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Studies of water and solvents at liquid/solid interfaces by sum frequency generation vibrational spectroscopy Yang, Zheng


This dissertation studies the surface chemistry of water and organic solvents at liquid/mineral interfaces using IR-visible sum frequency generation (SFG) vibrational spectroscopy. The solvents studied include pentane, heptane, tetradecane and toluene, and the minerals include silica and mica. These liquid/mineral interfaces are relevant for environmental and industrial processes, such as ice nucleation and oilsands extraction. Structures of water at water/silica interfaces were studied in the presence of alkali chloride in solution. Perturbations of the interfacial water structures were observed with NaCl concentrations as low as 1x10⁻⁴ M. Different alkali cations produce different magnitudes of perturbation, with K⁺ > Li⁺ > Na⁺. This order was explained by the different effective ionic radii and electrostatic interactions between the cations and silica surfaces. The adsorption of water at solvent/silica interfaces was studied at room temperature. A water layer without detectable free OHs was discovered at toluene/silica interface. This water layer showed resistance against further adsorption of water molecules and was very stable at room temperature. However, similar structure of water was not observed at heptane/silica interfaces. Water structures on mica were studied with atmospherically relevant sulphuric acid concentrations. Experimental data showed that ordered water structures on mica completely disappeared when the concentration of sulfuric acid reached 5 mol/L. The results partially explain why sulfuric acid coatings influence the ice nucleation properties of mineral dust particles in the atmosphere. The competitive adsorption of toluene and n-alkane at solvent/silica interfaces was studied. The surface coverage of toluene for toluene-pentane, toluene-heptane, and toluene-tetradecane mixtures were measured over the complete mole fraction range from 0 to 1. Overall, toluene competes favorably on silica, but the molar adsorption free energy of alkanes increases as the chain length increases. Finally, experiments were conducted to study the effect of interfacial water on bitumen liberation from mineral surfaces in water. The bitumen liberation rate increases when the water content between bitumen and the mineral increases. The liberation also highly depends on the surface properties of minerals. At the same water content, the rate of bitumen displacement on different mineral surfaces is: freshly cleaved mica > rinsed mica > silica.

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