Quasi-Elastic Light Scattering Study of Five-Component Microemulsions

A family of five-component microemulsions made with sodium 4-(l-heptylnonyl)benzenesulfonate, isobutyl alcohol, dodecane, and NaCl brines of various concentrations has been studied with quasi-elastic light scattering techniques. It is found that when the water volume fraction Φ_w in the microemulsion phase is either less than 0.2 or larger than 0.75, the size of well-defined droplets (water-in-oil or oil-in-water) increases linearly as the volume fraction of dispersed phase increases. Both the direct and hydrodynamic interactions between droplets in these two composition ranges have been considered. When Φ_w lies between 0.25 and 0.7, however, the measured apparent diffusion coefficient remains nearly constant. This observation favors a model of bicontinuous structure. The light scattering measurements also suggest that the onset of a structure change from water-in-oil droplets to a bicontinuous structure occurs at Φ_w ~ 0.2 and that the transition corresponds to a change in the electrical conductivity of the microemulsion. In addition, the effect of H₂O/D₂O substitution upon microemulsion structure is examined and the problem of multiple scattering of light from middle phase microemulsions is discussed.