sufficient to chelate unwanted polyvalent metals but enough not affect the surface tension, was chosen and this was EDTA:surfactant 1:333. For both compounds the surface excess Γ obtained from NR data was consistently higher than that derived from tensiometry, using a Gibbs pre-factor of 2. However, better agreement was found if this was changed to 1.7, which would be consistent with around 30% dissociation of counterions from the film. At the critical micelle concentration (cmc) the fully fluorinated surfactant adsorbs most strongly, in that NR gave areas per molecule for the Na PFN and H Na PFN of 41 and 44 Å2, respectively, whereas drop volume tensiometry gave 43 and 51 Å2. Taking together all the different methods, at their cmc’s the average values were 43.1±3.4 and 48.4±5.2 Å2 for the Na PFN and H Na PFN, respectively. These changes are consistent with weaker intermolecular interactions in the layer, and stronger adsorption, in the absence of the H–CF2 dipole. Aggregation in the bulk was also investigated by small-angle neutron scattering (SANS), and the data were modelled in terms of charged spherical micelles. At a volume fraction φmic∼0.05, the micellar radii and net charges were similar, being 15 and 14 Å, −10 and −13, for Na PFN and H Na PFN, respectively. These values suggest that the ion dissociation in micelles is between 25 and 45%, and this is similar to the ionisation at the surface suggested by the adsorption measurements. These results will help improve understanding about properties of fully- and partially-fluorinated surfactants, which are often used in speciality applications, such as water-in-CO2 microemulsions.

Dicationic magnetic ionic liquids with heteroanionic anions allow for tunability of physicochemical properties while retaining magnetic susceptibility.

This article reviews approaches for modification of solvent properties of supercritical carbon dioxide (scCO2), with particular reference to self-assembly of oligomeric and polymeric solute additives. Of special interest are viscosity modifiers for scCO2 based on molecular self-assembly. Background on polymers and surfactants with CO2-compatible functionalities is covered, leading on to the attempts made so far to increase the scCO2 viscosity, which are described in detail. The significance of this field, and the implications a breakthrough could bring environmentally and economically will be addressed.

A new isothermal approach to the recovery of inorganic nanoparticles (NPs) is demonstrated. The NPs can be incorporated into a background microemulsion (ME) supporting fluid, and they can be recovered by addition of non-adsorbing polymer. A clean liquid–liquid (L–L) phase transition can be readily induced by addition of polymer to the MEs. Furthermore, the L–L transitions are also observed in the presence of added NPs, but now the nanoparticles concentrate in the lower co-existing ME phases. Once recovered, the NPs can be redispersed by adding extra ME as a solvent.

Here it is shown that the chemical nature of outer organic surfactant layers, used to stabilize inorganic nanoparticles (NPs), is a key factor controlling solubility in a mixed liquid CO2—heptane (10% vol) solvent.

Gold particles have been formed in water-in-oil microemulsions doped with a photodestructible surfactant. UV light-induced nanoparticle flocculation has been achieved after photolysis of the photosurfactant, leading to a reduction in the steric stabilization provided by the surfactant layer.

The formation of ions in nonpolar solvents (with relative permittivity εr of approximately 2) is more difficult than in polar liquids; however, these charged species play an important role in many applications, such as electrophoretic displays. The low relative permittivities of these solvents mean that charges have to be separated by large distances to be stable (approximately 28 nm or 40 times that in water). The inverse micelles formed by surfactants in these solvents provide an environment to stabilize ions and charges. Common surfactants used are sodium dioctylsulfosuccinate (Aerosol OT or AOT), polyisobutylene succinimide, sorbitan oleate, and zirconyl 2-ethyl hexanoate. The behavior of charged inverse micelles has been studied on both the bulk and on the microscopic scale and can be used to determine the motion of the micelles, their structure, and the nature of the electrostatic double layer. Colloidal particles are only weakly charged in the absence of surfactant, but in the presence of surfactants, many types, including polymers, metal oxides, carbon blacks, and pigments, have been observed to become positively or negatively charged. Several mechanisms have been proposed as the origin of surface charge, including acid–base reactions between the colloid and the inverse micelle, preferential adsorption of charged inverse micelles, or dissolution of surface species. While most studies vary only the concentration of surfactant, systematic variation of the particle surface chemistry or the surfactant structure have provided insight into the origin of charging in nonpolar liquids. By carefully varying system parameters and working to understand the interactions between surfactants and colloidal surfaces, further advances will be made leading to better understanding of the origin of charge and to the development of more effective surfactants.