SPONTANEOUS CURVATURE OF SURFACTANT FILMS

We employ a mean-field model for monolayers of short-chain surfactants at an oil-water interface, and include a quantitative consideration of the head-group interactions (both steric and electrostatic) and the interfacial geometry. The behavior is then analyzed numerically, giving values for the spontaneous curvature, the bending moduli, and the neutral plane position, as the molecular parameters are varied. The spontaneous curvature is found to have a "universal" behavior for different type of head groups, which leads to a confirmation of the geometric theory of the surfactant parameter. The values for the bending moduli are consistent with experiments, and for the case of electrostatic head group interactions we predict that k(c) is only weakly dependent on electrolyte concentration. The local continuum analysis of the bending energy of monolayers, on which other theories depend, is shown to be only valid close to the spontaneous curvature.