Rigidities of cationic surfactant films in microemulsions

The bending rigidities of dichain cationic surfactant monolayers in Winsor II water-in-cyclohexane microemulsions have been studied using surface light scattering (SLS) and small-angle neutron scattering (SANS). The elastic moduli (2K + Kbar) are typically between 1.0 and 2.5 k(B)T and increase with both surfactant chain length and added NaBr electrolyte, For salt free systems the scaling relationship observed with surfactant film thickness t, 2K + Kbar similar to t(2.3), agrees reasonably well with theory. The separate electrostatic contribution (2K + Kbar)(elec) has been calculated using known equations, and this has been used to provide estimates for inherent surfactant rigidities (2K + Kbar)(surf). This work shows that the film bending elasticity model is a valid description of these microemulsions.