Excess Water Phases Research Articles

Hexosomes formed from glycerate surfactants—Formulation as a colloidal carrier for irinotecan

A new class of amphiphiles with a glycerate headgroup, recently shown to form reverse hexagonal phase in excess water, have been dispersed to form Hexosome dispersions comprising sub-200 nm particles retaining the internal nanostructure of the parent H II phase. The application of these novel materials to the development of a new injectable formulation of irinotecan was investigated. The formulation of irinotecan with a small percentage of oleic acid in oleyl glycerate permitted a clinically relevant dose of irinotecan to be dissolved in the glycerate surfactant and dispersed in aqueous medium to form an injectable particle-based dose form of irinotecan. Importantly, incorporation of irinotecan into Hexosomes at neutral pH did not result in conversion from the active lactone to the inactive carboxylate form on storage, and is hence a promising alternative to the current low pH formulation of irinotecan required to inhibit this conversion. Although release of irinotecan from the Hexosomes was shown to be virtually instantaneous from the Hexosomes on substantial dilution, the retention of the drug in lactone form at neutral pH demonstrates a potential application of these novel nanostructured particles in injectable drug delivery.

Lyotropic liquid crystalline phases formed from glycerate surfactants as sustained release drug delivery systems

A new class of surfactants with glycerate headgroups, that form viscous lyotropic liquid crystalline phases in excess water, have been investigated for their potential to provide sustained release matrices for depot drug delivery. Oleyl glycerate and phytanyl glycerate were used as representative surfactants of this new class, and their behaviour compared with that of glyceryl monooleate (GMO). The surfactants were found to form reverse hexagonal phase (H II) in excess water, and the matrices were loaded with a series of model hydrophobic and hydrophilic drugs, (paclitaxel, irinotecan, glucose, histidine and octreotide), and the release kinetics determined. In all cases, the release behaviour obeyed Higuchi kinetics, with linear drug release versus square root of time. The H II phases released model drugs slower than the GMO cubic phase matrix. The oleyl glycerate matrix was found to consistently release drug faster than the phytanyl glycerate matrix, despite both matrices being based on H II phase. To further demonstrate the potential utility of these materials as drug depot delivery systems, an injectable precursor formulation for octreotide was also prepared and demonstrated to provide controlled release for the peptide. The stability of the H II phase to likely in vivo breakdown products was also assessed.

Synthesis, characterization and evaluation of amphiphilic star copolymeric emulsifiers based on methoxy hexa(ethylene glycol) methacrylate and benzyl methacrylate

Amphiphilic star copolymers were synthesized by sequential monomer and cross-linker additions using group transfer polymerization (GTP). Benzyl methacrylate (BzMA) and methoxy hexa(ethylene glycol) methacrylate (HEGMA) served as the hydrophobic and hydrophilic monomers, respectively, whereas the also hydrophobic ethylene glycol dimethacrylate (EGDMA) was used as the cross-linker. In total, twelve star copolymers were prepared, covering three different overall hydrophobic compositions, 39, 53 and 70% w/w, and four different architectures, AB star-block, BA star-block, heteroarm star and random star. The theoretical molecular weight of each arm was kept constant at 5000 g mol −1. The molecular weights and molecular weight distributions of the linear precursors and of all the star copolymers were characterized by gel permeation chromatography (GPC) in tetrahydrofuran (THF), while their compositions were confirmed by proton nuclear magnetic resonance ( 1H NMR) spectroscopy. Moreover, all the star copolymers were characterized by static light scattering (SLS) in THF to determine the absolute weight-average molecular weight, M w, and the weight-average number of arms. After polymer characterization, xylene–water and diazinon (pesticide)–water emulsions were prepared using these star copolymers as stabilizers at 1% w/w copolymer concentration and at different overall organic phase/water ratios. The most important factor in determining the emulsion type was the star copolymer composition in hydrophobic units. The four most hydrophilic star copolymers (39% w/w hydrophobic composition) always formed o/w emulsions, while the four most hydrophobic star copolymers (70% w/w hydrophobic composition) always formed w/o emulsions. The type of the emulsion in the case of the star copolymers with the more balanced composition, 53% w/w hydrophobic units, also depended on the emulsion content in the organic solvent, similar to particulate-stabilized emulsions. Considering that the best o/w emulsifier is that star copolymer which can emulsify the largest quantity of organic phase in water resulting in low viscosity, o/w emulsions without excess oil or water phase, it appeared that the most hydrophilic random copolymer star is the optimal emulsifier. Moreover, this star copolymer presented the smallest droplet size in its emulsions. It is also noteworthy that the resulting emulsions almost never had high viscosity, a feature attributable to the compact nature of star polymers.

Clean-up of Oily Wastewater by Froth Flotation: Effect of Microemulsion Formation III: Use of Anionic/Nonionic Surfactant Mixtures and Effect of Relative Volumes of Dissimilar Phases

ABSTRACTFroth flotation, a surfactant-based separation process, can be used to remove emulsified oil from water. In previous work, the maximum removal of ortho-dichlorobenzene from water using froth flotation was achieved when a Winsor Type III microemulsion was formed. However, the exact relationship between the equilibrium microemulsion characteristics and the froth flotation operation is still not clear. In the Winsor Type III microemulsion, three phases are present: an excess water phase containing little surfactant or oil, an excess oil phase containing little water or surfactant, and a middle phase containing almost all of the surfactant and large volume fractions of both oil and water (even equal volumes of oil and water). The Winsor Type III microemulsion also corresponds to a minimum in interfacial tension between liquid phases at equilibrium. In order to elucidate which aspect of the microemulsion is responsible for flotation of oil, flotation experiments were performed with three different combinations of phases: water and middle phases (w-m); water and oil phases (w-o); and water, middle, and oil phases (w-m-o). Since it was deduced that most oil being recovered when in the Winsor Type III microemulsion region is in the excess oil phase (not the middle phase), the reason why the Winsor Type III microemulsion results in excellent oil removal in flotation operation is probably the ultralow interfacial tensions present, and the formation of the middle phase is incidental.

Effect of de Novo Designed Peptides Interacting with the Lipid-Membrane Interface on the Stability of the Cubic Phases of the Monoolein Membrane

Elucidation of the mechanism of transitions between the cubic phase and the liquid-crystalline (Lα) phase and between different cubic phases is essential for an understanding of the dynamics of biomembranes and the development of new crystallization techniques. Recently, we found that electrostatic interactions due to the surface charges of lipid membranes induce transitions between the cubic phase and Lα phase and between different cubic phases (Aota-Nakano, Y.; Li, S. J.; Yamazaki, M. Biochim. Biophys. Acta 1999, 1461, 96; Li, S. J.; Yamashita, Y.; Yamazaki, M. Biophys. J. 2001, 81, 983). In the present study, we used small-angle X-ray scattering to investigate the effects of a de novo designed peptide (WLFLLKKK, peptide-1, which has positive charges and a site that is partitioned in electrically neutral lipid-membrane interfaces) on the stability of the cubic phase of the monoolein (MO) membrane. As the peptide-1 concentration increased, a transition from the Q224 to the Q229 phase in the MO membrane at a 30 wt % lipid concentration occurred at R = 0.0090 (R is the molar ratio of peptide to MO), and at R ≥ 0.040, the MO/peptide-1 membranes were in the Lα phase. As the NaCl concentration in the bulk phase increased, for the MO/peptide-1 membranes in the Q229 phase in excess water, a Q229 phase to Q224 phase transition occurred at low concentrations of NaCl. Similarly, for the MO/peptide-1 membranes (R = 0.080) in excess water, at low NaCl concentrations, they were in the Lα phase, but at ≥0.40 M NaCl, they were in the Q224 phase. These results indicate that peptide-1 was partitioned in the membrane interface of the MO membrane, electrostatic interactions due to peptide-1 in the membrane interface make the Q229 phase more stable than the Q224 phase, and with larger electrostatic interactions, the Lα phase is more stable than these cubic phases. Increased peptide-1 concentration reduced the absolute value of spontaneous curvature of the MO/peptide-1 monolayer membrane. On the basis of these results, we discuss the mechanism of the effect of peptide-1 on the phase stability of the MO membranes.

Thermal behaviour of cubic phases rich in 1-monooleoyl-rac-glycerol in the ternary system. 1-monooleoyl-rac-glycerol/n-octyl-beta-D-glucoside/water.

Using synchrotron X-ray diffraction the thermal behaviour was studied of the cubic phases in the 1-monooleoyl-rac-glycerol (MO)/n-octyl-beta-d-glucopyranoside (OG)/2H2O system with 58 or 45 wt % MO concentration and varying OG/2H2O contents. These MO contents correspond to a Pn3m cubic single-phase or a Pn3m cubic phase in excess water on the binary MO/water axis of the ternary phase diagram. The cubic liquid crystalline phases are stable with small fractions of OG, while higher OG concentrations trigger a cubic-to-lamellar phase transition. Moreover, with increasing OG concentration the initial Pn3m structure is completely converted to an Ia3d structure prior to the Lalpha phase being formed. Upon heating this effect is reversed, resulting in an Ia3d-to-Pn3m phase transition. For some samples additional peaks were observed in the diffractograms upon heating, resulting from the metastability notoriously shown by bicontinuous cubic phases. This judgement is supported by the fact that upon cooling these peaks were absent. Remarkably, both the Ia3d and the Pn3m cubic structures could be in equilibrium with excess water in this ternary system. A comparison is made with previous results on n-dodecyl-beta-d-maltoside (DM), showing that cubic phases with OG have higher thermal and compositional stability than with DM.

The Relation Between Phase Behavior and Formation of Narrow Size Distribution W/O Emulsions

ABSTRACT Water-in-oil (W/O) emulsions of the water/C12E5/isooctanol/isooctane system have been prepared at 25° C. Phase behavior studies of the system with constant (2.5 and 6 wt.%) isooctanol concentration showed that the surfactant becomes more lipophilic with the increase in the alkanol concentration. Emulsification was carried out using four low-energy emulsification methods using the slow addition of one or various components to the rest of them, with gentle agitation. Emulsions with low-polydis-persity were obtained when the emulsification process started with a single lamellar liquid crystalline phase. If in addition to a lamellar liquid crystalline phase, other phases, such as excess water phase, were initially present, emulsions with intermediate polydispersity were produced. When a lamellar liquid crystalline phase was not involved and the spontaneous natural curvature of the surfactant was not changed during emulsification, highly polydisperse emulsions were obtained.

Highly Concentrated Emulsions Based on the Reverse Micellar Cubic Phase.

The phase behavior of commercial lipophilic poly(oxyethylene)-poly(dimethylsiloxane) copolymer (PEOS), Me3SiO(Si(Me)2O)x-(SiMe(C3H6(OCH2CH2)nOH)O)ySiMe3 (x= 50-100, y= 1-5, n= 7-15, HLB value= 5), in water and water + oil systems was investigated by phase study and small angle X-ray scattering (SAXS). The lipophilic PEOS forms a reverse hexagonal liquid crystal (H2) phase in water, and the H2 + excess water phase exists at a low concentration. The H2 phase changes to a reverse micellar cubic (I2) phase with addition of oil. The I2 structure is identified to the Fd3m space group by SAXS. An excess water phase is also coexisted with I2 phase beyond the solubilization limit of water in the I2 phase and a large amount of water can be dispersed in I2 phase as emulsion droplets. As a result, highly viscous or stable gel-emulsion over 96% water as the internal phase could be formulated.

Formation of Microemulsions in Aqueous NaCl/Sodium (3-Dodecanoyloxy-2-Hydroxy-Propyl) Succinate/Glycerol Mono(2-Ethylhexyl) Ether/Oil Systems

ABSTRACT Sodium (3-dodecanoyloxy-2-hydroxy-propyl) succinate (SLGMS) forms microemulsions by mixing with cosurfactants such as glycerol mono(2-ethylhexyl) ether (MEH), although the combination with ordinary cosurfactant such as hexanol does not form a microemulsion of large solubilization. The middle-phase microemulsion coexists with excess water and oil (octane) phases at an optimum-mixing fraction of SLGMS and MEH in the presence of salt. The monomeric solubility of MEH in oil is low and MEH is mainly combined with SLGMS at an oil—water interface inside microemulsions. With decreasing salinity, the three-phase body shrinks and eventually disappears. The three-phase body may be terminated at a tricritical point, at which three phases simultaneously coexist. The effect of type of oil on the solubilization capacity of the microemulsions is also discussed.

The Influence of Lysolipids on the Spontaneous Curvature and Bending Elasticity of Phospholipid Membranes

The effects of lysolipids on phospholipid layer curvature and bending elasticity were examined using x-ray diffraction and the osmotic stress method. Lysolipids with two different head groups, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), and differing hydrocarbon chains were mixed with the hexagonal-forming lipid, dioleoylphosphatidylethanolamine (DOPE). With up to 30 mole% lysolipid in DOPE, the mixture maintains the inverted hexagonal (H II) phase in excess water, where increasing levels of lysolipid result in a systematic increase in the H II lattice dimension. Analysis of the structural changes imposed by lysolipids show that, opposite to DOPE itself, which has an spontaneous radius of curvature ( R 0) of −30 Å, PC lysolipids add high positive curvature, with R 0 = +38 to +60 Å, depending on chain length. LysoPEs, in contrast, add very small curvatures. When both polar group and hydrocarbon chains of the added lysolipid mismatch those of DOPE, the structural effects are qualitatively different from otherwise. Such mismatched lysolipids “reshape” the effective combination molecule into a longer and more cylindrical configuration compared to those lysolipids with either matching polar group or hydrocarbon chain.

Bending Frustration of Lipid−Water Mesophases Based on Cubic Minimal Surfaces1

Inverse bicontinuous cubic phases are ubiquitous in lipid-water mixtures and consist of a lipid bilayer forming a cubic minimal surface, thereby dividing space into two cubic networks of water channels. For small hydrocarbon chain lengths, the monolayers can be modeled as parallel surfaces to a minimal midsurface. The bending energy of the cubic phases is determined by the distribution of Gaussian curvature over the minimal midsurfaces which we calculate for seven different structures (G, D, P, I-WP, C(P), S and F-RD). We show that the free-energy densities of the structures G, D and P are considerably lower than those of the other investigated structures due to their narrow distribution of Gaussian curvature. The Bonnet transformation between G, D, and P implies that these phases coexist along a triple line, which also includes an excess water phase. Our model includes thermal membrane undulations. Our qualitative predictions remain unchanged when higher order terms in the curvature energy are included. Calculated phase diagrams agree well with the experimental results for 2:1 lauric acid/dilauroyl phosphatidylcholine and water.

Low concentration of DMSO stabilizes the bilayer gel phase rather than the interdigitated gel phase in dihexadecylphosphatidylcholine membrane

We have investigated effects of dimethylsulfoxide (DMSO) on the phase stability of multilamellar vesicles of the ether-linked 1,2-dihexadecyl- sn-glycero-3-phosphatidylcholine (DHPC-MLV), which is known to be in the interdigitated gel (L βI) phase in excess water at 20°C. The results of X-ray diffraction experiments indicate that the DHPC membrane was in the L β′ phase at X≥0.12 ( X=mole fraction of DMSO in DMSO/water mixture). The result of differential scanning calorimetry indicate that the gel to liquid-crystalline phase transition temperature increased, but the L βI to P β′ phase transition temperature decreased with an increase in DMSO concentration. These results show that DMSO stabilizes the bilayer gel phase rather than the L βI phase at its low concentration. The solubility of phosphorylcholine, which is the same structure as the headgroup of DHPC, decreased with an increase in DMSO concentration, indicating that the interaction free energy of the hydrophilic segments of the membrane with solvents increases with an increase in DMSO concentration. On the basis of the thermodynamic analysis, the mechanism of the stabilization of the bilayer gel phase of DHPC-MLV by DMSO is discussed. The decrease in the repulsive interaction between the headgroups of the phospholipid induced by the low concentrations of DMSO in water plays an important role in this stabilization.

O-Alkyl dioleoylphosphatidylcholinium compounds: the effect of varying alkyl chain length on their physical properties and in vitro DNA transfection activity.

1,2-Dioleoyl-sn-3-ethylphosphocholine (EDOPC) has been previously shown be a highly effective DNA transfection reagent in vitro. To assess the effect of alkyl chain length on transfection efficiency, the O-methyl, O-propyl, O-hexyl, O-decyl, and O-octadecyl derivatives have been prepared from dioleoylphosphatidylcholine using the corresponding alkyl trifluoromethylsulfonate. The methyl, ethyl, and propyl derivatives formed liposomes which were very large and unilamellar. The ethyl and propyl derivatives were equally efficient at mediating transfection (even in the presence of serum) of BHK cells, but the chemically labile methyl derivative was a much weaker transfection agent. The O-decyl and O-octadecyl compounds, which assume the inverted hexagonal phase in excess water (as determined by X-ray diffraction), were almost inactive after manual agitation in both water and in saline; however, after sonication, these compounds exhibited good transfection activity. The O-hexyl derivative displayed novel behavior, assuming the lamellar phase at low and a cubic phase at high ionic strength. All compounds, whether lamellar or not, formed lamellar structures when complexed with DNA. In water, where the hexyl compound dispersed well, sonication diminished transfection activity, whereas at physiological ionic strength, which led to poor manual dispersion, sonication was essential for good transfection. These results emphasize the importance of optimal dispersion of a cationic lipid: too little, and interaction with DNA is handicapped, too much, and the resultant particle transfects poorly. Lipid dispersibility is thus an important variable in assessing lipid transfection agents, and caution is advised in attributing too much significance to chemical structure until interaction with DNA has been optimized.

Dynamics of Interface Instabilities in Nonionic Lamellar Phases

The swelling and dissolution dynamics of a soluble (small miscibility gap) and an insoluble (large miscibility gap) lamellar phase (Lα) of nonionic surfactant in water have been qualitatively and quantitatively investigated. In the case of an insoluble Lα, we observe classical myelinic instabilities at the interface. (These are absent in the soluble case.) By doping the excess water phase with colloidal tracer particles, we show that water enters at the roots, rather than through the walls, of the myelins. We estimate the mean water flux and find it to be consistent with this picture. We also study the interface between a lamellar phase and pure surfactant liquid (L2). At this interface we observe a new dynamic instability in which “onion jets” appear to be ejected from Lα into L2, where they disappear. Closer investigation shows these onions to be focal conic II defects residing near the surface of a wedge of homeotropic Lα, which is in contact with the wall of the sample holder. The defects move much faster than the wedge. We offer some theoretical discussion of how these observations shed light on the mechanism of interfacial instabilities, particularly myelin formation, in surfactant dissolution.

Phase Behavior of Water/Sucrose Dodecanoate/Perfume Systems

The effects of added perfume compounds (d-limonene (LN) and linalool (LL)) on the phase behavior of sucrose dodecanoate in water were investigated. Following the addition of hydrocarbon-type perfume, LN, to L-1695 comprised mainly of sucrose monododecanoate, hexagonal-discontinuous cubic phase transition was noted to occur and an excess oil (LN) phase separated from discontinuous cubic phase beyond the solubilization limit. Surfactant layer curvature continued to change from positive to negative with the addition of amphiphilic polar perfume, LL, to the L-1695-water system. LL is thus shown miscible with L-1695 and an excess water phase separates from reversed micellar solution phase in the dilute region. The difference in phase behavior derived from the solubilization mechanism of each perfume in selforganizing structures and this mechanism may be clarified by small-angle X-ray scattering (SAXS) measurement. LN is thus solubilized mainly deep within surfactant aggregates, whereas LL penetrates the surfactant palisade layer. Based on a comparison of the results for lipophilic sucrose multi-dodecanoates (L-595) and L-1695, considerable penetration of perfume large into a self-organized structure of a lipophilic surfactant may be concluded to occur even if the perfume in question is solubilized.

Low pH Induces an Interdigitated Gel to Bilayer Gel Phase Transition in Dihexadecylphosphatidylcholine Membrane

We have investigated the influence of pH on the structures and phase behaviors of multilamellar vesicles of the ether-linked dihexadecylphosphatidylcholine (DHPC-MLV). This phospholipid is known to be in the interdigitated gel (L β I) phase in excess water at 20°C at neutral pH. The results of X-ray diffraction experiments indicate that a phase transition from L β I phase to the bilayer gel phase occurred in DHPC-MLV in 0.5 M KCl around pH 3.9 with a decrease in pH, and that at low pH values, less than pH 2.2, DHPC-MLVs were in L β′ phase. The results of fluorescence and light scattering method indicate that the gel to liquid-crystalline phase transition temperature ( T m) of DHPC-MLV increased with a decrease in pH. On the basis of a thermodynamic analysis, we conclude that the main mechanism of the low-pH induced L β I to bilayer gel phase transition in DHPC-MLV and the increase in its T m is connected with the decrease in the repulsive interaction between the headgroups of these phospholipids. As pH decreases, the phosphate groups of the headgroups begin to be protonated, and as a result, the apparent positive surface charges appear. However, surface dipoles decrease and the interaction free energy of the hydrophilic segments with water increases. The latter effect dominates the pure electrostatic repulsion between the charged headgroups, and thereby, the total repulsive interaction in the interface decreases.

Phase Behavior and Salt Partitioning in Two- and Three-Phase Anionic Surfactant Microemulsion Systems: Part II, Partitioning of Salt

The partitioning of salt (chlorides of Na, K, Ca, Mg) between an excess water phase and a microemulsion phase were studied for two five-component anionic microemulsion systems at 20°C. Three-phase microemulsions contain two phases that include inorganic salts, that is, excess brine and the microemulsion phase. The aim of the investigation was to describe the partitioning of Na+, K+, Ca++, Mg++, and Cl− between water in the microemulsion phase and water in the excess water phase. Both surfactant systems were anionic with Na+ as counterion. It was found that Na+ and Cl− partitioned more strongly toward the excess water phase while K+, Ca++, and Mg++ show more preference for the microemulsion phase. The validity of considering (NaCl + water) as pseudo-component brine has been examined. Including a chloride depleted region, caused by electrostatic repulsion from the negatively charged surfactant layer, in the surfactant pseudo-component, is found to describe the NaCl partitioning. When discussing partitioning of K+, Ca++, and Mg++ between the microemulsion phase and the excess water phase in these systems, it is important to consider that there are two cations in the solutions, Na+ as counterion for the surfactant and either K+, Ca++, or Mg++ for the electrolyte. The preferential partitioning toward the microemulsion phase for K+, Ca++, and Mg++ seems to be caused by an ion exchange of Na+ in the surfactant layer.

Equilibrium phase behaviour and emulsion stability in silicone oil+water+AOT mixtures

We have investigated the equilibrium phase behaviour and the emulsion stability in mixtures of aqueous NaCl, anionic surfactant AOT and polydimethylsiloxane (PDMS) oils. For hexamethyldisiloxane, the transition of Winsor systems from I–III–II is effected by increasing the electrolyte concentration. Aggregated surfactant transfers from water to oil via a third phase, the composition of which resembles that of the L3 phase. The PDMS/water interfacial tension passes through a low minimum around conditions of three phase formation. In single phase microemulsions, we have determined the uptake of silicone oil into aqueous surfactant solutions and the solubilisation of water into surfactant solutions in oil, both as a function of salt concentration and temperature. Maximum uptake of oil (water) occurs at a salt concentration equal to that at the Winsor I/III (III/II) boundary. The partitioning of salt between dispersed and excess water phases is in favour of the latter for small drops but becomes more nearly equal for larger drops. Macroemulsions prepared from the equilibrium coexisting phases invert from oil-in-water (o/w) to water-in-oil (w/o) at intermediate salt concentrations. For o/w emulsions, the stability to both creaming and coalescence falls markedly approaching the Winsor I/III transition. An argument based on the influence of the interfacial tension on the deformability of the drops is consistent with the findings. For w/o emulsions, the stability to both sedimentation and coalescence increases close to the boundary where three phases form. Three phase emulsions are extremely unstable. In systems containing PDMS oils of higher molecular weight, inversion of equilibrium systems from water to oil continuous occurs at higher salt concentrations. Both the width of the three phase region and the minimum interfacial tension increase with oil molecular weight. Emulsion inversion takes place over virtually the same range of salt concentration as that for which three phases are formed.

FORMATION OF MICROEMULSION AND LIQUID CRYSTAL IN BIOCOMPATIBLE SUCROSE ALKANOATE SYSTEMS

Sucrose monoalkanoates (SE) form microemulsions by mixing with lipophilic cosurfactants such as the middle-chain alcohols, or sucrose polyalkanoates. In the case that sucrose monododecanoate is combined with hexanol, a middle-phase microemulsion is produced and coexists with excess water and oil phases at optimum mixing fraction of SE and hexanol. The bicontinuous structure of the microemulsion was confirmed by means of self-diffusion coefficient on NMR measurement. A larger solubilization capability of oil is attained to a mixed mono- and poly-dodecanoate system than that of the middle-chain alcohol. It is probably due to the location of most of surfactant molecules at the micro-interface inside the microemulsion. In this system, instead of forming a three-phase region, a lamellar liquid crystal intrudes into multi-phase region since the surfactant layer is rather rigid as which was supported by the results of SAXS and ESR measurements. Further addition of a short-chain alcohol such as propanol to the mixed system leads to the flexible interface, as a consequence three-phase microemulsion with large solubilization is formed.

A ring-opening reaction performed in microemulsions

Different types of microemulsion formulations have been explored as reaction medium for the ring opening of a lipophilic epoxide, 1,2-epoxyoctane, with sodium hydrogen sulfite. In two-phase systems without surfactant or phase-transfer reagent the reaction is very sluggish. Microemulsions based on alcohol ethoxylate proved to be efficient reaction media, giving almost complete conversion of epoxide to hydroxysulfonate in 2 h at 75°C. A microemulsion based on the anionic surfactant sodium dodecylsulfate gave a slower rate of epoxide consumption. In addition, the dodecylsulfate anion took an active part in the reaction, competing with the hydrogen sulfite ion as ring-opening reagent. Reactions were also carried out in Winsor III systems based on alcohol ethoxylate. Compared with the one-phase system, reaction in the Winsor III system was slow, probably due to unfavourable partitioning of the reagents between the middle-phase microemulsion, where the reaction occurs, and the excess oil and water phases. An attempt was made to increase the reaction rate in the Winsor III system by addition of a phase-transfer agent, tetrabutylammonium hydrogen sulfate. The phase transfer agent gave only a marginal accelerating effect, however.