Self-diffusion and polydispersity in water + AOT +p-xylene microemulsions. A dynamic light-scattering study

Abstract

The intermediate scattering function, F(q, t), has been obtained for water microemulsions stabilised by AOT (sodium di-2-ethylhexylsulphosuccinate) and dispersed in p-xylene with a mole ratio of water to surfactant of 10±0.5. For volume fractions of water plus surfactant, ϕ, in the range 0.01–0.53 the excess scattering over that caused by solvent is attributed to diffusional motion of microemulsion particles. The mean hydrodynamic radius, r_H, obtained from the diffusion coefficient extrapolated to infinite dilution is 2.7±0.2 nm. All measurements are in the range qr_H≪ 1, where q is the wavevector. At high concentrations (0.25 < ϕ < 0.5)F(q, t) fits to the sum of two exponentials, suggesting that there are two modes of relaxation. It is concluded that the slowly decaying exponential can be assigned to the relaxation by self-diffusion of spatial polydispersity fluctuations, as suggested recently in the literature. For ϕ < 0.4 the associated self-diffusion coefficients are very similar in value to the diffusion coefficients for surfactant molecules derived from n.m.r. data, showing that surfactant is bound to microemulsion particles on average at least for 10^–5 s. The volume-fraction dependence of self-diffusion can be explained on the basis of a hard-sphere model for the microemulsion particles. From the amplitudes of the two exponentials the estimated extent of polydispersity is 0.15±0.03 in the range 0.25 < ϕ < 0.5. For ϕ < 0.25 the two exponentials are not well resolved and the data are fitted to a cumulants expansion of order two. Results indicate that the timescale for local polydispersity fluctuations is at least three orders of magnitude longer than the estimated time between particle collisions.