NMR in microemulsions. NMR translational diffusion studies of a model microemulsion

The double chained surfactant didodecyldimethylammonium sulfate (DDAS) forms microemulsions when mixed, in appropriate amounts, with water and aliphatic hydrocarbons, such as dodecane and hexadecane. The microemulsions have been investigated by means of the NMR self-diffusion approach, by which the self-diffusion coefficients of the hydrocarbon and the surfactant can be measured. At low surfactant-to-oil ratios the aggregates are discrete and spherical in shape. As the surfactant-to-oil ratio is increased, the surfactant aggregates change shape and the structure evolves into a bicontinuous microemulsion. In extracting this information from the experimentally determined self-diffusion coefficients, we make use of reduced diffusion coefficients, which we obtain by dividing the observed diffusion coefficients with the values pertaining to diffusion in a system where there is no hindrance for the surfactant or oil diffusion (such a system would, for instance, be constituted by an oil-swollen lamellar phase in which the oil and surfactant-diffusion is measured parallel to the bilayer surface). The observation that the reduced diffusion coefficients for the surfactant and oil are equal at high surfactant-to-oil ratios indicates that the structure is truly bicontinuous over distances on the order of μm.