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

Istituto Nazionale di Fisica della Materia and Dipartimento di Fisica, Universita` di Perugia I-06100 Perugia, Italy Received 30 October 1997; revised manuscript received 24 July 1998The high-frequency dielectric response of sodium bis 2-ethylhexyl sulfosuccinate AOT reverse micelles inn-heptane has been measured in the range 0.01–20 GHz as a function of volume fraction of a dispersedphase at a fixed value of the water to surfactant molar ratio W 11. A relaxation phenomenon has beenobserved that is connected to the AOT ion pair diffusion at the water-surfactant interface. This dielectricrelaxation has been found to be markedly affected by the interparticle interactions that have been modulated byadding different amounts of NaCl. In order to reveal the connections between attractive interactions among thedroplets and the stereochemical configurations of AOT molecules, an ir study has been performed on thecarbonyl stretching band of AOT molecules in the micellar aggregates. A comparison with dielectric datashows that a correlation exists between the intensity of attractive interactions and the composition of AOTrotamers in the microemulsion system. These results support the hypothesis that attractive intermicellar inter-actions are favored by a particular orientation and packing of nonpolar tails of AOT molecules in the micellarmicroaggregate. S1063-651X 98 10312-4PACS number s : 82.70. y, 77.22.Gm, 77.84.Nh, 63.50. xI. INTRODUCTION