Droplet size and dynamics in water-in-oil microemulsions

Abstract

This paper is a short review of a systematic study of the effect of oil, surfactant and cosurfactant alkyl chain length (with n, m and n_c carbon atoms in the alkyl chain, respectively) and temperature (T) on the droplet size, rate constant k_c for exchange of material between colliding droplets, and attractive interactions between droplets in water-in-oil microemulsions. In agreement will current theories on the stability of water-in-oil microemulsions, the droplet size, k_c and attractive interactions between droplets increase as n and T increase or as m and n_c decrease. Correlations have been found between droplet size, value of k_c, magnitude of interdroplet attractive interactions, appearance of electrical conductivity percolation and water solubility as one of the parameters n, m, n_c or T is changed. It has been shown, for instance, that electrical conductivity percolation appears only for systems characterized by a value of k_c larger than (1–2)·10⁹ M⁻¹ s⁻¹. It has also been shown that electrical measurements as a function of the molar concentration ratio ω (equal to [water]/[surfactant]) can give qualitative information concerning the variation of droplet size, k_c and attractive interdroplet interactions with n, m and n_c.