Films of di-chained surfactants in microemulsions

The nature of negatively curved films of di-chained surfactants in water-in-oil microemulsions has been studied by small-angle neutron scattering (SANS). Zwitterionic and ionic amphiphiles with different tail structures were investigated: synthetic C-18:1 phosphatidylcholines (PCs) containing C=C bonds, n-alkyl-n-dodecyldimethylammonium bromides (Cn-C12s) and aerosol-OT (AOT). The oil was cyclohexane, but n-heptane was used for AOT. Various possibilities for solvent penetration into the surfactant film were investigated by model fitting contrast variation SANS data. The most consistent and realistic fit parameters were obtained using a sharp interface model. Although there was no evidence for any hydration layers, the degree of oil penetration was found to increase when the surfactant chains were made more disordered. With C12-C12 and AOT there was no evidence for oil mixing, whereas for PC and C18-C12 layers the results inferred a cyclohexane volume fraction phi(C6D12) of between 0.05 and 0.20 depending on the surfactant. (C) 1997 Elsevier Science B.V.