Molecular Interaction in Aqueous Solutions : Effects of Temperature and Solutes on Surface Structure and Hydrogen Bonding in Water
Abstract: Intermolecular forces in liquids can give rise to complex emergent structures, particularly in water, where hydrogen bonding causes the formation of extended networks, and surfaces, where intermolecular structures can be distinctly different from the bulk, leading to unique properties and behaviours. While such structures have been studied in detail, it is still relatively unknown how such structures respond and adapt to external changes to their environment. In this thesis, the structure formation behaviour, particularly at the water interface and in the hydrogen bond network, in response to external tunable parameters are investigated using a combination of spectroscopic techniques. Surface-sensitive X-ray photoelectron spectroscopy and Auger spectroscopy are employed to study samples of different sizes, ranging from single gas-phase water molecules where intermolecular interactions are negligible, to the infinite liquid bulk with its fully extended hydrogen bond network. Particular emphasis is placed on the influence of temperature on structural aspects of the studied systems, how they affect the hydrogen bond network as well as the surface enrichment of solutes.The first part focuses on structure formation at surfaces, and in the case of solutes in water we find that elevated temperature increases the surface enrichment of halide ions, and a diverse set of behaviour for cations depending on the nature of the counter ion. These results are surprising since it was previously predicted that surface enrichment should decrease with temperature, and our results superficially appear to contradict the notion that any emergent structure should become washed out at higher temperature as entropy and disorder begin to dominate. In addition, results on the differences between the surface composition of wet and dry saline aerosols are presented, where we find notable structural differences linked to the different efflorescence points of salts during the drying process.In the second part, results from studies exploring different aspects of the hydrogen bond network are presented, which are probed via the post-ionisation decay observed in the Auger spectrum. Firstly, the hydrogen bond network of water is studied at different temperatures and system sizes. We find both temperature, and confinement in the form of clusters, to measurably weaken the hydrogen bond network compared with the bulk liquid. Secondly, the influence of hydrophobic functional groups is studied in liquid methanol and mixed liquids containing the cryoprotectants glycerol and dimethyl sulfoxide. The presence of the hydrophobic groups leads to a fracturing of the intermolecular network into smaller domains of hydrophilic and hydrophobic groups.
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