In-situ spectroscopic investigations of molecular structure at aqueous/solid and aqueous/monolayer/solid interfaces

The ability to alter and control the surface properties of solids immersed in an aqueous phase has been a sought-after objective of scientists and industrialists for centuries. Key to these endeavors is the partitioning of surface active chemical species between a bulk aqueous phase and the solid surface. The adsorption and/or chemical reactivity of these species on a solid phase can lead to large changes in the properties of the interfacial region including: alteration of the interfacial electrical fields, changes in the hydrophobicity of the solid phase, inducement of interfacial water molecule structure, and disruption and reorientation of that structure as a function of interfacial conditions.; This dissertation reports on the in-situ studies of a variety of solid/liquid interfaces using the surface specific technique vibrational sum-frequency spectroscopy. The studies investigate how changes in the bulk aqueous phase constituents lead to large changes in the orientation and structuring of molecules in the interfacial region. These studies monitor the self-assembly of surface active molecules at the solid/liquid boundary leading to the formation of hydrophobic monolayers on the solid surface. The interaction of these monolayers with water molecules in the aqueous phase are examined in detail.; The studies are primarily focused on the interactions of water molecules with the sparingly-soluble, ionic solid CaF2, and the changes that occur to the interfacial properties as a function of aqueous phase composition. These studies are followed by an examination of the adsorption behavior of a series of long and short chain surfactant ions onto the CaF2 surface to elucidate the dependence of surfactant headgroup and chain length on the formation of hydrophobic monolayers on this surface. Adsorption of these ions induces significant changes in the properties of the interfacial region, leading to changes in the interfacial water molecule orientation and structuring. Further studies of the interaction of water molecules with well-ordered hydrophobic self-assembled monolayers on an oxide surface are then presented. The changes in the interactions between water molecules and these highly hydrophobic surfaces are examined as a function of monolayer coverage and order. Comparisons between monolayers produced at the salt/aqueous interface and the oxide/aqueous interface are then presented.