We used cryo-TEM and light scattering to study the micellar growth and subsequent network formation in aqueous solutions of the nonionic surfactant C12E5. Cryo-TEM shows the first direct evidence for the existence of connected topology in the vicinity of the critical point and the two-phase separation curve of the C12E5/water micellar system. The coexisting phases within the two-phase region consist of one concentrated and one dilute network of interconnected cylindrical micelles. These findings are consistent with the recent theoretical explanation of criticality and phase separation in certain nonionic surfactant systems as resulting from entropic attraction between network junctions. Away from the two-phase separation curve, we have identified uniaxial micellar growth, with increasing temperature and concentration, into long threadlike micelles. From the power-law dependence of the radius of gyration, RG,z, and the hydrodynamic diameter, D h H, on the molecular weight (RG,z, D h H ∼ Mw ν ), we find that these threadlike micelles have properties resembling those of flexible polymers in a good solvent (ν ∼ 0.6). Static and dynamic light scattering show that the mean micelle contour length L B increases as c 0.5 (c is the surfactant concentration) in agreement with theory. Finally, we show that the end-cap energy Ec increases linearly with temperature.

As a basis for crystallization studies, the solubilization of amino acids (glycine, l-histidine, and l-phenylalanine) in water-in-isooctane microemulsions stabilized by AOT (sodium di-2-ethylhexyl sulfosuccinate) was investigated. The maximum amount of amino acid that could be solubilized was determined by the solid-liquid extraction method, and the effect of the guest molecules (amino acids) on the size and shape of the microemulsion droplets and their thermal properties were determined using SAXS and DSC measurements, respectively. The solubilization of glycine molecules, which primarily dissolve in the water pool, was slightly lower than their solubility in pure water, decreasing with increasing concentration of AOT and increasing with increasing water content in the microemulsion. In contrast, the solubilization of phenylalanine, which is primarily located at the water/oil interface, exceeded several times the solubility in water, the solubilized amount increasing with increasing AOT and/or water concentrations. Histidine had characteristics intermediate between these two extremes. Solubilization of those molecules effected an increase in droplet size. The thermal analysis showed that loading of the microemulsion droplets with glycine has a much stronger effect on the thermal behavior of the emulsified water than has loading with phenylalanine. The low solubilization of glycine as compared to its solubility in pure water can be explained by the state of water within the microemulsion droplets, i.e., part of it is present as free water and part as water bound to the AOT headgroups. The loading of phenylalanine changed the shape of the microemulsion droplets from spherical to ellipsoidal, and with increasing droplet sizes, the [phenylalanine]/[AOT] molar ratio at the interface increased.

The crystallization of glycine and l-phenylalanine from water-isooctane microemulsions stabilized by AOT (sodium di-2-ethylhexyl sulfosuccinate) has been investigated. Crystallization phenomena were strongly affected by the localization of the solubilized molecules within the microemulsion droplets. In the case of glycine, which is solubilized within the water pools, a significant reduction in crystal size was observed in the temperature range investigated (Ti ) 35 °C, Tc ) 5 °C). While crystals formed in aqueous solution usually grow to millimeter sizes, glycine crystals grown from microemulsions were submicron to micron in size. In addition, the R-form was only observed from aqueous solution, while the γ-form was the predominant form from microemulsions. In the case of phenylalanine molecules, which are located primarily at the W/O interface, morphology and polymorphism were also affected in addition to crystal size. While phenylalanine crystallizes from aqueous solution in the form of two polymorphs, i.e., the needlelike R-form and platelike β-form, upon crystallization from microemulsions, only the β-form appeared. The different crystallization mechanisms of the two amino acids are discussed.