THEORY OF PHASE BEHAVIOR AND CONCENTRATION GRADIENTS PRODUCED BY GRAVITY IN MICROEMULSIONS
Doctor of Philosophy
The thermodynamic behavior of microemulsions is studied theoretically, both with and without the effect of a gravitational field. The gravity-free study consists of two parts. One is development of an improved theory for predicting drop sizes in microemulsions which coexist with a single excess phase. A lattice model of the surfactant films of the drops is considered which includes a hydrocarbon chain region containing tails from both surfactant and alcohol molecules as well as hydrocarbon molecules. Analytical equations are derived for minimizing system free energy which enable properties to be computed with greater accuracy than with a previous theory utilizing a similar model. Moreover, the behavior of microemulsions containing single-chain surfactants can be predicted by the present theory but not by the previous one. The second part of the gravity-free work utilizes a simple duplex film model for the surfactant films which includes both interfacial tension and bending effects. Ternary phase diagrams are constructed for oil-water-surfactant systems assuming that microemulsion phases alwasy contain monodisperse, spherical drops. The predicted phase diagrams are, for the most part, similar to those seen experimentally. However, regions of coexistence of oil-in-water and water-in-oil microemulsions are found under conditions where the hydrophilic and lipophilic properties of the surfactant films are nearly balanced. The theory also allows interfacial tensions to be calculated between microemulsions and excess phases. This theory of microemulsions is extended to include the effect of a gravitational or centrifugal field. The equations of sedimentation equilibrium are employed, and microemulsion phase continuity, drop radius and volume fraction, and the area per molecule in the surfactant films are obtained as a function of elevation. Far from phase inversion conditions drop sizes are relatively small and the predicted concentration variations within microemulsion phases are low in an ordinary gravitational field though they may be appreciable for high-speed centrifugation. Near phase inversion conditions the theory predicts that transformation from water continuous to oil continuous behavior may occur with increasing elevation. Generally speaking, gravity reduces solubilization within microemulsions and promotes separation of excess phases.