Film bending elasticity in microemulsions made with nonionic surfactants

The interfaces between water and alkanes in the presence of a monolayer of nonionic surfactant (polyethylene glycol alkyl ether) have been studied as a function of temperature for surfactants of different chain lengths. At low temperatures and moderate surfactant concentration, the aqueous phase is an oil in water microemulsion; at high temperatures, the oil phase is a water in oil microemulsion. In our study, the middle of the studied temperature range corresponds to the inversion temperature where a middle phase microemulsion coexists with both oil and water. — The interfacial tensions, which are very low, have been measured with surface light scattering and the interfacial film roughness with ellipsometry. From these data, the bending elastic constant of the surfactant monolayer is determined. The calculated persistence length of the monolayer is compared to the characteristic size in the middle phase microemulsion measured with neutron scattering.