Structures in a microemulsion system of an ethoxylated polymethylsiloxane surfactant, water, and oil studied by NMR self-diffusion measurements

Microemulsion samples of an ethoxylated polymethylsiloxane surfactant, water, and 1-dodecanol or 1-decanol as the oil component are investigated using pulsed field gradient NMR to determine the components' self-diffusion coefficients. It is demonstrated that the structure of the liquids depends heavily on their composition, in that, for low water content, the structure is water-in-oil (w/o), gradually changing to a bicontinous structure in a concentration range ca. 40-60 wt % water, and, finally, to an oil-in-water (o/w) structure for more water rich samples. In the water poor samples, the surfactant molecules apparently do not form extended aggregates (inverted micelles). In the water rich samples, the surfactant and oil (if present) form ordinary micelles, and it is demonstrated for the binary water/surfactant system that the micelles are spherical at very low surfactant concentrations and grow into oblate (disk) shaped aggregates at surfactant concentrations above ca. 5 wt %. From density and viscosity measurements of binary mixtures of oil (1-decanol) and surfactant, it is demonstrated that these components form solutions that are not far from ideal.