Adiabatic compressibility of AOT [sodium bis(2-ethylhexyl)sulfosuccinate] reverse micelles: Analysis of a simple model based an micellar size and volumetric measurements

The self-assembly of amphiphilic molecules into supramolecular aggregates i nvolves a number of complex phenomena and forces. Recent developments of hi ghly sensitive, densimetric and acoustic methods on small volume samples ha ve provided novel sensitive probes to explore the physical properties of th ese complex fluids. We have investigated, by high precision densimetry and ultrasound velocimetry, reverse micelles of [sodium bis(2-ethylhexyl)sulfos uccinate] in oil (isooctane and decane), at increasing water concentration and at variable micellar volume fractions. The size of these spherical mice lles has been determined by small angle x-ray scattering. Using these resul ts, in the framework of the effective medium theory, we have developed a si mple model of micellar compressibility, allowing the calculation of physica l parameters (aggregation number, volume, and compressibility) of the surfa ctant monomolecular film as well as that of the micellar waters. In particu lar, we shaw that the central aqueous core designated as " free'' water, lo cated at a distance from the oil-water interacting interface, is twice as c ompressible as "bulk" water. One notable feature of this work. is the influ ence of the nature of the oil on the above parameters.