Crystallization of organic compounds in reversed micelles. I. Solubilization of amino acids in water-isooctane-AOT microemulsions

As a basis for crystallization studies, the solubilization of amino acids ( glycine, l-histidine, and L-phenylalanine) in water-in-isooctane microemuls ions stabilized by AOT (sodium di-2-ethylhexyl sulfosuccinate) was investig ated. The maximum amount of amino acid that could be solubilized was determ ined by the solid-liquid extraction method, and the effect of the guest mol ecules (amino acids) on the size and shape of the microemulsion droplets an d their thermal properties were determined using SAXS and DSC measurements, respectively. The solubilization of glycine molecules, which primarily dis solve in the water pool, was slightly lower than their solubility in pure w ater, decreasing with increasing concentration of AOT and increasing with i ncreasing water content in the microemulsion. In contrast, the solubilizati on of phenylalanine, which is primarily located at the water/oil interface, exceeded several times the solubility in water, the solubilized amount inc reasing with increasing AOT and/or water concentrations. Histidine had char acteristics intermediate between these two extremes. Solubilization of thos e molecules effected an increase in droplet size. The thermal analysis show ed that loading of the microemulsion droplets with glycine has a much stron ger effect on the thermal behavior of the emulsified water than has loading with phenylalanine. The low solubilization of glycine as compared to its s olubility in pure water can be explained by the state of water within the m icroemulsion droplets, i.e., part of it is present as free water and part a s water bound to the AOT headgroups. The loading of phenylalanine changed t he shape of the microemulsion droplets from spherical to ellipsoidal, and w ith increasing droplet sizes, the [phenylalanine]/[AOT] molar ratio at the interface increased.