BENDING OF IONIC SURFACTANT MONOLAYERS

The electrostatic contribution to the bending moduli and spontaneous c urvature of monolayers formed by ionic surfactants in solution is calc ulated for all salt concentrations, ranging from no added salt (counte rions only) to excess salt. This is accomplished using a perturbative expansion in curvature of the free energy of the Poisson-Boltzmann cel l model, which is shown to give precisely the same results for the mod uli as would an alternative calculation employing moments of the trans verse pressure profile of the electric double layer. With this treatme nt, it is possible to quantify the dependence of the moduli and sponta neous curvature on surfactant concentration alone (i.e., with fixed av erage salt concentration), a point of central importance to the validi ty of the flexible surface model in the description of ionic surfactan t systems. A manifestation of the counterion condensation phenomenon i s also observed, as the monolayer rigidity saturates and becomes indep endent of the surface area per ionic headgroup.