Surfactant film bending elasticity in microemulsions: Structure and droplet polydispersity

Abstract

The surfactant film bending elasticity can be described by a spontaneous curvature C0 and two elastic constants K and K̄ associated with the mean curvature and the Gaussian curvature, respectively. These parameters are very important in the determination of the structure of the dispersions stabilized by the surfactant (droplets or sponge-like structures). We have studied ternary mixtures of oil, water, and nonionic surfactants of different chain lengths. Depending on the temperature, the microemulsions are in equilibrium with excess oil (o/w structure), excess water (w/o), or both excess oil and water (bicontinuous). We present neutron scattering data from which we determine the microemulsion structure and, in the case of droplet structures, of the droplet polydispersity. These results, in combination with those from earlier experiments using ellipsometry, are used to estimate K̄.