Thermodynamics of Microemulsion Formation

Outlines of complex ternary phase diagrams of cetyltrimethyl ammonium bromide/water/CHCl3, cetylpyridinium chloride/water/CHCl3, sodium bisethylhexyl sulfosuccinate/water/CHCl3, and octylphenoxy polyoxyethylene ether/water/CHCl3 systems have been presented. Enthalpies of solution of water-in-oil + amphiphile and oil-in-water + amphiphile media have been calorimetrically determined. Except for cetyltrimethylammonium bromide/water/CHCl3 system, all the systems have exhibited thermal inversion for water and oil dispersion near the phase separation point. The surfactant concentration at the inversion point bears a linear correlation with the concentration of the dispersed solvent. The thermodynamics of solubilization of the solvents in the form of microdroplets are presented. The measured heat capacities of all the systems agree very reasonably with the calculated values assuming ideal mixing behavior of the components.