Dynamic aspects of emulsion stability

This dissertation encompasses novel theoretical, experimental and computational advances in the understanding of the transient behavior of emulsions undergoing phase separation. Firstly, the kinetics of dissolution of single drops of pure hydrocarbons and their mixtures in aqueous solutions of a nonionic surfactant (C12E8) was studied theoretically and experimentally. At moderate surfactant concentrations, both interfacial resistance to mass transfer and diffusion of micelles carrying solubilized oil dictated the solubilization rates. At high surfactant concentrations, the onset of spontaneously generated convection in the aqueous phase was observed. In such cases, convection aided mass transport in the bulk phase and reduced the diffusional resistance, thus leaving interfacial resistance as rate-controlling. Data suggest that the adsorption/fusion of micelles at the interfaces was the elementary molecular step within the kinetic mechanism that dictated the interfacial resistance to mass transport. Experimental results for the solubilization of single droplets were correlated without adjustable parameters with a plausible mass transfer model in agreement with such mechanism. This model was extended to polydisperse emulsions of hydrocarbons in nonionic surfactant solutions, and it was successfully applied to correlate data from experiments on solubilization in emulsions, Ostwald ripening and compositional ripening. In addition, a new experimental technique based on nuclear magnetic resonance (NMR) was developed to characterize emulsions. The contributions of this work include a novel theory to interpret results from NMR restricted diffusion experiments and an original procedure that couples diffusion measurements with transverse relaxation rate experiments to determine drop size distributions with arbitrary shape, the water/oil ratio of the emulsion and the rate of decay of magnetization at the interfaces, i.e., the surface relaxivity. It is shown that the procedure also allows identification of whether the dispersion is oil-in-water (O/W) or water-in-oil (W/O) in a straightforward manner and is suitable to evaluate changes in drop size distributions in time steps of approximately five minutes without manipulation or destruction of the sample. Finally, the effect of chemicals of known structure and composition (alkylphenol polyalkoxylated resins and polyurethanes) on the stability and properties of brine-in-crude-oil emulsions was assessed experimentally. (Abstract shortened by UMI.