Studying the Effects of pH, Ion Concentration and Ion Valency on the Silica/Water Interface Using Nonlinear Optical Spectroscopy

• Author / Creator

  Darlington, Akemi M

• The silica/water interface is complex and depends on a multitude of properties like surface charge, ion concentration, ion identity, and surface structure. Yet because silica is an insulator, the interface it creates with water is often difficult to study. However, the nonlinear optical methods, such as second harmonic generation (SHG) and sum frequency generation (SFG) spectroscopy, are capable of probing the buried interfaces like the silica/water interface. This thesis focuses on perturbing the silica/water interface through different actions and observing the response of the interfacial species using SHG and SFG. Firstly, the effect of experimental starting pH on the acid-base titration curves at the silica/water interface using SHG was explored. Depending on the starting pH, either two or three pKa sites were observed as the pH was varied. Vibrational SFG was then used to monitor the response of water molecules to changes in NaCl concentration and experimental starting pH. At high ionic strength, the pH dependent SFG of NaCl was different than that observed by SHG, indicating that they probed different aspects of the interface. Additionally, the experimental starting pH had little effect on the resultant SFG signal in contrast with the SHG results, which did depend on the experimental starting pH. This difference provided further support that SHG and ssp polarized SFG are sensitive to different features of the interface. We observed that the minimum intensity of the ssp polarized SFG depended on the salt concentration; specifically it shifted from ~ pH 6 to pH 8 upon increasing the salt from 0.1 M to 0.5 M. In contrast, the SHG minimum was always at pH 2 independent of salt concentration. The peak observed at 3200 cm-1 that dominates the ssp polarized SFG has been proposed to be sensitive to water molecules farther from the surface. Consequently we theorized that this minimum in SFG intensity corresponded to a minimum in the zeta potential, which has been shown to shift to higher pH with higher salt concentration. Next, the same polarization combinations of light were utilized in both SHG and SFG to do a direct comparison of the two techniques. Unlike the ssp SFG pH dependence, the pss polarized SFG pH dependence was similar to the pss polarized SHG. Despite the similarities in trend with pH, we observed that the magnitude of the intensity changes of the SHG signal could not be fully explained by the change in the intensity of the SFG of water molecules. This led us to conclude that there is another interfacial species that contributes directly to the SHG signal intensity, most likely the siloxides. Finally, the effect of calcium on both SHG and SFG was explored. The presence of CaCl2 caused the SHG and both ssp and pss polarized SFG to behave differently compared to each other. Specifically, the SHG signal in the presence of calcium increased in response to increasing pH until a maximum intensity where upon it decreased with further increases in pH. In contrast SFG exhibited the opposite behavior where the signal intensity in the SFG decreased until a minimum was reached and then increased as the pH was further increased. To understand this unusual pH dependence of the ssp SFG, the SFG range was probed at higher wavenumbers. This expanded range allowed for the observation of a CaOH mode that grew in at high pH that interfered with SFG of the water molecules. Consequently, we attributed the increase in SFG intensity at higher pH to the appearance of this CaOH species. If these CaOH species were forming right at the surface, then it should disrupt the surface water molecules, which can be visualized using pss polarized SFG. Indeed, the pss polarized SFG decreased with increasing pH suggesting that the formation of CaOH at the interface disrupts the surface water. Moreover, the presence of a new mode in the SFG provides further support that there is another interfacial species that contributes to the SHG intensity, which explains the unusual pH dependence of the latter. Through this work, we have a better understanding of how pH, ion concentration and ion identity affect the silica water interface. These results can be useful for modelling of geochemical phenomena and industrial processing of silica sands/soils.

• Subjects / Keywords

  • Silica/Water
  • Second Harmonic Generation
  • Sum Frequency Generation

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R32R3PC9R

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.