Quasi-elastic light-scattering investigation of microemulsions

Abstract

Quasi-elastic light scattering was used to investigate W/O microemulsions of heptane-AOT-(bis(2-ethylhexyl)sulfosuccinate sodium salt)-water and W/O and O/W microemulsions of hexadecane-SDBS (sodium dodecyl benzene sulfonate)-n-pentanol-water systems as a function of temperature and dispersed-phase volume. The apparent molecular weights, distribution functions of radii, and diffusion coefficients were determined from the autocorrelation function of the scattered light. In the heptane-AOT-water system it was found that at constant temperature and constant AOT concentration the total interfacial area was independent of the water volume over a fairly large range. The polydispersity and the average hydrodynamic radii increased with temperature and disperse-phase volume for the W/O microemulsions. The average droplet radius ranged from 60 to 3000 A. The behavior of the O/W microemulsions was found to be more complex. In most of the 11 systems studied the distribution functions were bimodal, indicating the presence of surfactant micelles in equilibrium with microemulsion droplets. The apparent molecular weights determined from intensity measurements and from the average hydrodynamic radii agree well for the heptane-AOT-water systems but the agreement is not good for the other systems.