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 respect to the effect of the electric charge density on the bending elasticity of the amphiphilic film. For this an originally uncharged microemulsion system became charged by the substitution of the nonionic by an ionic surfactant (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 with the maximum particle radius R-m. p and R-m were measured by means of small-angle neutron scattering (SANS) experiments in the shell contrast. Neutron spin echo (NSE) has been employed to measure directly the dynamics of the shape fluctuations of the droplets. This method enables a separate determination 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 bending constants 2 kappa+<(<kappa>)over bar>. Also the change of the ionic strength 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 sum but separately the two bending constants stay within experimental error unchanged. This experimental observation is in contrast to simple electrostatic theories that would predict a much more pronounced influence of the electric charge density on the bending properties of the amphiphilic film. (C) 2001 American Institute of Physics.