Phase behavior and microstructure of water/trisiloxane E6 and E10 polyoxyethylene surfactant/silicone oil systems

The binary and ternary phase behavior of the trisiloxane E-6 and E-10 polyoxyethylene surfactants with water and three low molecular weight silicone oils has been determined. The silicone oils were the tetra- and pentacyclosiloxanes, D-4 and D-5, and the short linear tetrasiloxane oil, MD2M. Microstructures were investigated using small-angle X-ray scattering, polarized light microscopy, and cryogenic transmission electron microscopy. Our results illustrate the differences and similarities between the trisiloxane surfactants and conventional hydrocarbon surfactants. Both water/surfactant binary systems form the isotropic spongelike phase, L-3, in a narrow temperature band above the lamellar phase region. The L-3 phase is found at significantly higher temperatures for the E-10 homolog, In the water/M(D'E-6)M system, no two-phase region was detected between the region labeled L-3 and the isotropic surfactant-rich region, L-2. The different birefringent appearance as one progresses between the L-3, the L-2, and the lamellar liquid crystal phase, L-alpha, regions is photographically documented. A large region of lamellar phase is the dominating feature of both the binary water/M(D'E-6)M system and the ternary water/M(D'E-6)M/silicone oil systems. In addition to L-3 and L-alpha, the E-10 trisiloxane surfactant, with its larger hydrophilic group, also forms the normal hexagonal liquid crystal phase, H-1, in the binary water/M(D'E-10)M system and H-1 and the cubic liquid crystal phase, I-1, in the ternary systems. The d spacings of the lamellar phase in both the binary and ternary systems are inversely proportional to surfactant concentration and vary only weakly with oil content. Low molecular weight silicone oils shift the phase regions to higher temperature and lead to the formation of highly structured phases such as I-1. Higher molecular weight oils shift microemulsion regions to higher temperatures and surfactant concentrations.