Thermodynamic Stability of Water-in-oil Emulsions

It is well known that colloids can show phase transitions similar to the ones observed in molecular systems. Colloid-diluted and colloid-concentrated phases can coexist in thermodynamic equilibrium at certain conditions. In a previous work, the authors of this research have used this concept to study petroleum emulsions stability, applying thermodynamic perturbation theories to calculate a phase diagram of water-in-oil emulsions under the influence of an electric field. In this work, it was used a similar thermodynamic formalism coupled with the Lifshitz theory to perform a qualitative analysis of the influence of some variables on emulsion stability. Among the variables studied were temperature, applied electric field, and salt concentration. This research found that droplet diameter is a key parameter in studying thermodynamic stability. Variables like temperature and oil composition can have an opposite effect on thermodynamic and kinetic stability. The application of an electric field can destabilize thermodynamically the emulsion. It can also induce meta-stable transitions, which are important for the calculation of droplet coagulation rate. The results presented in this work are in accordance with trends of experimental and industrial observations, and seek to promote a better understanding of water-in-oil emulsion stability.