Model for water-in-oil microemulsions: Surfactant effects

We have simulated, using a Monte Carlo technique, a mixture of oil, water, and surfactant on a two-dimensional square lattice. The computations are performed for water-in-oil microemulsion (i.e., inverse emulsion) where we have explicitly included, besides the interaction energy epsilon between the water (oil) and the head (tail) of surfactant molecules, the molecular length m as a parameter. We find that as epsilon increases, the size of the water droplets decreases due to a reduction in the interfacial energy between water and oil in the presence of surfactant molecules. We further show that the effect of m on the surfactant efficiency in determining the water solubility in oil depends strongly on epsilon. When \epsilon\ is small, the influence of m is strong; however, when \epsilon\ is large, m virtually has no effect in determining the efficiency of the surfactant molecules, which is explained using a simple thermodynamic argument. Finally, the structure of the system is profoundly affected by the surfactant concentration and, most important, when this concentration is high enough the system evolves into a highly ordered lamellarlike structure.