Thermodynamic Approaches to Microemulsions

The thermodynamics of microemulsions is treated by decomposing the Helmholtz free energy into a sum of a free energy F0 of a dispersion in a continuous medium containing fixed, noninteracting globules and a free energy DeltaF due to the entropy of dispersion of globules in the continuous medium and to the interactions among them. The pressure p1 in the continuous medium of the system involving fixed, noninteracting globules is determined in two different ways. In one of them, it is calculated by minimizing the total free energy with respect to the volume fraction phi of the dispersed medium, while in the other it is considered equal to the external pressure. The equivalence between the conventional thermodynamics of a multicomponent mixture and a thermodynamics based on modeling a microemulsion as a dispersion is used to derive the basic equations. Equations are obtained for single microemulsions as well as microemulsions coexisting with an excess dispersed phase and with both excess phases. One demonstrates that the conventional Laplace equation is not valid for a microemulsion, and new equations are derived. One concludes that the approach involving the determination of p1 via the optimization of the total free energy is the proper one. Considering that DeltaF is dominated by the entropy of dispersion of globules in the continuous medium, equations are established relating the interfacial free energy at the surface of the globules to the radius of the globules and phi, for the case in which a microemulsion coexists with an excess dispersed phase. These equations reveal that for phi approximately 0.5 the above state cannot be stable and that a transition to another state involving a microemulsion coexisting with both excess phases is likely to occur. Copyright 1998 Academic Press.