Pressure-dependence of the bending modulus of surfactant monolayers in ternary microemulsion systems observed by neutron spin echo

Pressure effects on bending elasticities of surfactant monolayers in a ternary microemulsion composed of aerosol-OT∕D2O/decane

Pressure effects on the bending elasticities of surfactant monolayers have been investigated in a microemulsion system composed of aerosol-OT (AOT), D2O, and deuterated decane by means of small angle neutron scattering, neutron spin echo (NSE), and dynamic light scattering (DLS). In this system, a water-in-oil droplet structure, at ambient temperature and pressure, decomposes into two phases, under both increasing temperature and pressure. The authors' previous study showed that the bending modulus kappa of monolayers slightly decreased with increasing temperature, while it increased with increasing pressure. Temperature and pressure dependencies of kappa were explained in terms of a microscopic model, which takes into account the interactions between surfactant molecules. In this paper, the authors present the temperature and pressure dependencies of kappa obtained by the analysis combined with DLS and NSE experiments. The values of the bending modulus and mean displacement of the second-order droplet deformation are reasonable. It was further confirmed that an increase in the attractive interaction between hydrocarbon tails of AOT molecules with increasing pressure could be the origin of the pressure-induced phase transition.