Understanding Bulk and Interfacial Properties of Octane-Water-Silica Systems Using Monte Carlo Simulation
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In this dissertation, we use Monte Carlo simulation to study the bulk and interfacial properties of a model octane-water-silica system. We first investigate the effect of temperature, pressure, and composition on the bulk saturation properties of the octane-water system. We trace the water-rich liquid-vapor, octane-rich liquid-vapor, and liquid-liquid saturation curves for the binary mixture over a wide range of temperatures, pressures, and compositions. These curves are constructed via a combination of direct grand canonical simulation and various expanded ensemble schemes. We also study the wetting properties of the octane-water-silica system, including the octane-water interfacial tension and the contact angle of a water droplet on a silica substrate in a mother octane fluid, over a wide range of temperatures, pressures, and fluid compositions. Two versions of the interface potential approach are employed in this work: 1) water spreading interface potential, which is defined as the surface excess free energy associated with the growth of a water film from a silica surface in the presence of octane; 2) octane spreading interface potential, which is defined as the surface excess free energy associated with the growth of an octane film from a silica surface in the presence of water. These two approaches provide a direct measure of water and octane spreading coefficients, respectively, which allow us to calculate interfacial properties, such as contact angle and octane-water surface tension. In the next step, we investigate the effect of the hydrophilicity of the silica surface on the wetting behavior. The results show that when continuously increasing the hydrophilicity scale, the contact angle of the water droplet on the silica/octane interface first increases then decreases. This change in the wettability can be related to the manner in which water molecules interact with and organize near the silica surface. The structure of the interfacial water for varying hydrophilicity is investigated in terms of density profiles, in-plane radial distribution function and orientation distribution of interfacial water molecules. Finally, we study the effect of acetic acid on the wetting behavior. The results show that the adsorption of acetic acid at the octane/water interface reduces the oil-water interfacial tension while adsorption at the silica surface shifts the wetting condition to more oil-wet.