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) w ith various hydrocarbons have been characterized by means of small-ang le-neutron-scattering (SANS) (carried out at the Laboratoire Leon Bril louin, 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 rela ted to the bending elasticity of the amphiphilic film that can be desc ribed 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 experim ent. Both p and gamma lead independently for all systems to very simil ar 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 perfo rmed SANS experiments fora dilution series, where from the concentrati on dependence of R and p we are able to deduce independently this sum of the elastic constants that agrees well with the values obtained bef ore. The elastic theory of the amphiphilic film allows for a self-cons istent interpretation of structural and interfacial tension data and a lso to reliably predict one of these quantities from the knowledge of the other ones.