X-ray study of ice and water on surfaces and in aqueous solutions [electronic resource]

Since antiquity, water has been considered as the most important substance on Earth. In fact, the abundance of liquid water on this planet allowed for the emergence and evolution of life. At the molecular level, the structure of an individual water molecule may seem simple; however, the intermolecular interaction between water molecules in the form of hydrogen bonding gives rise to water's various properties that can be considered anomalous when compared to other liquids. In this dissertation, several synchrotron x-ray techniques were used to investigate water in its liquid and solid phases in terms of its structure in and interaction with different model systems that mimic the environments in which water is usually found in nature. First, the effect of ion solvation on the local structure of water was explored using x-ray absorption spectroscopy (XAS) and small angle x-ray scattering (SAXS). There is a charge density dependence of the ability of cations in the restructuring of the water molecules in their vicinity. The monovalent Na+ tends to weaken H-bonds between water molecules, while the divalent Mg2+ and trivalent Al3+ form stable, highly ordered, high-density hydration shells. Fluoride (F-) anion was found to act as a nucleation site for the formation of tetrahedral, low-density structures in liquid water due to its ability to form H-bonds with water, similar to the effect of temperature decrease. XAS spectra of alkali halide solutions confirm that all alkali cations break water-water H-bonds to a varying degree while Cl- and Br- have no distinguishable effect on water and I- tends to break H-bonds. Second, the structure of water confined in AOT reverse micelles was studied using x-ray Raman scattering spectroscopy (XRS), which shows spectral changes that are indicative of an increasing fraction of weakened/distorted H-bonds. Third, the effect of ice adsorption on the electronic structure of hydrophobic methyl-terminated Si(111) surface was studied by C 1s XAS. Despite macroscopic observations of hydrophobicity (i.e. the formation of three-dimensional ice clusters on the surface to maximize water-water H-bonding and minimize surface-water interaction), water molecules were found to directly interact with and increase the asymmetric character of the excited state orbitals of the methyl groups. Finally, x-ray photoelectron spectroscopy (XPS) and XAS were used to study the growth and structure of ice on Pt(111). Pt 4f and O 1s XPS spectra show that the crystalline ice prepared by isothermally heating amorphous ice on Pt(111) forms three-dimensional crystallites, and a significant area of the first monolayer on Pt(111) is exposed to vacuum even at relatively high total coverages. In addition, experimental data from XAS and infrared reflection absorption spectroscopy (IRAS) confirm that the free OH groups on the surface of ice give rise to the XAS pre-edge feature, but there are also contributions from disordered H-bonding in bulk ice, likely due to the presence of various phases of amorphous ice.

Keywords: Chemistry.
Advisor: Nilsson Anders
Andersen Hans
Chidsey Christopher E. D. (Christopher Elisha Dunn)