Interparticle interactions in water-in-oil microemulsions: Dielectric and ir investigations

Abstract

The high-frequency dielectric response of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles in n-heptane has been measured in the range 0.01--20 GHz as a function of volume fraction \ensuremath{\varphi} of a dispersed phase at a fixed value of the water to surfactant molar ratio $W=11.$ A relaxation phenomenon has been observed that is connected to the AOT ion pair diffusion at the water-surfactant interface. This dielectric relaxation has been found to be markedly affected by the interparticle interactions that have been modulated by adding different amounts of NaCl. In order to reveal the connections between attractive interactions among the droplets and the stereochemical configurations of AOT molecules, an ir study has been performed on the carbonyl stretching band of AOT molecules in the micellar aggregates. A comparison with dielectric data shows that a correlation exists between the intensity of attractive interactions and the composition of AOT rotamers in the microemulsion system. These results support the hypothesis that attractive intermicellar interactions are favored by a particular orientation and packing of nonpolar tails of AOT molecules in the micellar microaggregate.