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Water at the silica surface : effect of ions and temperature Lovering, Kaitlin

Abstract

This dissertation studies the silica/water interface using sum frequency generation spectroscopy. The effects of alkali chloride ions and temperature on the hydrogen bonding network at the interface are examined. We observed that the structure of water in the Stern layer depends on the identity of the cation. The ability of a cation to displace the hydration water on silica surface is in the order of Mg²⁺ > Ca²⁺ > Li⁺ > Na⁺, consistent with the trend of the acid dissociation constant of the salt. We conclude that ions with a high pKa, such as Mg²⁺ and Ca²⁺, have a local electrostatic field strong enough to polarize water molecules in the hydration shells of the ions. These partially hydrolyzed water molecules form linkages with the negative charges on the silica, forming solvent shared ion pairs. During freezing of pure water, we observed a transient phase of ice at water/mineral interfaces, which had enhanced IR-visible sum frequency generation intensity for several minutes. Most forms of ice are centrosymmetric but a possible explanation of for the transient phase is the formation of stacking-disordered ice during the freezing process. Stacking-disordered ice, which has only been observed in the bulk ice at temperatures lower than -20 °C, is a random mixture of layers of hexagonal ice and cubic ice. The transient phase at the ice/mineral interface was observed at temperatures as high as -1 °C. This observation suggests that the mineral surface may play a role in promoting the formation of the stacking-disordered ice at the interface. The effect of ions during freezing at the silica/water interface was investigated. Ice is the first phase to form. NaCl·2H₂O forms below the eutectic temperature, indicating that the formation and growth of ice does not push the ions out of the interfacial region. We compared the surface freezing diagram with the bulk equilibrium phase diagram of aqueous sodium chloride solutions. Although the concentration of ions is higher at a charged surface, we observe that freezing point depression at the surface is analogous to freezing point depression for homogeneous freezing and bulk equilibrium phase diagram.

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