LIQUID CRYSTAL TO MICROEMULSION TRANSITIONS IN ANIONIC SURFACTANT-OIL-BRINE SYSTEMS
Doctor of Philosophy
Various anionic surfactant-oil-brine systems were studied to understand the transition between liquid crystal and microemulsion phases produced by changes in composition. This work involved equilibrium phase behavior studies and included optical microscopy and measurements of electrical conductivity, density, viscosity, and interfacial tension. These studies provide information important in designing slugs for oil recovery processes and should be useful in understanding certain detergency processes. The effects of alcohol on surfactant-alcohol-brine systems were determined in the composition range pertinent to oil recovery. A general relationship was found between apparent viscosity and phase behavior for lamellar liquid crystals of high water content (>85%) and low viscosity and their dispersions which were prominent in these systems. A simple model was developed to explain the transition from lamellar to isotropic phases with increasing salinity in anionic surfactant systems, and with increasing temperature in nonionic surfactant systems. Basically, a decrease in tension at surfaces of plate-like particles allows more particles with smaller diameters to be formed with a resulting increase in the entropy of dispersion. The effect of added oil in several systems, including petroleum sulfonate, pure sulfonate (Texas-1), and salt-tolerant ethoxylated, propoxylated sulfate was studied. A general pattern of behavior was found at and above optimum salinity, where oil first caused transformation of liquid crystal to a water-continuous isotropic phase, then gradual inversion of this phase. At low salinities behavior was similar but without the inversion. Just below optimum, the transition was more complex with two three-phase regions found in the petroleum sulfonate and Texas-1 systems and a four-phase region in the salt-tolerant system. Lamellar liquid crystals in equilibrium with excess oil were discovered at equal water-to-oil ratios and low salinities on equilibrating petroleum sulfonates with longer chain hydrocarbons. These liquid crystals have low viscosities and exhibit low interfacial tensions with oil (about 0.02 dyne/cm) despite their minimal oil solubilization. Finally, phase behavior of alcohol-free Aerosol-OT-oil-brine systems were studied. The results with n-dodecane were plotted on a series of ternary diagrams for different salinities. Brine was found to be a satisfactory pseudocomponent in substantial portions of the diagrams for salinities well below and above optimum.