The effect of the charge density of microemulsion droplets on the bending elasticity of their amphiphilic film

Oil-in-water (O/W) microemulsion droplets have been investigated with respe ct to the effect of the electric charge density on the bending elasticity o f the amphiphilic film. For this an originally uncharged microemulsion syst em became charged by the substitution of the nonionic by an ionic surfactan t (up to 5 mol %). The sum of the bending constants, 2 kappa+<(<kappa>)over bar>, has been determined from the polydispersity index p of the droplets and alternatively from the macroscopic interfacial tension gamma together w ith the maximum particle radius R-m. p and R-m were measured by means of sm all-angle neutron scattering (SANS) experiments in the shell contrast. Neut ron spin echo (NSE) has been employed to measure directly the dynamics of t he shape fluctuations of the droplets. This method enables a separate deter mination of kappa on its own. It is found that the effect of the increasing charge density leads only to a fairly small increase for the sum of the be nding constants 2 kappa+<(<kappa>)over bar>. Also the change of the ionic s trength for a charged microemulsion system has almost no influence on this sum. NSE measurements show no measurable difference in the dynamics of the charged and uncharged system leading to the conclusion that not only the su m but separately the two bending constants stay within experimental error u nchanged. This experimental observation is in contrast to simple electrosta tic theories that would predict a much more pronounced influence of the ele ctric charge density on the bending properties of the amphiphilic film. (C) 2001 American Institute of Physics.