The use of novel water-in-oil microemulsions in microemulsion electrokinetic chromatography

We describe the novel use of water-in-oil (W/O) microemulsions to achieve unique separations in microemulsion electrokinetic chromatography (MEEKC). The choice and concentration of the buffer type, surfactant and co-surfactant were all examined and optimized. Separations of a range of neutral and acidic analytes was shown to be markedly different to that obtained by (oil-in-water) O/W MEEKC. Neutral solutes are separated by virtue of their solubility (log P) values in O/W MEEKC with the more water-insoluble solutes migrating last. This separation process does not occur in W/O, as neutral solutes are not separated in order of log P.     

Hydrated electrons in water-in-oil microemulsion: a pulse radiolysis study

Half-life and yield of hydrated electron in water-in-oil microemulsion of cetyl trimethylammonium bromide-water-1-pentanol-cyclohexane system has been studied. Dose distribution calculation suggests scavenging of electrons that are generated in cyclohexane by water pools and hydroxyl radical generation by direct water radiolysis only. Dose distribution, half-life of e_aq⁻, generation and solvation of e_aq⁻ in this system and its comparison with other systems have been discussed.     

Effect of counterions on the activity of lipase in cationic water-in-oil microemulsions

This paper delineates how the different counterions affect the physicochemical properties of the aqueous aggregates and thereby the lipase activities at the interface of cationic water-in-oil microemulsions. To this end, we have synthesized a series of cetyltrimethylammonium-based surfactants, 1-14, having aliphatic, aliphatic with aromatic substitution at the alpha position, and aromatic carboxylate anion as the counterion. The physicochemical characterizations of these aqueous aggregates were done by conductometric, tensiometric, fluorometric techniques to determine counterion binding (beta), critical micelle concentration (cmc), and micropolarity at the microenvironment. It has been found that the activity of lipase mainly increases with hydrophobicity (which is directly proportional to the counterion binding (beta) of the surfactant) of the counterion and reaches a maximum when the beta value is around 0.5. Increase in hydrophobicity as well as beta leads to the attachment of more counterions at interface resulting in enhancement of interfacial area. Consequently, the enzyme may attain flexible secondary conformation at the augmented surface area and also allow larger population of substrates and enzyme molecules at the interface leading to the enhancement in lipase activity. After an optimum value of beta, further increase probably produces a steric crowding at the interface, hindering the smooth occupancy of enzyme and the substrate in this region leading to decrease of enzyme activity, while molecular surface area of the counterion did not show any virtual influence on the lipase activity. Thus, the variation in the counterion structure and hydrophobicity plays a crucial role in modulating the lipase activity.     

Tailoring of horseradish peroxidase activity in cationic water-in-oil microemulsions

Horseradish peroxidase (HRP) in cationic water-in-oil (W/O) microemulsions has always been ignored in reverse micellar enzymology, mainly because cationic surfactants are inhibitors of enzyme peroxidase. In the present study, for the first time, we have successfully introduced the cationic W/O microemulsion as an attractive host for efficient HRP activity. To this notion, much improved activity of HRP was observed in the W/O microemulsion of cetyltrimethylammonium bromide (CTAB) with an increase in n-hexanol concentration and W0 ([water]/[surfactant]), presumably due to the increased interfacial area of the microemulsions. In support of our above observation, six surfactants were synthesized with an increased headgroup size where the methyl groups of CTAB were subsequently replaced by the n-propyl and 2-hydroxyethyl groups, respectively, to prepare mono-, di-, and tripropylated/hydroxyethylated n-hexadecylammonium bromide. The peroxidase activity enhanced with headgroup size and also followed an overall trend similar to that found in the case of CTAB. Possibly, the reduced positive charge density at the augmented interfacial area by means of increase, either in headgroup size, cosurfactant concentration, and/or W0, is not capable of inactivating HRP. Also, the larger space at the interface may facilitate easier solubilization of the enzyme and increase the local concentration of enzyme and substrate, leading to the higher activity of HRP. The best activity was obtained with surfactant N-hexadecyl-N,N,N-tripropylammonium bromide, the highest ever found in any cationic W/O microemulsions, being almost 3 times higher than that found in water. Strikingly, this observed highest activity is comparable with that observed in an anionic bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT)-based system, the best W/O microemulsions used for HRP.     

"Black spots" in a surfactant-rich Belousov-Zhabotinsky reaction dispersed in a water-in-oil microemulsion system

The Belousov-Zhabotinsky (BZ) reaction dispersed in water-in-oil aerosol OT (AOT) microemulsion has been studied at small radius R(d) of water nanodroplets (R(d) (nm) congruent with0.17omega,omega = [H(2)O][AOT] = 9). Stationary spotlike and labyrinthine Turing patterns are found close to the fully oxidized state. These patterns, islands of high concentration of the reduced state of the Ru(bpy)(3) (2+) catalyst, can coexist either with "black" reduction waves or, under other conditions, with the "white" oxidation waves usually observed in the BZ reaction. The experimental observations are analyzed with the aid of a new Oregonator-like model and qualitatively reproduced in computer simulations.     

Synthesis of CeO2@SiO2 core-shell nanoparticles by water-in-oil microemulsion. Preparation of functional thin film

Synthesis of nanoparticles under restricted environment offered by water-in-oil (W/O) microemulsions provides excellent control over particle size and shape, and inter-particle spacing. Such an environment has been involved to synthesize silica nanoparticles with a CeO2 core, so-called CeO2@SiO2. Aqueous fluids made up of ceria nanoparticles with a size close to 5 nm have been used as the water phase component. The starting CeO2 sols and obtained CeO2@SiO2 nanoparticles have been characterized by dynamic light scattering (DLS), X-ray diffraction, scanning and transmission electron microscopy, and specific surface area measurements. The microemulsion process has been characterized by DLS. Preliminary results on CeO2@SiO2 thin films are presented.     

The effects of anionic surfactants on the conductivity of a water-in-oil emulsion in a dielectric hydrophobic capillary

The effect of anionic surfactants on the conductivity of a water-in-crude oil emulsion in a laminar flow inside a dielectric hydrophobic capillary is experimentally investigated in an alternating electric field with strengths ranging from 4 to 10 kV/cm and a frequency of 50 Hz. Conductivity is analyzed as a function of aqueous phase concentration, electric field strength, and surfactant concentration in the dispersed phase.     

Small-angle-neutron-scattering from giant water-in-oil microemulsion droplets. II. Polymer-decorated droplets in a quaternary system

Amphiphilic block copolymers of the type poly(ethylenepropylene)-co-poly(ethyleneoxide) dramatically enhance the solubilisation efficiency of non-ionic surfactants in microemulsions that contain equal volumes of water in oil. Consequently, the length scale of the microstructure of such bicontinuous microemulsions is dramatically increased up to the order of a few 100 nm. In this paper, we show that this so-called efficiency boosting effect can also be applied to water-in-oil microemulsions with droplet microstructure. Such giant water-in-oil microemulsions would provide confined compartments in which chemical reactions of biological macromolecules can be performed on a single molecule level. With this motivation we investigated the phase behavior and the microstructure of oil-rich microemulsions containing D(2)O, n-decane(d22), C(10)E(4) and the amphiphilic block copolymer PEP5-PEO5 [poly(ethylenepropylene)-co-poly(ethyleneoxide), weight per block of 5000 g/ mol]. We found that 15 wt % of water can be solubilised by 5 wt % of surfactant and block copolymer when about 6 wt % of surfactant is replaced by the block copolymer. Small-angle-neutron-scattering experiments were performed to determine the length scales and microstructure topologies of the oil-rich microemulsions. To analyze the scattering data, we derived a novel form factor that also takes into account the scattering contribution of the hydrophobic part of the block copolymer molecules that reside in the surfactant shell. The quantitative analysis of the scattering data with this form factor shows that the radius of the largest droplets amounts up to 30 nm. The novel form factor also yielded qualitative information on the stretching of the polymer chains in dependence on the polymer surface density and the droplet radius.     

Effect of the spacer group of cationic gemini surfactant on microemulsion phase behavior

The phase behavior of a system of n-butanol/n-octane/water/cationic gemini surfactant, alkanediyl-alpha,omega-(dimethydodecyl-ammonium bromide)(12-n-12, n=3,4,6), has been investigated by determination the pseudo-ternary phase diagrams. The results have shown that the spacer group of gemini surfactant has a great effect on the phase behavior. The longer the spacer group for the geminis, the more similar the geminis properties to the traditional ones. The mixing content of surfactant and cosurfactant needed for forming microemulsions increases with the geminis' spacer group. The study has also shown that the shorter spacer group of geminis is favorable for the formation of higher ordered surfactant aggregates such as liquid crystals. Furthermore, the microstructures of each region for the studied systems have been investigated by electrical conductivity measurements, UV-visible absorbance spectra of pyrene probe, and dynamic light scattering (DLS). All the results are in accord with each other. DLS makes use of the sensitivity of DLS to structural changes and as expected the hydrodynamic diameter of the microemulsion droplet changes as the transformation of microemulsion microstructures take place. Moreover, the spherical and network structures of microemulsion were further verified by freezing-etching TEM.     

Cross-diffusion in a water-in-oil microemulsion loaded with malonic acid or ferroin. Taylor dispersion method for four-component systems

We describe an improved Taylor dispersion method for four-component systems, which we apply to measure the main- and cross-diffusion coefficients in an Aerosol OT water-in-oil microemulsion loaded with one of the reactants of the Belousov-Zhabotinsky (BZ) reaction, water(1)/AOT(2)/R(3)/octane(4) system, where R is malonic acid or ferroin. With [H(2)O]/[AOT] = 11.8 and volume droplet fraction phi d = 0.18, when the microemulsion is below the percolation transition, the cross-diffusion coefficients D(13) and D(23) are large and positive ( D(13)/ D(33) congruent with 14, D(23)/ D(33) congruent with 3) for malonic acid and large and negative for ferroin ( D(13)/ D(33) congruent with -112, D(23)/ D(33) congruent with -30) while coefficients D(31) and D(32) are small and negative for malonic acid ( D(31)/ D(33) congruent with -0.01, D(32)/ D(33) congruent with -0.14) and small and positive for ferroin ( D(31)/ D(33) congruent with 5 x 10(-4), D(32)/ D(33) congruent with 8 x 10(-3)). These data represent the first direct determination of cross-diffusion effects in a pattern-forming system and of the full matrix of diffusion coefficients for a four-component system. The results should provide a basis for modeling pattern formation in the BZ-AOT system.     

Cosurfactant effects on the microemulsion polymerization of styrene

The polymerization of styrene in o/w microemulsions stabilized with dodecyltrimethylammonium bromide (DTAB) with or without cosurfactant (n-butanol, n-hexanol or n-octanol) is examined here. The addition of a cosurfactant enhances the one-phase region in the order: n-butanol > n-hexanol > n-octanol. The kinetics of polymerization slows down in the presence of the alcohol. With the alcohol, the molar masses increase, but no particular trend was noticed on particle size of the lattices. However, by changing the surfactant counter-ion to chloride, alcohol effects on the kinetics almost vanish. Possible explanations to these results are given here. To cite this article: J.E. Puig et al., C. R. Chimie 6 (2003).     

The effects of some electrolytes on flocculation with a cationic polyacrylamide

The effects of sodium chloride, sodium sulfate, and alum (aluminum sulfate) on the performance of a cationic polyacrylamide flocculant in a papermaking suspension consisting of bleached (hardwood: softwood, 50∶50) kraft wood-pulp fibres and anatase (TiO₂) were investigated. Sodium chloride and sodium sulfate, 1×10^?5 to 1×10^?2 M, in the presence of polymer, caused negligible changes in the electrophoretic mobility of the TiO₂ and in the first-pass retention of TiO₂ (heteroflocculation of TiO₂ and fibres). Alum at concentrations from 1×10^?5 to 1×10^?4 M at pH 4.0 and 4.5 increased retention with polymer; higher alum concentrations resulted in lower retentions. At pH 4.0 the electrophoretic mobility of the TiO₂ was positive over the entire range of alum concentrations investigated (1×10^?5?3.2×10^?3 M) whereas at pH 4.5 the mobility was negative at 1×10^?5 M alum and charge reversal was observed at about 4×10^?5 M alum. The intrinsic viscosity of the cationic polyacrylamide was decreased by the addition of alum, sodium chloride or sodium sulfate. The effect of alum on the polymer conformation appeared to be that of the non-specific interaction of sulfate ions with a cationic polyelectrolyte. Retention results are discussed in terms of the colloidal stability of TiO₂, the adsorption of polyacrylamide on TiO₂ and the conformation of adsorbed polymer.     

The effects of cationic and zwitterionic micelles on the keto-enol interconversion of 2-phenylacetylfuran and 2-phenylacetylthiophene

The presence of micelles from cationic and zwitterionic surfactants increases the apparent acidity of either the keto and the enol forms of 2-phenylacetylfuran (2PAF) and 2-phenylacetylthiophene (2PAT). This effect can be attributed to the affinity of the surfactant micelles for the enolate of the two substrates. Although the equilibrium constants for keto-enol tautomerism of 2PAF and 2PAT, K_T=[enol]/[ketone]=pK_a^KH−pK_a^EH, do not change much, the presence of micelles provides an efficient method for producing appreciable quantities of the enolates under mild experimental conditions and in aqueous solutions. The obtained rate-profiles for the ketonisation reactions and the consistency of the kinetic rate constants over a wide range of ‘pH’ in several overlapping buffers indicate that the pH of the aqueous pseudophase (but not that at the micellar surface) can be controlled by buffers. Moreover, the increase of the acidity and the decrease of the ‘water’ rate of ketonisation of the enols of 2PAF and 2PAT upon addition of surfactants allow the uncovery of a metal ion catalysed pathway that cannot be observed in absence of surfactants.     

Electrokinetic capillary chromatography in a polar continuous-phase water-in-oil microemulsion constituted by water, sodium dodecyl sulfate, and n-pentanol

A water-in-oil (w/o) microemulsion (ME) constituted by 15% Tris buffer, pH 8.4, in water and 85% sodium dodecyl sulfate (SDS)/n-pentanol 1:4 mixture, capable of dissolving up to 30% vegetable oils and lard, was used as background electrolyte in reverse microemulsion electrokinetic capillary chromatography (RMEEKC). Owing to the free SDS ions in the continuous phase and some degree of percolation, the ME showed a high conductivity (0.65 mS. cm(-1) at 25 degrees C) and sustained a very stable capillary current. Previous rinsing of the capillary with a quaternary ammonium salt for electroosmotic flow (EOF) reduction, a series of nonionic and anionic solutes dissolved either in the ME or in fat samples diluted with the ME (1:4 ratio), were injected. Using -20 kV, fair separations of the solutes in the migration time order singly charged anions < nonionic solutes < doubly charged anions approximately pyromellitate were obtained, salicylate (I) showing by far the shortest migration time, and phthalate (II) and pyromellitate the longest. Separation was attributed to partition between the aqueous droplets, where pyromellitate and II were assumed to be trapped, and the n-pentanol continuous phase, where the mobilitites could be also modified by association of the solutes with SDS ions. Adequate EOF markers were not found, thus the relative mobility of any solute with respect to the mobility of the droplets, mu(r), was expressed as a fraction of the mobility of I with respect to that of the droplets, being mu(r) = (t(II) - t(R)) t(I) / [(t(II) - t(I)) t(R)], where t(R), t(I), and t(II) are the migration times of the solutes I and II, respectively. The application of RMEEKC to the analysis of both hydrophilic and hydrophobic samples, including edible fats, was demonstrated.     

Poly(n-hexyl methacrylate) polymerization in three-component microemulsion stabilized by a cationic surfactant

The polymerization of n-hexyl methacrylate (n-HMA) in three-component microemulsion stabilized with dodecyltrimethylammonium bromide (DTAB) is reported as a function of monomer and initiator concentrations and temperature. The obtained latices were bluish, transparent, and translucent. Particle sizes and molar masses were on the order of 20 nm and 3 x 10(6) g/mol, respectively. In all cases, high reaction rates and final conversions of 98% were obtained. Polymerization temperature has a strong effect on reaction rate and conversion.     

阴/阳离子表面活性剂复配体系的中相微乳液研究

阴离子表面活性剂双-2-乙基己基磺化琥珀酸钠(简称AOT), 和阳离子表面活性剂十六烷基三甲基溴代铵(简称CTAB), 在有醇、正辛烷、盐水存在的情况下,能形成多相微乳液。本文系统地研究了阴/阳离子表面活性剂配比、醇的种类、醇的浓度对该体系的中相微乳液的形成及特性的影响, 得到了中相微乳液的特性参数(最佳含盐量S^*, 最佳中相微乳液体积V^*, 界面张力r~E、盐宽△S等)。这些性质对与阴/阳离子表面活性剂复配体系, 三次采油及日用化工上的应用开发具有重要意义。最后还开展单独阴离子表面活性剂体系和阴/阳离子表面活性剂复配体系进行了比较, 得到一些有价值规律, 并从理论上进行了探讨。     

Synergistic effects of CuO and Au nanodomains on Cu2O cubes for improving photocatalytic activity and stability

Cu₂O is a promising photocatalyst, but it suffers from poor photocatalytic activity and stability, especially for Cu₂O cubes. Herein, we report the deposition of CuO and Au nanodomains on Cu₂O cubes to form dual surface heterostructures (HCs) to improve photocatalytic activity and stability. The apparent quantum efficiency of Au/CuO/Cu₂O HCs was ca. 123 times that of pristine Cu₂O. In addition, the Au/CuO/Cu₂O HCs maintained nearly 80% of its original activity after eight cycles in contrast to five cycles for the Au/Cu₂O material. Therefore, CuO and Au domains greatly improved the photocatalytic activity and stability of the Cu₂O cubes due to the synergistic effect of the HCs.     

The phase diagrams of the CdCl2-NaCl-H2O system at 298 K

Solid-liquid equilibrium of ternary system Cd²⁺, Na⁺//Cl^?-H₂O at 298 K were studied by an isothermal solution saturation method. Experimental results indicate that there are three univariant curves AE, EF, and FB, two invariant points E, F, and three crystallization fields in the ternary system. The ternary system has one double salt Na₂CdCl₄ · 3H₂O. The crystallization zones of equilibrium solid phases are CdCl₂ · H₂O (AEG field), Na₂CdCl₄ · 3H₂O (EFM field), and NaCl (FBN field), respectively. The composition of the invariant point E is CdCl₂ · H₂O and Na₂CdCl₄ · 3H₂O of which content was 52.70 and 4.11%, respectively. The composition of the invariant point F is Na₂CdCl₄ · 3H₂O and NaCl of which content was 27.92 and 14.95%, respectively. The density of solution in the ternary system show regular changes along with the increased cadmium concentration. The results indicated that CdCl₂ · H₂O possessed the highest solubility among those three salts, which means a strong transfer of Cd ion and a high pollution risk of soil environment. And the solubility of NaCl would be restrained as the three salts existing together.     

Head-group size or hydrophilicity of surfactants: the major regulator of lipase activity in cationic water-in-oil microemulsions

To determine the crucial role of surfactant head-group size in micellar enzymology, the activity of Chromobacterium Viscosum (CV) lipase was estimated in cationic water-in-oil (w/o) microemulsions of three different series of surfactants with varied head-group size and hydrophilicity. The different series were prepared by subsequent replacement of three methyl groups of cetyltrimethylammonium bromide (CTAB) with hydroxyethyl (1-3, series I), methoxyethyl (4-6, series II), and n-propyl (7-9, series III) groups. The hydrophilicity at the polar head was gradually reduced from series I to series III. Interestingly, the lipase activity was found to be markedly higher for series II surfactants relative to their more hydrophilic analogues in series I. Moreover, the activity remained almost comparable for complementary analogues of both series I and III, though the hydrophilicity was drastically different. Noticeably, the head-group area per surfactant is almost similar for comparable surfactants of both series I and III, but distinctly higher in case of series II surfactants. Thus the lipase activity was largely regulated by the surfactant head-group size, which plays the dominant role over the hydrophilicity. The increase in head-group size presumably allows the enzyme to attain a flexible conformation as well as increase in the local concentration of enzyme and substrate, leading to the higher efficiency of lipase. The lipase showed its best activity in the microemulsion of 6 probably because of its highest head-group size. Furthermore, the observed activity in 6 is 2-3-fold and 8-fold higher than sodium bis(2-ethyl-1-hexyl)sulfosuccinate (AOT) and CTAB-based microemulsions, respectively, and in fact highest ever in any w/o microemulsions.