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Reverse micellar mobile phases based on poly(oxyethylene) (4) lauryl ether (Brij 30) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) were used as mobile phases in HPLC. The chromatographic behavior of the model compounds was studied on the basis of reverse micellar mobile phases modified by salt additives. The binding of o-nitroaniline (o-NA) to the micelles of Brij 30 was determined by UVvisible spectroscopy. The different influences of kosmotropic and chaotropic anions on the binding constant K b was revealed. This may be controlled by the different arrangement of kosmotropic and chaotropic ions in the water core of reverse micelles.Reverse micelles represent one of the normal membranous structures in cells. The biological processes occurring in a reverse micellar system mimic the membranous environment. Reverse micelles are isolated, surfactant-coated water droplets, which have arisen as an appropriate model for confined water in biological systems.1 The structure of water is the subject of basic studies in physics, chemistry, and biology. The complexity of the water structure becomes enormous when it is confined to nanometer-scale cavities. Such environments include biological molecules and membranes, porous rocks and clays, and zeolites. Therefore, investigation of the properties of water core of the reverse micelles is urgent and topical. 1,2 Ion additives significantly influence the structure of a water core in the preparation of a reverse microemulsion from oil, surfactant, and water. Ions are classified as kosmotropes (structure makers) or chaotropes (structure breakers) according to their abilities to affect the structure of water. The ionic kosmotropic and chaotropic additives influence the water structure in water pockets of reverse micelles because of ion water interactions.3 Kosmotropic ions are small or multiply charged ions with a high charge density. They bind water molecules in the first hydrated layer, and they are considered to influence the water structure. Chaotropic ions are large singly charged ions with a low charge density. They retain water molecules weakly in the second hydrated layer and randomize the structure of the liquid water. 4 Direct and reverse microemulsions are used in HPLC for modeling the membrane structure. In particular, frequently, oilin-water microemulsions are used for the estimation of hydrophobicity of compounds with biomedical significance. Water-inoil microemulsions are used rarely despite their interesting structure. Due to the unique two-phase composition of water-inoil microemulsions, their use in HPLC as mobile phases may provide useful information about the solubilization of hydrophilic and hydrophobic compounds into the water nanocages of reverse micelles on the basis of the chromatographic retention factor (k). Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium dodecyl sulfate are more common surfactants for the preparation of mobile phases in microemulsion liquid chromatography. 5,6 Absorption probe investigation in reverse micellar systems is a p...

Instead of water reversed micelles can also be formed with polar organic solvents possessed with high dielectric constant and very low solubility in oil phase. Nonaqueous reverse micelles or microemulsions represent an interesting microreactors for various reactions, especially for reactions, where reagents can react with water. Study of localization places of molecular probes in organic polar pockets of reverse micelles is topical. The solvatochromic behavior of optical probes ortho-nitroaniline (o-NA) and methyl orange (MO) was studied in nonaqueous reverse micelles on the basis of surfactants sodium bis (2-ethylhexyl) sulfosuccinate (AOT) and polyoxyethylene (4) lauryl ether (C12E4) and polar organic solvents (acetonitrile, dimethylformamide, glacial acetic acid, etc.) insoluble in oil phase hexane. The strength of binding of o-NA and MO to AOT and C12E4 reversed micelles was assesssed via binding constant (K b ) and association degree (α) respectively. Donor, acceptor, or dipole-dipole interactions ability of the solvent to the head groups of surfactant was taken into account in order to explain results obtained with UV–visible spectroscopic method. The binding constants of o-NA with reverse AOT micelles in the presence of various solvents in the pockets of reversed micelles increase in the following row water < glacial acetic acid < acetonitrile < dimethylformamide < dimethyl sulfoxide, but this sequence is reversed when o-NA binds to C12E4 reverse micelles. The high value of the proton donor or acidity parameter in the water molecule (x d  = 0.37) determines the weak binding of o-NA to the head AOT groups (K b  = 20.8) in case of aqueous reverse micelles. The high value of the dipole parameter in the dimethylformamide molecule (x n  = 0.40) promotes its strong interaction with nonionic polyoxyethylene groups of C12E4, which results in low value of binding constant (K b  = 26.5) in case of optical probe o-NA and low value of association degree (α = 0.60) using MO as absorption probe. The results of this article will contribute to the improvement of the concept of interfacial processes, viz.: (i) some issues of supramolecular chemistry, (ii) revealing the contribution of parameters of donor, acceptor or dipole-dipole interaction in a polar organic solvent at the surfactant/nonpolar organic solvent interface, and (iii) features of the dissolution of optical probes in non-aqueous reverse micelles.

The microenvironment of water droplets of nonionic polyoxyethylene (4) lauryl ether reverse micelles was investigated with infrared and ultraviolet-visible spectroscopy, also by viscosity and dynamic light scattering measurements in the presence of biological surfactant sodium cholate as additive. Influence of concentrations of polyoxyethylene (4) lauryl ether and sodium cholate on the microstructure of mixed reverse micelles was studied. Influence of sodium cholate as additive on the ratio of free, bound and trapped water fractions was studied via deconvolution of the O-H stretching vibrational absorption spectra in the region of 3 000–3 800 cm−1 into three subpeaks with a Monte Carlo method. Several characteristic parameters of reverse microemulsions were calculated on the basis of measurements of their kinematic viscosity. A different influence of concentration of sodium cholate on the binding constant and association degree of optical probes (o-nitroaniline and methyl orange respectively) to ethylene oxide groups of nonionic surfactant was observed. It was found that the bound water fraction, the binding constants and association degrees, the relative density of water pockets, etc. change their values passing through an extremum under the influence of sodium cholate.