Microstructures in aqueous solutions of a polyoxyethylene trisiloxane surfactant and a cosurfactant studied by SANS and NMR self-diffusion.

Microemulsion samples of a polyoxyethylene trisiloxane surfactant, water, and 1-decanol are investigated using pulsed field gradient NMR and small-angle neutron scattering (SANS) to determine the solution structure. The surfactant/decanol weight ratio has been kept constant at values of 10:1, 8:1, and 6:1 under variation of water content. The temperature was 32 degrees C for the measurement series at the weight ratio of 10:1 to avoid phase separation at high water content. Also, aqueous surfactant solution samples have been investigated as a function of composition and temperature. Water-rich samples consist of micelles that are close to spherical at very low surfactant concentration and grow into anisometric, that is, oblate formed aggregates, at higher surfactant (or surfactant and decanol) concentration. The aggregates grow with increasing temperature, most probably due to dehydration of the hydrophilic groups. In a concentration range around 50 wt % water, the systems form bicontinuous structures. SANS data are used to estimate surfactant film properties using a model developed for interpretation of neutron scattering data from related systems.

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.

Aqueous microemulsions of a fluorinated surfactant and oil studied by PFG-NMR: transformation from threadlike to spherical micelles.

The aqueous microemulsion system consisting of the fluorinated surfactant tetraethylammonium perfluorooctylsulfonate (TEAFOS) and the fluorinated oil 1H-perfluorohexane (PFH) has been investigated using the pulsed field gradient NMR self-diffusion method on both 1H and 19F. Neat TEAFOS(aq) builds threadlike micelles from rather low surfactant concentrations up to ca. 80 mmol kg(-1). The addition of PFH to TEAFOS(aq) solutions induces a transition from threadlike micelles to spherical micelles solubilizing the oil. In this paper, information from the self-diffusion coefficients of oil (PFH), surfactant counterion (TEA+), surfactant ion (FOS-), and water (HDO) during the transition is presented.