Effects of Alkane Chain Length on the Bending Elasticity Constant K of AOT Monolayers at the Planar Oil-Water Interface

In systems containing AOT + aqueous NaCl + normal alkane, the change of the microemulsion type from oil-in-water to water-in-oil through a surfactant-rich phase can be effected by increasing the aqueous phase salt concentration. We have investigated the multiphase behaviour (in Winsor systems) for a range of alkanes. The low oil-water interfacial tensions enable us to measure the bending elasticity constant K of the monolayer by ellipsometry. Values of K are independent of salt concentration but decrease with increasing alkane chain length. This is discussed in terms of the differing extents of oil penetration into the surfactant chain region and its consequences on the structure of the third phase.