Pathway and energetics of the thermally-induced structural changes in microemulsions

Microemulsions are thermodynamically stable dispersions of nano-droplets stabilized by surfactants. In water-in-oil microemulsions, the nano-droplets exist in the form of swollen reverse micelles. Recent experiments demonstrated very high critical heat fluxes (CHF) in boiling of water-in-oil microemulsions, which makes these microemulsions ideal heat transfer fluids for high heat-flux thermal management applications. While the nanoscale structure of the microemulsions is inevitably changed during their boiling, little is known about such change and its energetics, which are critical for understanding the microemulsion boiling. Here we report a study of the pathway and energetics of thermally-induced structural change in water-in-oil microemulsions using microsecond-long molecular dynamics simulations. We show that, for microemulsions that are thermodynamically stable at room temperature, upon increasing their temperature to similar to that found in boiling experiments, their spherical reverse micelles transition to cylindrical wormlike reverse micelles through merging and splitting of individual reverse micelles. These structural transitions are accompanied by a gradual release of water molecules from the reverse micelles. These transitions are endothermic with the heat absorption dominated by the release of water molecules from the water core of the reverse micelles. The release rate of water molecules from the interior of reverse micelles, hence the heat absorption rate, slows down as the structural transition proceeds because the remaining water molecules in the interior bind more strongly to the polar moieties of the surfactants.