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The Behaviour and Interactions of the Silica/Water Interface Studied through Nonlinear Optics
- Author / Creator
- Rehl, Benjamin
The chemistry of silica in contact with water is rich and diverse. These two substances are among the most ubiquitous on the surface of the Earth. Yet, despite many investigations into the interactions between silica and water, there is still more to discover. In aqueous solution, the silica surface can interact with itself, water, and ions. These interfacial processes are linked to the behaviour of water bound to the surface. Such water and related interfacial phenomena are difficult to study since silica, an insulator, is not amenable to electrochemical techniques, and conventional spectroscopic methods are overwhelmed by bulk responses. For this reason, nonlinear optical techniques, such as sum frequency generation (SFG) spectroscopy, have proven useful due to their surface specificity. This thesis focuses on understanding the response of interfacial water near silica in the presence of ions or solution pH. Sum frequency generation is primarily used in this thesis to measure the silica/water interface, however, complementary techniques such as second harmonic generation (SHG) and streaming potential are also used.
Under the model of the 𝜒(3) method, both SFG and SHG are believed to measure the amount of aligned waters at the silica/water interface, yet different pH dependent behaviours have been observed by these techniques. Through a comparison of nonresonant SHG and resonant SFG, we aimed to shed light on this difference. It was found that the nonresonant signal generated at the silica/air interface was substantial, which led to the conclusion that both the silica and aligned waters contribute strongly to the SHG signal. Furthermore, comparison of SFG spectral features and molecular dynamics simulations suggested the existence of at least two populations of water at the silica surface, which are oppositely aligned at low pH. Destructive interference to SHG signal from oppositely aligned waters may also result in the differences observed.
Although ions and pH are known to play large roles in the processes of the silica/water interface, the response of surface bound water remains elusive. To observe spectral changes arising from these surface waters, SFG spectra were deconvoluted into waters aligned by the static electric field of silica and waters aligned by hydrogen bonding to silica. To do so, complex spectra were obtained from the intensity measurements by using the maximum entropy method, and then compared to zeta potential ζ, measurements at the same interface. An orientation flip of waters resonating at low wavenumbers (less than 3200 cm-1) was observed as ionic strength was increased. In a similar manner, the surface water response to changes in solution pH were investigated. A change in water orientation contributing to the lower wavenumbers in the SFG surface spectra was again observed. The surface spectral features were assigned to hydrogen bond donors and acceptors and comparisons of the pH-dependent and ionic strength-dependent trends were made to macroscopic properties such as the metastability of silica colloids near their point of zero charge.
The acetonitrile-water mixture is used in chemical separation techniques such as hydrophilic interaction chromatography (HILIC), which employs silica as a stationary phase. The structure of the interface determines the retention times of the technique. Although pH can be varied to tune the separations, it is difficult to predict the retention times of some solutes when the pH is increased to high values. SFG spectra at the silica/acetonitrile-water interface of the methyl, water, and nitrile stretching regions demonstrated a change to the interfacial structure occurring at pH 10, which was highlighted by a sudden loss in signal originating from acetonitrile. Through orientation analysis of the SFG spectra, it was found that interfacial acetonitrile did not reorient over the pH adjustment, but rather was displaced from the interface by water. These observations may aid in predicting HILIC retention times at high solution pH.
These nonlinear optical studies serve to further the knowledge and understanding of the silica/water interface. As silica is a highly studied, model surface, the observations and methodologies within this thesis may prove insightful for understanding other mineral oxides or charged surfaces in contact with water, and may improve the understanding of SHG and SFG at these interfaces.
- Graduation date
- Fall 2020
- Type of Item
- Doctor of Philosophy
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