Coated droplet model of microemulsions: Optical matching and polydispersity

• Published in: The Journal of chemical physics Vol. 94; no. 12; pp. 8503 - 8509
• Main Authors: Ric̆ka, J., Borkovec, M., Hofmeier, U.
• Format: Journal Article
• Language: English
• Published: Woodbury, NY American Institute of Physics 15.06.1991
• Subjects: Exact sciences and technology; Fluid dynamics; Fundamental areas of phenomenology (including applications); Physics; Water; Binary mixture; Hydrodynamics; Chemical reaction; Micropolar fluid
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.460083

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 ω. Only two free parameters are needed to fit the ω dependence of the other three quantities: δ represents approximately the thickness of the surfactant layer and γ=〈r 2〉/〈r〉2−1 reflects the radius polydispersity. Our results suggest a polydispersity index (γ)1/2 of only 12%. This value is much smaller than previously estimated by small angle scattering.