COATED DROPLET MODEL OF MICROEMULSIONS - OPTICAL MATCHING AND POLYDISPERSITY

Dilute water-in-oil microemulsions, such as the ternary system water-AOT-hexane, can be modeled as suspensions of spherical droplets coated with surfactant. There is a general agreement that the radius of the droplets is not uniform but the magnitude of the polydispersity is subject to discussions. The polydispersity influences strongly optical properties of the microemulsions, in particular it determines the characteristic features of scattering data observed in the vicinity of optical matching. We present here analytical expressions and data for the refractive index increment, the average scattering intensity, the apparent hydrodynamic radius, and the second cumulant of dynamic light scattering. Our model reproduces without any free parameters the dependence of the refractive index increment on the water-to-surfactant ratio omega. Only two free parameters are needed to fit the omega dependence of the other three quantities: delta represents approximately the thickness of the surfactant layer and gamma = <r2>/<r>2 - 1 reflects the radius polydispersity. Our results suggest a polydispersity index square-root gamma of only 12%. This value is much smaller than previously estimated by small angle scattering.