Nonionic-anionic mixed surfactants cubic mesophases. Part I: Structural chaotropic and kosmotropic effect

We prepared and investigated cubic bicontinuous (V) phase from mixtures of nonionic monoolein (GMO) and anionic oleyl lactate (OL) surfactants in the presence of ethanol and water. The isotropic region and the composition of the V phase in the pseudoternary phase diagram vary with the nature of the hydrophilic headgroups and their charge. We examined three anionic species, acidic (HOL), partially neutralized (OL), and totally ionized (NaOL) forms. The largest swollen V region within the phase diagram was formed from the partially neutralized form. The V formation is dependent on the GMO/OL ratio. The largest isotropic region in the phase diagrams was found with GMO/OL at a 70:30 wt % ratio and in the presence of 5.0 and 38.5 wt % ethanol and water, respectively. The structural effect of OL was determined by small-angle X-ray spectroscopy, differential scanning calorimetry, and Fourier transform infrared. The results revealed that the structure is curvature-dependent. Mesophases made from 90:10 wt % GMO/OL showed phase transition from gyroid (G) to diamond (D) symmetry. Preparations made from 30:70 wt % GMO/OL exhibited coexistence of two mesophases, one (of low order) cubic and the other lamellar. Because of the overall gauche deformation growth, the hydrocarbon order decreased with the OL content increase. The GMO, sn₂ and sn₃ headgroups, and water structure vibration bands indicate a chaotropic effect as a result of the interdigitation of OL anions and Na⁺ and H⁺ counterions.