Studying Specific Ion Effects at the Silica/Aqueous Interface

Abstract

Nonlinear optical techniques are widely used for surface analysis, as they are surface specific and can be used to probe interfaces of insulators such as silica. Silica is commonly found in nature such as reservoirs of natural water and oils. Nonetheless, it is not clear how silica interacts with the water at a molecular level. In this thesis, second harmonic generation (SHG) and sum frequency generation (SFG) techniques were used to probe silica/aqueous interface under different environmental conditions. Firstly, broadband vibrational SFG spectroscopy was used to study specific ion effects (SIEs) and investigate how different ions affect the ordering of interfacial water molecules at the silica/aqueous interface as a function of pH. SFG spectra were measured for 0.5 M CsCl, KCl, NaCl and LiCl from pH 2-12 to specifically study the pH-dependent effect of monovalent cations on the interfacial water structure at a silica surface. SFG results show that at neutral pH~ 7.0, Cs+ and K+ adsorb more strongly than Li+ and Na+, following the direct Hofmeister series. The reverse Hofmeister trend was observed when pH was greater than 10 where Li+ had the greatest adsorption and Cs+ had the least adsorption. The results show that cation adsorption is dependent on the type of ions as well as the pH of the solution. Secondly, broadband vibrational SFG spectroscopy and non-resonant SHG spectroscopy were used to investigate the effect of 0.1 M NaHCO3, one of the most commonly found salts in natural water and in industrial water used in oil sands. SFG and SHG of 0.1 M NaHCO3 were compared with 0.1 M NaCl, a salt which has been previously investigated by our group. SFG results show that HCO3- anions likely partition within diffuse layer towards siloxide-Na+ coordinated surface rather than directly adsorbing to silica surface. Overall, both of SIEs results could be used to better understand many environmental, geochemical and industrial processes and applied in modeling of pollutant transport in water treatment.