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 CₙEⱼ 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 γ were related to the bending elasticity of the amphiphilic film that can be described by the spontaneous curvature and two elastic constants, bending modulus κ and saddle-splay modulus κ. 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 γ lead independently for all systems to very similar values for the elastic moduli. Their sum 2κ+κ is proportional to the chain length of the surfactant but independent of our employed hydrocarbons (hexane{→}decane). Finally, we also performed SANS experiments for a 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.