In this contribution, we (i) link the mesoscopic structuring of the binary structured solvent mixture H2O/tert-butanol (TBA) to the kinetics and the efficacy of the oxidation of benzyl alcohol (BA) to the corresponding aldehyde catalyzed by H5PMo10V2O40. We also compare the catalytic efficacy of this reaction in the mesoscopically structured solvent H2O/TBA to an unstructured (or very weakly structured) solvent H2O/ethanol (EtOH). In this context, we (ii) also give a methodological outline on how to study systematically the catalytic efficacy of chemical reactions as a function of the mesoscale structuring of a binary solvent. We demonstrate that the obtained yields of benzyl aldehyde depend on the type of mesoscopic structuring of the binary solvent H2O/TBA. An elevated catalytic performance of at least 100% is found for unstructured binary mixtures H2O/TBA compared to compartmented binary mixtures H2O/TBA. We conclude that compartmentation of both the organic substrate and the catalyst in TBA and water-rich micro phases seems to be unfavorable for the catalytic efficacy.

In the present contribution, the pre-structuring of binary mixtures of hydrotropes and H2O is linked to the solubilisation of poorly water miscible compounds. We have chosen a series of short-chain alcohols as hydrotropes and benzyl alcohol, limonene and a hydrophobic azo-dye (Disperse Red 13) as organic compounds to be dissolved. A very weak pre-structuring is found for ethanol/H2O and 2-propanol/H2O mixtures. Pre-structuring is most developed for binary 1-propanol/H2O and tert-butanol/H2O mixtures and supports the bicontinuity model of alcohol-rich and water-rich domains as already postulated by Anisimov et al. Such a pre-structuring leads to a high solubilisation power for poorly water miscible components (limonene and Disperse Red, characterized by high octanol/water partition coefficients, log(P) values of 4.5 and 4.85), whereas a very weak pre-structuring leads to a high solubilisation power for slightly water miscible components (benzyl alcohol). This difference in solubilisation power can be linked to (i) the formation of mesoscale structures in the cases of ethanol and 2-propanol and (ii) the extension of pre-structures in the cases of 1-propanol and tert-butanol. Three different solubilisation mechanisms could be identified: bulk solubilisation, interface solubilisation and a combination of both. These supramolecular structures in binary and ternary systems were investigated by small-and-wide-angle X-ray and neutron scattering, dynamic light scattering and conductivity measurements (in the presence of small amounts of salt).

The influence of electrolytes on liquid-liquid equilibria (LLE) of water/1-butanol and on the partitioning of 5-hydroxymethylfurfural (HMF) between water-rich and 1-butanol-rich phases was investigated in this study. For that purpose, the LLE of the ternary systems water/1-butanol/HMF, water/1-butanol/salt, and the LLE of the quaternary system water/1-butanol/HMF/salt were measured at 298.15 K under atmospheric pressure. The investigated salts were composed of one of the anions Cl−, CH3COO−, NO3− and SO42− and either Li+ or Na+. By investigating the LLE of the system water/1-butanol/salt it was found that 1-butanol was salted-out from the aqueous phase by all salts, and the strength of the salting-out increased in the following order NO3− < CH3COO− ≈ Cl− < SO42−, independently of the cation.Based on the LLE data, the partition coefficient KHMFw of HMF between 1-butanol and aqueous phase was determined. Li2SO4 caused a pronounced salting-out of HMF from the aqueous phase, whereas only a moderate influence was observed for NaCl and CH3COONa. LiCl even caused a salting-in at LiCl molalities above 6 mol/kgH2O. electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT) was successfully used to model the influence of salts on the LLE water/1-butanol. Without fitting parameters to LLE data of the quaternary system water/1-butanol/HMF/salt, ePC-SAFT allowed predicting the salt influence on the partitioning of HMF in these systems in good agreement with the experimental data.

Ternary solutions containing one hydrotrope (such as ethanol) and two immiscible fluids, both being soluble in the hydrotrope at any proportion, show unexpected solubilization power and allow strange but yet unexplained membrane enzyme activity. We study the system ethanol-water-octanol as a simple model of such kinds of ternary solutions. The stability of “detergentless” micelles or microemulsions in such mixtures was proposed in the pioneering works of Barden and coworkers [Smith GD, Donelan CE, Barden RE (1977) J Colloid Interface Sci 60(3):488–496 and Keiser BA, Varie D, Barden RE, Holt SL (1979) J Phys Chem 83(10):1276–1281] in the 1970s and then, neglected, because no general explanation for the observations was available. Recent direct microstructural evidence by light, X-ray, and neutron scattering using contrast variation reopened the debate. We propose here a general principle for solubilization without conventional surfactants: the balance between hydration force and entropy. This balance explains the stability of microemulsions in homogeneous ternary mixtures based on cosolvents.

•Nano-structures were found in water/alcohol/mosquito repellent systems.•DLS/SLS confirmed findings for both natural and synthetic repellents.•Nano-structures can be found in commercial formulations.•Potential consequences for formulation efficacy.It was recently demonstrated that nano-structures were present in water/ethanol/oil systems, where the oil was either octanol or fragrance molecules. The goal of the present work is to check if such structures exist also in other, related systems and if a general concept can be deduced from these observations. To this purpose, natural and synthetic mosquito repellent molecules were investigated, which represent nearly all repellents used on the market. For the ternary water/alcohol (ethanol or isopropanol)/repellent systems ternary phase diagrams were established. The presence and the ordering of the nano-droplets were checked and characterized with dynamic and static light scattering and conductivity measurements. Based on these results it can be concluded that a nano-ordering with generally an organic continuum exists in hydro-alcoholic commercial mosquito repellents, and thus that these systems are not simply molecular solutions. This might have a consequence for diffusion processes in the skin.

The realms of existence of “green” microemulsions are reported and their pseudo-ternary phase diagrams as well as electrical conductivities. For the investigations, the model system water/sodium oleate/citronellol/limonene is used. Herein, sodium oleate is the surfactant and citronellol the cosurfactant. The optimal surfactant-to-cosurfactant mass ratio is found to be 1:1. By replacing successively citronellol with ethanol the homogeneous single phase can be extended. At first, an anti-percolative behaviour of the system is obtained that can be converted into a percolative one. For low ethanol content microemulsions remain of water-in-oil (w/o) type. With increasing ethanol, between 30 and 90 wt% of ethanol in the blend, the monophasic region is diminished whereas liquid crystalline phases (lamellar and mixed lamellar and cubic ones) extend in the water-rich area. Above 90% the realms of existence of microemulsions are increased again and bicontinuous structures and oil-in-water (o/w) microemulsions were found. For a certain percentage of ethanol (92.5%) in the blend with citronellol and a surfactant + cosurfactant-to-oil ratio equal to or higher than 4:1 a highly water dilutable concentrated microemulsion is formed which can incorporate 99 wt% of water. By adding water rapidly to the water-rich areas of this system, thermodynamically unstable translucid nanoemulsions can occur, which turn into transparent microemulsions with time.Graphical abstractHighlights► Formulation of green microemulsions. ► Formulation of highly water dilutable microemulsions. ► Investigation of the influence of mixed cosurfactants on the microemulsion area.

Mineralization of alkaline-earth carbonates in silica-rich media at high pH leads to fascinating crystal morphologies that strongly resemble products from biomineralization, despite the absence of any organic matter. Recent work has demonstrated that elaborate CaCO3 structures can be grown in such systems even at high supersaturation, as nanoparticles of amorphous calcium carbonate (ACC) were spontaneously coated by skins of silica and thus served as temporary storage depots continuously supplying growth units for the formation of crystalline calcite. In the present study, we have precipitated barium carbonate under similar conditions and found surprisingly different behavior. At low silica concentrations, there was no evidence for an amorphous carbonate precursor phase and crystallization occurred immediately, resulting in elongated crystals that showed progressive self-similar branching due to the poisoning influence of silicate oligomers on the growth process. Above a certain threshold in the silica content, rapid crystallization was in turn prevented and amorphous nanoparticles were stabilized in solution. However, in contrast to previous observations made for CaCO3, the particles were found to be hybrids consisting of a silica core that was surrounded by a layer of amorphous barium carbonate, which was then again covered by a an outer shell of silica. These self-assembled core–shell–shell nanoparticles were characterized by different techniques, including high-resolution transmission electron microscopy and elemental analyses at the nanoscale. Time-dependent studies further evidence that the carbonate component in the particles can either be permanently trapped in an amorphous state (high silica concentrations, leading to impervious outer silica skins), or be released gradually from the interstitial layers into the surrounding medium (intermediate concentrations, giving porous external shells). In the latter case, enhanced particle aggregation induces segregation of silica hydrogel with embedded amorphous BaCO3 precursors, which later crystallize in the matrix to yield complex ultrastructures consisting of uniform silica-coated nanorods. The spontaneous formation of core–shell–shell nanoparticles and their subsequent development in the system is discussed on the basis of local pH gradients and inverse pH-dependent trends in the solubility of carbonate and silica, which link their chemistry in solution and provoke coupled mineralization events. Our findings depict a promising strategy for the production of multilayered nanostructures via a facile one-pot route, which is based on self-organization of simple components and may be exploited for the design of novel advanced materials.Keywords: amorphous precursor phases; barium carbonate; biomimetic self-assembly; biomorphs; core−shell nanoparticles; layer-by-layer mineralization; silica;

In this study, we show a strategy of formulating highly and fully water dilutable sustainable microemulsions with dibasic esters as oil pseudophase. First, dimethyl or diethyl DBEs with different numbers of carbon atoms between the ester functions (succinate C2, glutarate C3, and adipate C4) or DBE blends and a surfactant–cosurfactant melt consisting of sodium dodecylsulfate and 1-pentanol with a constant weight ratio of 1:2 are used. We investigated the extent of the realms of existence of homogeneous single phases and represent them with pseudoternary phase diagrams. It was possible to formulate fully water dilutable oil-rich single-phase solutions with all dimethyl dibasic esters but not with the diethyl ones. By mixing the most hydrophilic dibasic ester with the most hydrophobic one with a mass ratio of 3:2, a fully water dilutable oil-rich single phase could also be obtained. The presence of water-in-oil, bicontinuous, and oil-in-water microemulsions was checked with conductivity and dynamic light scattering measurements. A percolative behavior was observed for all systems. Second, to formulate green and sustainable microemulsions, we replaced successively sodium dodeylsulfate by sodium oleate and 1-pentanol by ethanol or 1,5-pentanediol. With sodium oleate, highly water dilutable microemulsions were obtained. Homogeneous and translucid samples in the very diluted area turned bluish after several days. Longtime pH measurements of systems containing sodium oleate showed no degradation of the dibasic esters at low water content and a rapid hydrolysis at high water content with 1-pentanol as cosurfactant. Always in presence of sodium oleate, the substitution of 1-pentanol by ethanol or 1,5-pentanediol drives the system to microemulsions showing an instability of the ester at any water concentrations. Thermal gravimetric analysis measurements at 32 °C for 24 h confirmed a rapid evaporation of ethanol and water and negligible evaporation of 1,5-pentanediol.Keywords: 1,5-Pentanediol; Dibasic ester; Dilutable; Ethanol; Microemulsion

In this paper we consider clusters in the ternary systems water–benzyl alcohol and ethanol, ethyl lactate or γ-valerolactone as found with the help of dynamic and static light scattering experiments. These ternary mixtures are powerful solvent media and consist only of low-toxic solvents of natural origin. In a recent work we have shown that surfactantless microemulsions are formed in the water–ethanol–n-octanol system. By contrast, in the systems studied here the sizes of the aggregates are too small to be considered as micelles. It can be postulated that the presence of clusters or larger structures as in surfactantless microemulsions is strongly influenced by the most hydrophobic compound. The phenomenon of facilitated hydrotropy is also investigated in the above-mentioned systems. In this context, ethanol is considered as the primary hydrotrope and the more hydrophobic benzyl alcohol as the facilitating secondary hydrotrope. The hydrophobic dye Disperse Red 13 is used as a marker of facilitated hydrotropy. The results suggest that the degree of self-association of eco-solvent has a significant influence on the hydrotropic efficiency of benzyl alcohol.

Under certain conditions, mineralization of barium carbonate in silica-containing media at high pH results in the formation of complex crystal aggregates called “silica biomorphs”. These self-assembled, purely inorganic materials exhibit hierarchical structure and curved morphologies much reminiscent of certain living forms, thus constituting an interesting model system for the study of the biomineralization processes. In this paper, we report on the influence of the bulk pH on the morphogenesis of silica biomorphs in alkaline solutions. To that end, crystallization experiments were carried out at initial pH values between about 9.8 and 11.9, using atmospheric CO2 as a carbonate source. Formed aggregates were characterized quantitatively by statistical analyses of their morphology, number and size. Corresponding data evidence that well-developed polycrystalline architectures with elaborate shapes occur only when the starting pH is adjusted to values within a rather narrow corridor, ranging roughly from 10.2 to 11.1. Otherwise, merely ill-defined, globular or dumbbell-shaped particles were obtained. In addition, the pH of the mother solutions was monitored continuously during growth and correlated with time-dependent Ba2+ concentration profiles determined by in-situ X-ray fluorescence spectroscopy. By combining the collected data, temporal progressions of the bulk supersaturation were estimated for different conditions and used to re-evaluate the role of pH in the formation of silica biomorphs, particularly with regard to the recently proposed model of the growth mechanism. It is shown that a suitable starting pH is required not only to allow for dynamically coupled co-precipitation of the components, but also to maintain continuous CO2 uptake and hence adequate levels of supersaturation over extended periods of time during growth. Our experiments rationalize previous observations concerning the effect of pH on the formation of silica biomorphs, and disclose fundamental differences between growth in solutions and gels.

Using choline as a counterion in fatty acid surfactants substantially increases their water solubility as compared to classical sodium and potassium soaps, and thereby enables the application of desirable longer-chain derivatives at ambient temperature. Since choline can be decomposed both physiologically and environmentally, corresponding fatty acid soaps are considered to be highly biocompatible. Recent toxicity and biodegradability studies of choline ionic liquids, including anions such as short- and middle-chain alkanoates, have verified the expected low toxic impact. However, according to the European Cosmetic Directive 76/768/EEC, all salts of choline are forbidden in cosmetic products, mainly just due to its classification as a quaternary ammonium ion. In order to facilitate their application in the future, we have investigated the biodegradability of choline soaps (ChCm) with alkyl chain lengths of m = 12–18 according to the OCDE 301F standard. Further, the cytotoxicity of ChCm surfactants with m = 8–16 was determined, both for odd- and even-numbered fatty acids. Studies were carried out using two different human cell lines, namely cervix carcinoma cells (HeLa) and keratinocytes (SK-Mel-28). For a better comparability to common soaps and to shed light on the influence of the cation, sodium and potassium homologues were also investigated. Results reveal an unexpected non-linear relationship between the hydrophobic chain length and the IC50 value. Most importantly, the presented data show that IC50 values of ChCm surfactants coincide with those of the widely applied sodium and potassium soaps. This demonstrates that choline carboxylate surfactants are harmless and thus strongly supports their applicability in customer end products.

Highlights•Formulation of green microemulsions.•Limitations of using of ethanol as cosolvent to extend the microemulsion area.•Investigation of the film flexibility.In a previous work, we showed the extension of the microemulsion area towards highly or fully water dilutable systems by adding a short chain alcohol as cosolvent, like ethanol, to the system water/sodium oleate/citronellol/limonene. It was possible to convert an anti-percolative system to a percolative one by making the interfacial film more flexible. The question arises if this is a general concept. For this reason, we investigated pseudo-ternary systems water/surfactant/cosurfactant/ethanol/oil with different cosurfactants (1-pentanol, 1-heptanol, dodecanol, Guerbet alcohols) and fatty acid methyl ester-rapeseed biodiesel as oil phase. Sodium oleate was used as surfactant. Ethanol was added as cosolvent to enhance the film flexibility and so to increase the microemulsion area. With increasing hydrophobicity of the cosurfactants and the oil and without further addition of ethanol, only very restricted single phase areas were obtained. However, with a certain amount of ethanol, sodium oleate, and 1-heptanol a distinct path towards the water-rich corner was found. By replacing 1-heptanol by Guerbet alcohols this extension was lost and monophasic areas only in the surfactant-rich region were preserved. For all systems a limited “optimal” formulation was obtained for a specific percentage of ethanol in the ethanol–cosurfactant blend. With DLS the homogeneous single phase areas were checked to distinguish real solutions from microemulsions. The nano-structure and film flexibility was investigated using electrical conductivity measurements. In the choice of the solvents, we focused on sustainable solvents.Graphical abstract

In preceding studies, we demonstrated that choline carboxylates ChCm with alkyl chain lengths of m = 12 – 18 are highly water-soluble (for m = 12, soluble up to 93 wt % soap and 0 °C). In addition, choline soaps are featured by an extraordinary lyotropic phase behavior. With decreasing water concentration, the following phases were found: micellar phase (L1), discontinuous cubic phase (I1′ and I1″), hexagonal phase (H1), bicontinuous cubic phase (V1), and lamellar phase (Lα). The present work is also focused on the lyotropic phase behavior of choline soaps but with shorter alkyl chains or different alkyl chain properties. We have investigated the aqueous phase behavior of choline soaps with C8 and C10 chain-lengths (choline octanoate and choline decanoate) and with a C18 chain-length with a cis-double bond (choline oleate). We found that choline decanoate follows the lyotropic phase behavior of the longer-chain homologues mentioned above. Choline octanoate in water shows no discontinuous cubic phases, but an extended, isotropic micellar solution phase. In addition, choline octanoate is at the limit between a surfactant and a hydrotrope. The double bond in choline oleate leads also to a better solubility in water and a decrease of the solubilization temperature. It also influences the Gaussian curvature of the aggregates which results in a loss of discontinuous cubic phases in the binary phase diagram. The different lyotropic mesophases were identified by the penetration scan technique with polarizing light microscope and visual observations. To clarify the structural behavior small (SAXS) and wide (WAXS) angle X-ray scattering were performed. To further characterize the extended, isotropic micellar solution phase in the binary phase diagram of choline octanoate viscosity and conductivity measurements were also carried out.

In this paper we discuss the influence of chemical structures of renewable feedstock oils (RFOs) on the domains of existence and the nano-structures of microemulsions. We compare the results to those of classical microemulsions containing classical n-alkanes. First, the domains of microemulsions obtained from the melt of water, sodium dodecyl sulfate (SDS) as surfactant, 1-pentanol as co-surfactant and different RFOs (or RFO melts) in pseudo-ternary phase diagrams are presented. A surfactant–co-surfactant mass ratio of 1:2 is kept constant and the RFO (or RFO melt) is considered as a pseudo-constituent. Two different fatty methyl ester (FAME) biodiesels from rapeseed and cuphea oils, rapeseed oil, “TBK” biodiesel from rapeseed oil, limonene, and different mixtures of limonene to FAME-rapeseed biodiesel and FAME-rapeseed biodiesel to FAME-cuphea biodiesel are used as RFOs or RFO melts. Second, conductivity data are shown along an experimental path having a constant RFO or RFO melt:(surfactant–co-surfactant) mass ratio, whereas the water content is varied. All obtained data are then compared to data from previous studies with a series of n-alkanes (from n-hexane to n-hexadecane). As the main conclusion it is found that RFOs or RFO melts can easily substitute n-alkanes. From the chemical structure of the oils, it appears that not only the polarity of the oil plays an important role but also does the absolute size of the oil molecules. In all cases microemulsion systems exhibit percolative behavior.

Microemulsions are stable mixtures of a polar solvent, surfactant and an unpolar solvent. Ionic liquids (ILs, i.e. salts with melting points below 100 °C) are a huge class of potentially promising solvents. We discuss here published structural or thermodynamic investigations concerning microemulsions in which one or more of the three classical components are ILs.In microemulsions IL can replace respectively the “oil”, the “surfactant” and the “water” phase. Experimental proofs of the existence and stability of microemulsions are given as well as hints at their microstructure. While the four regimes initially defined by Winsor are all accessible, most of the examples of microemulsions containing ionic liquids belong to the class of “rigid” microemulsions. Since additional solutes have characteristic distribution coefficients for each pseudo phase, IL based microemulsions may provide a useful tool for solubilization (reaction medium) and separation, thus allowing the recovery of a large variety of reaction products, but also waste. Further to a discussion of phase diagrams and thermodynamics, we will show some application examples and propose challenges for future studies, in this vast but only emerging domain.Small angle neutron scattering data of microemulsions containing 6 wt.% ethylammonium nitrate, 40 wt.% surfactant + cosurfactant ([C16mim][Cl] + decanol mixture (1:4, molar ratio) and 54 wt.% [D26]dodecane at different temperatures.Highlights► Critical review of newest developments in the field. ► Critical review of possible applications. ► Discussion of cat-anionic interfacial films as two-dimensional Ionic Liquids

Crystal architectures delimited by sinuous boundaries and exhibiting complex hierarchical structures are a common product of natural biomineralization. However, related forms can also be generated in purely inorganic environments, as exemplified by the existence of so-called “silica-carbonate biomorphs”. These peculiar objects form upon coprecipitation of barium carbonate with silica and self-assemble into aggregates of highly oriented, uniform nanocrystals, displaying intricate noncrystallographic morphologies such as flat sheets and helicoidal filaments. While the driving force steering ordered mineralization on the nanoscale has recently been identified, the factors governing the development of curved forms on global scales are still inadequately understood. In the present work, we have investigated the circumstances that lead to the expression of smooth curvature in these systems and propose a scenario that may explain the observed morphologies. Detailed studies of the growth behavior show that morphogenesis takes crucial advantage of reduced nucleation barriers at both extrinsic and intrinsic surfaces. That is, sheets grow in a quasi-two-dimensional fashion because they spread across interfaces such as walls or the solution surface. In turn, twisted forms emerge when there is no foreign surface to grow on, such that the evolving aggregates curve back on themselves in order to use their own as a substrate. These hypotheses are corroborated by experiments with micropatterned surfaces, which show that the morphological selection intimately depends on the topology of the offered substrate. Finally, we demonstrate that, with the aid of suitable template patterns, it is possible to directly mold the shape (and size) of silica biomorphs and thus gain polycrystalline materials with predefined morphologies and complex structures.

Choline carboxylate surfactants are powerful alternatives to the well-known classical alkali soaps, since they exhibit substantially increased water solubility while maintaining biocompatibility, in contrast to simple quaternary ammonium ions. In the present study, we report the aqueous binary phase diagrams and a detailed investigation of the lyotropic liquid crystalline phases formed by choline carboxylate surfactants (ChCm) with chain lengths ranging from m = 12–18 and at surfactant concentrations of up to 95–98 wt%. The identification of the lyotropic mesophases and their sequence was achieved by the penetration scan technique. Structural details are elucidated by small-angle X-ray scattering (SAXS). The general sequence of mesophases with increasing soap concentration was found to be as follows: micellar (L1), discontinuous cubic (I1), hexagonal (H1), bicontinuous cubic (V1) and lamellar (Lα). The main difference to the phase behavior of alkali soaps or of other mono-anionic surfactants is the appearance and large extent of a discontinuous cubic phase with two or even more different symmetries. The obtained phase diagrams further highlight the extraordinarily high water solubility of ChCm soaps. Finally, structural parameters of ChCm salts such as the cross-sectional area at the polar–nonpolar interface are compared to those of alkali soaps and discussed in the terms of specific counterion binding and packing constraints.

In this work, we report on the phase behavior of 1-ethyl-3-methyl-imidazolium-ethylsulfate ([emim][etSO4])/limonene/polyethylene glycol tert-octylphenyl ether (Triton X-114 or TX-114) microemulsions as a function of ionic liquid (IL) content and temperature. Phase diagrams, conductivity measurements, and small angle X-ray scattering (SAXS) experiments will be presented. A hydrophilic IL, instead of water is used with the goal to enlarge the temperature range on which stable microemulsions can be formed. Indeed, the system shows remarkably large temperature stability, in particular down to −35 °C. We will emphasize on a comparison with a recently published work about microemulsions composed of [emim][etSO4], limonene, and Triton X-100 that to some extent are stable at temperatures well below the freezing point of water. The key parameter responsible for the difference in phase behavior, microstructure, and temperature stability is the average repeating number of ethylene oxide units in the surfactant head group, which is smaller for Triton X-114 compared to Triton X-100. Among the fundamental interest, how the amphiphilicity of the surfactant influences the phase diagram and phase behavior of IL-based microemulsions, the exchange of Triton X-100 by Triton X-114 results in one main advantage: along the experimental path the temperature where phase segregation occurs is significantly lowered leading to single phase microemulsions that exist at temperatures beneath 0 °C.Graphical abstractIL-based “cold” microemulsions have been studied in order to determine the effect of surfactant amphiphilicity on these ternary systems.Highlights► “Cold” microemulsions have been formulated and investigated. ► Microemulsions with IL as continuous phase do not phase separate down to −35 °C. ► A general concept for the formulation of “cold” microemulsions is proposed.

Research on nonaqueous microemulsions containing ionic liquids as polar and/or apolar phase, respectively, is growing at a fast rate. One key property of ionic liquids that highlights their potential and their diversification compared to water is their wide liquid temperature range. In this emerging-area review article we survey recent developments in the field of nonaqueous micellar solutions and microemulsions containing ionic liquids in general with a strong emphasis on the effect of temperature in particular. Various systems are discussed and compared to their aqueous counterparts.

Choline carboxylates (ChCm with m = 12−18) are simple biocompatible anionic surfactants with very low Krafft temperatures, possessing a rich aqueous phase behavior. In the present work, we have investigated the thermotropic mesomorphism of anhydrous ChCm salts for m = 12−18. Transition temperatures and enthalpies determined by differential scanning calorimetry reveal that all investigated compounds exhibit three different phases between −20 and 95 °C. The phases were further characterized by optical polarizing microscopy, NMR spin−spin relaxation, and X-ray scattering measurements. The nature of the phases was identified with increasing temperature as crystalline, semicrystalline, and liquid−crystalline lamellar. Even long-chain choline carboxylates (m = 18) were found to melt into a lamellar liquid−crystalline phase below 100 °C. Accordingly, with choline as counterion in simple fatty acid soaps, not only the water solubility is considerably enhanced but also the melting points are substantially reduced, hence facilitating thermotropic mesomorphism at temperatures between 35 and 95 °C. Thus, simple choline soaps with m = 12−18 may be classified as ionic liquids.

Recently, a new family of ionic liquids based on oligoether carboxylates was introduced. 2,5,8,11-Tetraoxatridecan-13-oate (TOTO) was shown to form room-temperature ionic liquids (RTILs) even with small alkali ions such as lithium and sodium. However, the alkali TOTO salts suffer from their extremely high viscosities and relatively low conductivities. Therefore, we replaced the alkali cations by tetraalkylammonium (TAA) ions and studied the TOTO salts of tetraethyl- (TEA), tetrapropyl- (TPA), and tetrabutylammonium (TBA). In addition, the environmentally benign quaternary ammonium ion choline (Ch) was included in the series. All salts were found to be ionic liquids at ambient temperatures with a glass transition typically at around −60 °C. Viscosities, conductivities, solvent polarities, and Kamlet–Taft parameters were determined as a function of temperature. When using quaternary ammonium ions, the viscosities of the resulting TOTO ionic liquids are >600 times lower, whereas conductivities increase by a factor of up to 1000 compared with their alkali counterparts. Solvent polarities further reveal that choline and TAA cations yield TOTO ionic liquids that are more polar than those obtained with the, per se, highly polar sodium ion. Results are discussed in terms of ion-pairing and structure-breaking concepts with regard to a possible complexation ability of the TOTO anion.

We demonstrate here that microemulsions with an IL as the continuous phase can be formed so that they are stable over a wide temperature range and have intermediary properties between flexible and stiff microemulsions. Three components (1-ethyl-3-methylimidazolium ethylsulfate ([emim][etSO4]), limonene, and octylphenol ethoxylate (Triton X 100, abbreviated as TX-100)) were used. This ternary system has been characterized from ambient temperature down to −10 °C by means of conductivity, viscosity, and small-angle X-ray scattering (SAXS) measurements. The SAXS data exhibit a characteristic single, broad scattering peak in conjunction with a typical q−4 decay at large q values. The SAXS data have also been interpreted in terms of a dimensionless dilution plot, demonstrating that microstructures are neither isolated droplets nor a random flexible film structure but resemble molten liquid crystals (i.e., they are formed from locally cylindrical or planar structures). This semirigidity is attributed to a good match between the surfactant and the ionic liquid; this holds in a temperature range well below 0 °C.

We report on the effects of electrolytes spanning a range of anions (NaOc, NaSCN, NaNO3, NaBr, NaCl, NaBu, NaOAc, Na2SO4, Na2HPO4, and Na2CO3) and cations (LiCl, NaCl, KCl, CsCl, and choline chloride) on the aqueous solubility of an extended surfactant. The surfactant is anionic with a long hydrophobic tail as well as a significant fraction of propylene oxide groups and ethylene oxide groups (C12−14-PO16-EO2−SO4Na, X-AES). In the absence of electrolytes, X-AES exhibits a cloud-point temperature that decreases with increasing surfactant concentration. After the addition of salts to the surfactant solutions, various shifts in the solubility curves are observed. These shifts follow precisely the same Hofmeister series that is found for salting-in and salting-out effects in protein solutions. In the presence of different concentrations of sodium xylene sulfonate (SXS), the solubility of the surfactant increases. In this context, SXS can be considered to be a salting-in salt. However, when the electrolytes are added to an aqueous solution of X-AES and SXS the Hofmeister series reverses for divalent anions such as Na2SO4, Na2HPO4, and Na2CO3. Studies on the phase behavior and micelle structures using polarization microscopy, freeze-etch TEM, and NMR measurements indicate a dramatic change in the coexisting phases on the addition of SXS.

Biodiesel has gained more and more attention in recent years resulting from the fact that it is made of renewable resources. Parallel to its environmental compatibility, biodiesel also exhibits a high thermal stability. We demonstrate here that biodiesel can replace conventional oils as apolar phase in nonaqueous microemulsions containing the room temperature ionic liquid ethylammonium nitrate as polar phase. In addition to the phase diagram and the viscosity of the microemulsions, we study the thermal stability of these systems. Along an experimental path in the phase diagram, no phase change could be observed between 30 °C and 150 °C. Conductivity measurements confirm the high thermal stability of these systems. The microemulsion exhibits a percolative behavior between 30 °C and 150 °C. Small angle X-ray scattering spectra show a single broad scattering peak similar to aqueous microemulsions. The spectra could well be described by the Teubner–Strey model. Furthermore, the adaptability of different models ranging from bicontinuous structures to ionic liquid in oil spheres as well as disordered open connected cylinders has been checked. These high temperature stable, nonaqueous, free of crude oil based organic solvent microemulsions highlight an efficient way towards the formulation of environmentally compatible microemulsions and open a wide field of potential applications.

In biomineralization, living organisms carefully control the crystallization of calcium carbonate to create functional materials and thereby often take advantage of polymorphism by stabilizing a specific phase that is most suitable for a given demand. In particular, the lifetime of usually transient amorphous calcium carbonate (ACC) seems to be thoroughly regulated by the organic matrix, so as to use it either as an intermediate storage depot or directly as a structural element in a permanently stable state. In the present study, we show that the temporal stability of ACC can be influenced in a deliberate manner also in much simpler purely abiotic systems. To illustrate this, we have monitored the progress of calcium carbonate precipitation at high pH from solutions containing different amounts of sodium silicate. It was found that growing ACC particles provoke spontaneous polymerization of silica in their vicinity, which is proposed to result from a local decrease of pH nearby the surface. This leads to the deposition of hydrated amorphous silica layers on the ACC grains, which arrest growth and alter the size of the particles. Depending on the silica concentration, these skins have different thicknesses and exhibit distinct degrees of porosity, therefore impeding to varying extents the dissolution of ACC and energetically favored transformation to calcite. Under the given conditions, crystallization of calcium carbonate was slowed down over tunable periods or completely prevented on time scales of years, even when ACC coexisted side by side with calcite in solution.

During the last ten years significant progress has been made in the understanding of specific ion effects. On the one hand new ideas about the origin of these effects came up, and on the other hand new experimental techniques were developed so that now even the ion concentration profile near surfaces can be measured with some confidence. In the present review some of the most important new progresses are summarised and critically discussed, especially in the context of colloidal and biological systems.

The influence of inorganic anions (NO3-, I−, Br−, Cl−, SO42-, and S2O32-) and of divalent cations (Ca2+ and Mg2+) on the zeta potential and on the isoelectric point of α-alumina in aqueous medium has been studied. The effect of the anions is highly ion specific even at salt concentrations as low as 5 × 10−4 M. This unexpected finding is in line with a recent report [Böstrom et al., J. Chem. Phys. 128 (2008) 135104]. It is also in agreement with an earlier theoretical prediction [B.W. Ninham, V.V. Yaminsky, Langmuir 13 (1997) 2097]. The results are consistent with the classical Hofmeister series, except for the case of NO3-. Divalent anions (SO42- and S2O32-) decrease the magnitude of the zeta potential of α-alumina in aqueous medium, more precisely; S2O32- produced large negative zeta potential (∼−12 to −47 mV) within the pH range of the study without the isoelectric point (IEP) of α-alumina. However, the SO42- decreased the zeta potential of α-alumina of different magnitudes (maximum ∼25 mV at both ends of the experimental acidic and basic pH scale) with a minor shift of the IEP (∼0.5 unit) toward lower pH. Ca2+ and Mg2+ produce zeta potentials of α-alumina roughly equal to that of neat α-alumina but slightly higher than that of Na+ at both sides of the IEP. We have shown further that the same ion specificity or equivalently competitive ion effects occur with the adsorption density of p-hydroxybenzoate onto α-alumina surfaces. The sequence of anions (with common cation) for the adsorption density of p  -hydroxybenzoate on the α-alumina surfaces follows the Hofmeister series sequence: S2O32- < SO42- < Cl− > Br− > I− > NO3-. The divalent cations (Ca2+ and Mg2+) exhibit a roughly equivalent effect on the adsorption of p-hydroxybenzoate onto α-alumina surfaces. Using the frequency shifts of νas(–COO−) and νs(–COO−) in the DRIFT spectra of p-hydroxybenzoate after adsorption and other characteristic peaks, we have demonstrated that p-hydroxybenzoate forms outer-sphere complexes onto α-alumina surfaces at pH 5 and 6 and inner-sphere complexes at pH 7, 8, and 9 in the presence of 5 × 10−4 M NaCl(aq).The effect of anions on the zeta potential of α-alumina is highly ion specific. The same ion specificity occurs with the adsorption density of p-hydroxybenzoate onto α-alumina surfaces.

The increasing number of publications reflects the still growing interest in nonaqueous microemulsions containing room-temperature ionic liquids. Recently, we characterized microemulsions composed of the room-temperature ionic liquid ethylammonium nitrate (EAN) as polar phase, dodecane as continuous phase and 1-hexadecyl-3-methyl imidazolium chloride ([C16mim][Cl]), an IL that exhibits surfactant properties, and decanol as cosurfactant at ambient temperature. We demonstrate here the high thermal stability of these microemulsions. Along an experimental path, no phase change could be observed visually within a temperature range between 30 °C and 150 °C. The microemulsions are characterized with quasi-elastic light scattering measurements at ambient temperature and temperature dependent small angle neutron scattering (SANS) experiments between 30 °C and 150 °C. DLS measurements at ambient temperature indicate a swelling of the formed structures with increasing amount of EAN up to a certain threshold. The SANS experiments were performed below this threshold. The data evaluation of such concentrated systems like microemulsions is possible with the “generalized indirect Fourier transformation” method (GIFT). We evaluated the small angle scattering data via the GIFT method, for comparison we also applied the model of Teubner and Strey (TS) which was often used to describe scattering curves of microemulsions. The GIFT method gives good fits throughout the experimental path, while the TS model gives relatively poor fits. Both, light scattering and SANS results are in agreement with the existence of EAN droplets stabilized by surfactant with dodecane as continuous phase along the whole investigated temperature range. Moreover, these results clearly demonstrate the possibility to formulate high temperature stable microemulsions with ionic liquids at ambient pressure.By replacing water in microemulsions by a room-temperature ionic liquid, a thermal stability between 30 °C and 150 °C at ambient pressure can be obtained.

The formation of microemulsions with triglycerides at ambient conditions can be improved by increasing the surfactant−water and surfactant−oil interactions. Therefore, extended surfactants were developed, which contain hydrophilic/lipophilic linkers. They have the ability to stretch further into the oil and water phase and enhance the solubility of oil in water. In this work, the phase behavior of a chosen extended surfactant (C12−14−PO16−EO2−SO4Na, X-AES) in H2O/D2O at high surfactant concentrations (30−100 wt %) and at temperatures between 0 and 90 °C is studied for the first time. The lyotropic liquid crystals formed were determined by optical microscopy, small-angle X-ray scattering (SAXS), and 2H and 23Na NMR, and a detailed phase diagram of the concentrated area is given. The obtained mesophases are a hexagonal phase (H1), at low temperatures and small concentrations, a lamellar phase (Lα) at high temperatures or concentrations, a bicontinuous cubic phase (V2) as well as a reverse hexagonal phase (H2). To our knowledge, this is the first surfactant that forms both H1 and H2 phases without the addition of a third compound. From the 2H NMR quadrupole splittings of D2O, we have examined water binding in the Lα and the H2 phases. There is no marked difference in the bound water between the two phases. Where sufficient water is present, the number of bound water molecules per X-AES is estimated to be ca. 18 with only small changes at different temperatures. Similar results were obtained from the 23Na NMR data, which again showed little difference in the ion binding between the Lα and the H2 phases. The X-ray diffraction data show that X-AES has a much smaller average length in the Lα phase compared to the all-trans length than in the case for conventional surfactants. At very high surfactant concentrations an inverse isotropic solution (L2), containing a small fraction of solid particles, is formed. This isotropic solution is clearly identified and the size of the reversed micelles was determined using 1H NMR measurements. Furthermore, the solid particles within the L2 phase and the neat surfactant were analyzed. The observed results were compared to common conventional surfactants (e.g., sodium dodecyl sulfate, sodium lauryl ether sulfate, and sodium dodecyl-p-benzene sulfonate), and the influence of the hydrophilic/lipophilic linkers on the phase behavior was discussed.

The effect of the anion, namely dicyanamide, hexafluorophosphate, trifluoroacetate, or trifluoromethanesulfonate, on the conductivity (κ) of 1-N-butyl-3-N-methylimidazolium-based room-temperature ionic liquids (RTILs) was studied over the temperature range (248 to 468) K. The uncertainty in κ was estimated to be less than 0.5 %. The conductivity values obtained are well-described by the Vogel−Fulcher−Tammann equation. Additionally, densities (ρ) and the corresponding molar conductivities (Λ) are reported for the temperature range (278 to 338) K. The data for Λ and the associated viscosities (η) were found to fit fractional forms of the Walden relationship.

The formation of microemulsions with triglycerides under ambient conditions has been a challenge for scientists for many decades. For this reason, so-called extended surfactants were developed that contained hydrophilic/lipophilic linkers to stretch further into the oil and water phase, and enhance the solubility of triglycerides in water. Currently, only limited information about the properties of these surfactants and its behavior in water is available. Therefore, in this work, mixtures of a chosen extended surfactant (C12−14−PO16−EO2−SO4Na, X−AES) with H2O/D2O over the whole concentration range were studied by optical microscopy. A schematic phase diagram has been obtained, which shows two isotropic liquid phases at the lowest and highest surfactant concentrations. Furthermore, between the isotropic solutions, four liquid-crystalline phases occur: a hexagonal phase (H1), a lamellar phase (Lα) with a change in birefringence, a bicontinuous cubic phase (V2), and a reverse hexagonal phase (H2). The structure of the micellar solution (L1) was determined by cryo-TEM, dynamic light scattering, and 1H NMR, which gave information about the size, the aggregation number, and the area per molecule of the micelles. Liquid-crystal formation occurs from the micellar solution in two different ways. The first route appeared by increasing the temperature, going from an L1 to an Lα phase. By increasing the surfactant concentration (at low temperatures), a second route showed a transition from L1 to H1. In addition, the effect of sodium chloride on the cloud point of the extended surfactant was examined, indicating that small amounts of NaCl have no influence on the phase behavior. The monolayer behavior of the extended surfactant at the air−water interface was also determined. Despite its water solubility, an isotherm on the water subphase was found, showing slow kinetics of the molecules to go into the bulk. Thus, the determination of the cmc of the extended surfactant using conventional methods was found to be impossible.

In this investigation we present for the first time microemulsions comprising an ionic liquid as surfactant and a room-temperature ionic liquid as polar pseudo-phase. Microemulsions containing the long- chain ionic liquid 1-hexadecyl-3-methyl-imidazolium chloride ([C16mim][Cl]) as surfactant, decanol as cosurfactant, dodecane as continuous phase and room temperature ionic liquids (ethylammonium nitrate (EAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]][BF4]), respectively) as polar microenvironment have been formulated. The phase diagrams of both systems were determined at a constant surfactant/cosurfactant molar ratio. EAN microregions in oil have been confirmed with conductivity measurements. In presence of EAN a model of dynamic percolation could be applied. Dynamic light scattering (DLS) measurements indicated a swelling of the formed nano-structures with increasing amount of EAN, a linear dependence of the hydrodynamic radii on the EAN weight fraction was observed. Both systems exhibited a single broad peak in SAXS and follow a characteristic q−4 dependence of the scattering intensity at large q values. The Teubner-Strey model was successfully used to fit the spectra giving fa, the amphiphilic factor, and the two characteristic length scales of microemulsions, namely the periodicity, d, and the correlation length, ζ. Furthermore, the specific area of the interface could be determined from the Porod limit and the experimental invariant. The amphiphilic factor clearly demonstrated structural differences between the two systems confirming that the nature of the polar ionic liquid plays an important role on the rigidity of the interfacial film. The adaptability of three different models ranging from spherical ionic liquid in oil over repulsive spheres to bicontinuous structures has been checked.

The stability of mixed surfactant solutions of sodium dodecylsulfate (SDS) with cetyltrimethylammonium bromide (CTAB) and with dodecyltrimethylammonium bromide (DTAB) was studied as a function of time. These specific mixtures are shown to have a solubility temperature below that of pure surfactant solutions in the anionic-rich region. The stability of such supersaturated solutions was studied with and without different additives. Surfactant mixtures without additives are shown to destabilize with time varying between 3 and 28 days, depending on the surfactant ratio. Generally, the stability of solution increases by increasing the percentage of the anionic surfactant. The variation of the chain length of the cationic surfactant produces a large effect on the stability of such mixed surfactant systems. The presence of simple electrolytes decreases, while the addition of middle-chain alcohols increases its stability. Bluish solutions corresponding to a vesicular region were observed at ratios close to equimolarity in samples without salt, and in the anionic-rich region upon the addition of middle-chain alcohols. Fluorescence and dynamic light scattering measurements showed that the destabilization of the solutions is not due to the formation of bigger aggregates, but rather due to a shift of the equilibrium between micelles and monomers, leading to the liberation of monomers, which precipitate. The lifetime of vesicles and micelles can therefore be controlled by varying the composition of the surfactant solutions and by additives. Controlling the precipitation phenomena is of importance for a large number of industrial processes, such as oil/solute recovery processes after extractions or chemical reactions.

In a previous study we have shown that the substitution of alkali ions in common fatty acid soaps by choline as a counterion of biological origin increases the solubility of the respective soaps without lowering the biocompatibility. Nevertheless, while choline dodecanoate (ChC12), myristate (ChC14), and palmitate (ChC16) have Krafft points below room temperature or even under 0 °C, choline stearate (ChC18) was not soluble below 40 °C. In the present contribution we show that an excess of choline hydroxide is able to solubilise choline stearate at temperatures as low as 14 °C. Furthermore, we compare our results to those obtained for the sodium and potassium salts of fatty acids, with molar ratios of base to acid higher than 1:1. In order to elucidate the solubilisation process regarding the different ion binding to the carboxylic headgroup, we further investigated the effect of different added chloride salts on the solubility of choline stearate. Our findings indicate that the cation affinity to the carboxylate headgroup follows the trend Na+ > K+ ≫ Ch+. The results are discussed in terms of hydrolysis of the fatty acids in combination with Collins’ concept of “matching water affinities”. As a feasible application of choline base, we present the saponification and simultaneous solubilisation of butter as an example of a hardly soluble triglyceride.

We show in this paper that three ways of characterizing “spontaneous” lateral packing of amphiphiles are equivalent: the spontaneous curvature, the molecular packing parameter, and the refined hydrophilic−lipophilic balance known as HLD (hydrophilic−lipophilic deviation). Recognition of this equivalence, with its underlying hypothesis of incompressible fluid with lowest surface energy, reinforces the single parameter bending energy expression implicit in the classical papers by Ninham and Israelachvili, as well as all the predictive models of solubilization developed as yet.

We show here the influence of n-alcohols (C2OH-C8OH) on the solubility behavior of cationic-anionic surfactant mixtures, so-called “catanionics”. We studied catanionics of different compositions composed of sodium dodecyl sulfate (SDS)/cetyltrimethylammonium bromide (CTAB) and sodium dodecanoate (SDod)/CTAB mixtures. Interestingly, with a molar excess of SDS, long chain n-alcohols (C4OH-C8OH) significantly depress the solubility temperature of the SDS+CTAB catanionic and increase the kinetic stability of the solution. The visual observations of solubility temperatures of catanionics were further confirmed by differential scanning calorimetry (DSC) measurements. For the catanionics a multistep solubilization was observed by DSC, for which the sulfate headgroup is responsible. This was probed by replacing SDS by SDod. A remarkable analogy was found between the influence of the alcohols on the solubility patterns of the catanionic mixtures and on the anesthesia of tadpoles. Possible reasons for this analogy are discussed also in this paper.

With choline as a beneficial counterion of biological origin, long-chain carboxylates become water soluble at room temperature.

A new type of intermediate structure was found in the salt-induced micelle-to-vesicle transition in a catanionic system composed of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in aqueous solution with an excess of anionic surfactant. The appearance of symmetrically shaped hollow structures, which we named blastulae vesicles, is presented.The formation of symmetrically shaped intermediate structures, i.e. blastulae vesicles, in non-equimolar catanionic systems.

31P NMR difference spectra of sodium caseinate sols with and without silicate ions provide direct evidence of interactions between silicate ions and casein serine phosphate groups. The addition of Ca2+ to sodium caseinate solution without silicate ions and, subsequently, the diffusion of atmospheric CO2 to the resulting mixture do not lead to CaCO3 mineralization, whereas comparable experiments in the presence of silicate ions induce the precipitation of hemispherical three-component microstructures composed of silica, casein, and calcium carbonate. Apparently, the silicate−protein interaction plays a role as promoter for calcium carbonate mineralization in aqueous sols. X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) analyses reveal that vaterite is the crystalline phase of the composites. The observed materials are flat on one side and curved outward on the other side. In time, the flat surface cracks to display a star-like shape. Occasionally, in the center of the crack, layer-by-layer sphere-like particles grow, probably due to a secondary nucleation. These spheres are composed of a large number of two-dimensional aragonitic sheets, which are densely packed and form a multilayered structure.

Salt addition to enzymes in buffer always induce the problem of the respective influences of electrostatic interactions and anion specificity on buffer pH and enzyme kinetics. In the present paper the influence of some sodium salts (Na2SO4, NaCl, NaBr and NaNO3) on the pH of a citrate buffer (c = 0.025 M), and on the catalytic constants of horseradish peroxidase (HRP) is studied. First, the pH changes due to the presence of salts in the buffer are examined; second, catalytic constants, KmABTS, VmaxABTS and VmaxABTS/KmABTS, are studied as a function of pH in buffer with and without added salt, at various salt concentrations and different pH values due to the salt additions. With a simple electrostatic model, it is possible to show that the glass electrode yields reasonable pH values even in the presence of fairly high 1 : 1 salt concentrations. For the catalytic efficiency, VmaxABTS/KmABTS, a Hofmeister series is found with opposite deviations from the pure pH effect for salting-in and salting-out ions over a large range of salt concentrations. This usual Hofmeister series is a consequence of three, for the moment inseparable salt concentration and specific ion-induced phenomena: global bulk effects, local active site effects and surface effects.

Lipases from Candida antarctica and Mucor miehei were encapsulated in lecithin water-in-oil (w/o) microemulsion-based organogels (MBG). These gels were formulated with either hydroxypropylmethyl cellulose (HPMC) or gelatin. The esterification of lauric acid and 1-propanol catalyzed by these MBGs was examined in supercritical carbon dioxide (scCO2; 35° C, 110 bar) as solvent for the substrates. The results were compared to those obtained with the reference substrate solvent isooctane. It turned out that the initial rates of this model reaction in scCO2 were higher than those observed in the reference system. Various parameters affecting the biocatalysis such as pressure, alcohol and acid chain length, and gel composition were investigated. Kinetic studies showed that the ester synthesis catalyzed by the immobilized C. antarctica lipase occurs via a Ping Pong Bi Bi mechanism in which only inhibition by excess of alcohol was identified. Values of all kinetic parameters were determined. In addition, experiments on the reusability of these gels in scCO2 were carried out and the state of water within the organogel was examined with the help of differential scanning calorimetry. The present study shows that biocatalysis using MBGs in scCO2 is a promising alternative to other bioconversion processes.

The activity of the enzyme horse radish peroxidase (HRP) is studied in a series of reverse microemulsions composed of dodecane, aqueous buffer, sodium dodecylsufate (SDS) and alcohols of the homologous series 1-butanol to 1-octanol. The HRP catalyzed reaction is the oxidation of a classical water soluble substrate, the 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) by hydrogen peroxide. In parallel electrical conductivity measurements are performed on the same solutions. The structural changes in the microemulsions, as inferred by the conductivity measurements, correlate remarkably well with the changes in the enzymatic activities. In particular it is found that (a) the maximum activity of the enzyme is always related to its optimum hydration and that this hydration can be related to the microemulsion structures, (b) the enzyme inhibition caused by the alcohols in microemulsions is a consequence of both the solubility of the alcohols in the buffer and the rigidity of the interfacial film. Consequently, it can be concluded that enzymatic activity measurements are a valuable tool to study confined systems such as microemulsions and, in particular, the amount of available hydration water. Enzymatic activities can be finely tuned by small changes in microemulsion structures, probably in a predictive way. Reverse SDS microemulsion containing HRP enzyme molecules, ions, alcohols and surfactant molecules.

The present state of affairs, theory and experiment with Hofmeister effects is reviewed.

In this study the influence of temperature and water on the dissolution of cellulose in the solvent system LiCl/DMAc is examined. The phase diagrams performed in the system without water show that higher amounts of cellulose can be dissolved at 5°C, whereas at 25°C smaller concentrations of LiCl in DMAc are sufficient to solubilize some cellulose. Furthermore a relation between the concentration of LiCl and the maximum possible water content is established. The analysis shows that the ratio between water and LiCl in the mixture has to be smaller than 2∶1. Otherwise complete dissolution of cellulose cannot be achieved. The relation between LiCl and anhydroglucose unit is examined as well. The results imply the breakdown of two hydrogen bonds during the solution process because the interaction of two LiCl/DMAc complexes with one glucose unit is required. The viscosity of the system is investigated also but no special behavior is observed.

The conductivity of water/ClPFPE-NH4/scCO2 systems is studied over a wide range of pressures and droplet volume fractions at 35°C and at two constant water-to-surfactant molar ratios, W0=10.3 and 18.7, respectively. The results are interpreted according to three different models for low, intermediate and high concentrations of water and surfactant. Information is obtained about a possible bulk-like behaviour of water, about the approximate size of the micelles, about micellar percolation, and about changes of CO2–water droplet interactions with pressure.

The mean spherical approximation (MSA) approach for electrolyte solutions is combined with a modified non-random two-liquid (NRTL) approach. The resulting model is suitable for a description of the thermodynamic properties of electrolyte-multisolvent systems. The ability of this MSA-NRTL model is investigated by examining activity and osmotic coefficients of binary and ternary electrolyte solutions. Especially for non-aqueous solutions, the model is superior to standard semi-empirical calculations used in the chemical industry.

In this paper we consider clusters in the ternary systems water–benzyl alcohol and ethanol, ethyl lactate or γ-valerolactone as found with the help of dynamic and static light scattering experiments. These ternary mixtures are powerful solvent media and consist only of low-toxic solvents of natural origin. In a recent work we have shown that surfactantless microemulsions are formed in the water–ethanol–n-octanol system. By contrast, in the systems studied here the sizes of the aggregates are too small to be considered as micelles. It can be postulated that the presence of clusters or larger structures as in surfactantless microemulsions is strongly influenced by the most hydrophobic compound. The phenomenon of facilitated hydrotropy is also investigated in the above-mentioned systems. In this context, ethanol is considered as the primary hydrotrope and the more hydrophobic benzyl alcohol as the facilitating secondary hydrotrope. The hydrophobic dye Disperse Red 13 is used as a marker of facilitated hydrotropy. The results suggest that the degree of self-association of eco-solvent has a significant influence on the hydrotropic efficiency of benzyl alcohol.

In the present contribution, the pre-structuring of binary mixtures of hydrotropes and H2O is linked to the solubilisation of poorly water miscible compounds. We have chosen a series of short-chain alcohols as hydrotropes and benzyl alcohol, limonene and a hydrophobic azo-dye (Disperse Red 13) as organic compounds to be dissolved. A very weak pre-structuring is found for ethanol/H2O and 2-propanol/H2O mixtures. Pre-structuring is most developed for binary 1-propanol/H2O and tert-butanol/H2O mixtures and supports the bicontinuity model of alcohol-rich and water-rich domains as already postulated by Anisimov et al. Such a pre-structuring leads to a high solubilisation power for poorly water miscible components (limonene and Disperse Red, characterized by high octanol/water partition coefficients, log(P) values of 4.5 and 4.85), whereas a very weak pre-structuring leads to a high solubilisation power for slightly water miscible components (benzyl alcohol). This difference in solubilisation power can be linked to (i) the formation of mesoscale structures in the cases of ethanol and 2-propanol and (ii) the extension of pre-structures in the cases of 1-propanol and tert-butanol. Three different solubilisation mechanisms could be identified: bulk solubilisation, interface solubilisation and a combination of both. These supramolecular structures in binary and ternary systems were investigated by small-and-wide-angle X-ray and neutron scattering, dynamic light scattering and conductivity measurements (in the presence of small amounts of salt).