Electrolyte-moderated interactions in water/oil microemulsions

Abstract

Interactions between the disperse phase droplets of water-in-oil (W/O) microemulsions of Aerosol-OT are profoundly affected by the addition of small amounts of electrolyte. We find that the effect of added electrolyte in W/O microemulsions is opposite to that which is observed in aqueous micellar solutions and W/O microemulsions, here the addition of electrolyte decreases attractive interactions rather than increasing them. Dynamic light scattering measurements show that the addition of salt decreases (and even eliminates) the dependence of the diffusion coefficient on disperse phase concentration. Time-averaged intensity measurements and shear viscosity measurements provide further evidence of increased hard-sphere-like behavior as the salt concentration increases. Analysis of the time-averaged intensity using a formalism developed by [23.] suggests that the attractive part of the interaction potential is significantly larger than that expected between pure water droplets. The attractive potential decreases as the electrolyte concentration increases. Based on interactions between fluctuating dipoles, a semiquantitative explanation for the effect of salt on attractive interactions is provided.