Phase behaviour and physical properties of mixtures of calcium salts of α-sulfonated fatty acid methyl ester and a cosurfactant

Abstract

We have studied the phase behaviour of mixtures of calcium salts of α-sulfonated fatty acid methyl ester Ca(C_x-α-MES)₂ and a monoglycerinether as cosurfactant (EHG). The molecular interactions between the surfactant and the cosurfactant and the influence on the phase behaviour have been studied systematically. With increasing cosurfactant concentration the micellar L₁-phase shows a transition to an L_α-phase and an L₃ sponge phase. The L₃-phase is observed for the first time in a ternary phase diagram of a Ca-salt of a α-sulfonated fatty acid methyl ester, the cosurfactant 2-ethylhexyl-monoglyceride and water. As for other ternary surfactant/cosurfactant systems, the L₃-phase occurs with increasing cosurfactant/surfactant ratios after the L_α-phase. Some properties of the newly observed L₃-phase are the same as for other known L₃-phases. It is a low viscous, optically isotropic fluid with a low flow birefringence. The time constants τ of the electric birefringence results scale with τ ∼ Φ⁻³. Its conductivity is very much higher than the conductivity of the neighbouring L_α-phase. However there are some marked differences from known normal L₃-phases. The novel L₃-phase is thermodynamically stable in spite of ionic charges on the bilayer. There is no two phase region between the L_α and the L₃-phase in the system Ca(C₁₄-αMES)₂/EHG and the L₃-phase is stable over a wide cosurfactant/surfactant ratio between one and two. In SANS measurements it shows a broad correlation peak that occurs at about the same position as the sharper peak in the L_α-phase. The structure of the L₃-phase is demonstrated by FF-TEM micrographs.