SIZE DISTRIBUTION MODEL FOR CHEMICAL INTERDIFFUSION IN WATER AOT HEPTANE WATER-IN-OIL MICROEMULSIONS

The Taylor dispersion method has been used to measure ternary interdiffusion coefficients for water-in-oil microemulsions prepared from water + AOT (sodium bis(2-ethylhexyl) sulfosuccinate) + heptane. In this system the water and AOT components diffuse together through the heptane-continuous medium as surfactant-coated water droplets. Yet the diffusion coefficient of AOT is up to 3 times larger than the diffusion coefficient of water. Although concentration gradients in AOT produce cocurrent coupled flows of water, gradients in water produce counterflows of AOT. This unexpected behavior is attributed to changes in the average droplet size along the diffusion path. Whereas an increase in the concentration of AOT produces a larger number of smaller, more rapidly diffusing droplets, an increase in the concentration of water has the opposite effect. The analysis confirms the experimental result that the diffusion of water and surfactant in a microemulsion produces two distinct diffusional relaxation times, even in cases of negligible droplet polydispersity.