Droplet Size Distributions and Phase Equilibria with Ionic Microemulsions
The free energy of droplet type microemulsions containing an ionic surfactant and a non-ionic cosurfactant is formulated on the basis of a curvature dependent interfacial free energy, the pressure difference between the droplets and the continuous medium, and the entropy of mixing of the spherical droplets with the medium. The equilibrium among droplets of various sizes is dealt with via a mass-action approach.
The analysis has been worked out for W/O and for O/W both for saturated (in equilibrium with excess W or O, respectively) and for unsaturated microemulsions.
The calculated free energy of curved electrical double layers forms the basis for understanding the influence of added electrolytes on the curvature dependence of the interfacial tension and on the phase behavior.
Quantitative aspects of the theory are adapted to experimental data obtained for microemulsions consisting of water, NaCl, sodiumdodecylsulphate, pentanol and cyclohexane.
Droplet size distributions and interfacial tensions between the microemulsion and excess water or oil follow from the theoretical treatment. It is stressed that the droplet size distribution is essential for understanding the existence of a finite range of salt concentrations (and cosurfactant concentrations), where a microemulsion may be in equilibrium with both W and O (Winsor III).
KeywordsEntropy Surfactant Toluene Hydrocarbon Cyclohexane
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