Experimental investigation of the structure of nonionic microemulsions and their relation to the bending elasticity of the amphiphilic film

Oil-in-water microemulsions of nonionic surfactants of type C(n)E(j) with various hydrocarbons have been characterized by means of small-angle-neutron-scattering (SANS) (carried out at the Laboratoire Leon Brillouin, CEA-CNRS, Saclay, France) and interfacial tension measurements. The obtained structural parameters (radius R and polydispersity index p) of this droplet system and the interfacial tension gamma were related to the bending elasticity of the amphiphilic film that can be described by the spontaneous curvature and two elastic constants, bending modulus kappa and saddle-splay modulus <(kappa)over bar>. The validity of our method to extract p from the shape analysis of the scattering curves has been confirmed by an independent contrast variation experiment. Both p and gamma lead independently for all systems to very similar values for the elastic moduli. Their sum 2 kappa+<(kappa)over bar> is proportional to the chain length of the surfactant but independent of our employed hydrocarbons (hexane-->decane). Finally, we also performed SANS experiments fora dilution series, where from the concentration dependence of R and p we are able to deduce independently this sum of the elastic constants that agrees well with the values obtained before. The elastic theory of the amphiphilic film allows for a self-consistent interpretation of structural and interfacial tension data and also to reliably predict one of these quantities from the knowledge of the other ones.