Liquid crystal and solution phases of sodium dodecyl-p-benzene sulfonate (LAS) and hexa-oxyethylene glycol dodecyl ether (C12E6); 1:1 mixtures in water

The liquid crystals and other phases formed when the mixed surfactant system sodium dodecyl-p-benzene sulfonate (LAS) and hexa-oxyethylene glycol dodecyl ether (C12E6, 1:1 mixtures by weight) is dispersed in water have been investigated using optical microscopy, X-ray diffraction and differential scanning calorimetry. Despite the fact that neat LAS is a multi-phase solid and C12E6, is a low melting crystalline solid, the mixed surfactants (no water) form a liquid phase. On addition of water there is a mixed mesophase coexistence region, followed by a lamellar phase and a micellar solution that extends up to ca. 50 wt.% surfactant. In the mixed mesophase region (ca. 88–95% surfactant) there appear to be “intermediate” and lamellar phases present. After heating, the system takes several days to relax to the original state. Based on simple “packing constraints” concepts hexagonal, cubic and lamellar phases are expected. The absence of the more viscous phases points to an additional contribution from the alkyl chains to the surfactant bilayer curvature.