(673b) Molecular Thermodynamic Modeling of Three Component Microemulsions

There is an increasing interest in microemulsions largely due to their scientific importance and technological potential. The pledge of the use of low volumes of amphiphile molecules compared to large amounts of bulk modifiers in a variety of chemical and industrial processes, from enhanced oil recovery to nanoparticle synthesis, fosters continuous investigation and improved understanding of these systems. Microemulsions are nanoheterogeneous, thermodynamically stable, spontaneously formed mixtures of oil and water by means of surfactants, with or without cosurfactants. Different approaches have been used to describe the thermodynamics of microemulsions in the past, from conventional liquid-liquid equilibrium using the excess Gibbs energy expression with activity coefficients as adjustable model parameters from regression of experimental data, to macroscopic phenomenological theories of globular microemulsions. To the best of our knowledge Nagarajan and Ruckenstein in 2000 published the only work on molecular thermodynamic modeling form microemulsions up to this date. The main limitation of their formulation is that it does not allow the verification against experimental data. The overall composition of the system cannot be easily related to the control variables, which are the mole fractions of surfactant and cosurfactant in the water phase at the equilibrium. In our work, we have developed a molecular thermodynamic theory for droplet-type microemulsions, both water-in-oil and oil-in-water based on direct minimization of the Gibbs Free Energy. We present the theoretical formulation for three-component microemulsions. Our thermodynamic model predicts the structural and compositional features of microemulsions. Model predictions are compared with experimental data for droplet size for water-alkane-DDAB systems.