Structure and Dynamics of Water Next to Mineral Surfaces

Abstract

Vibrational sum frequency generation (vSFG) spectroscopy is a second order non-linear technique that can efficiently and selectively investigate an interface between two media. vSFG spectroscopy has been frequently used to interrogate the mineral/water interface by probing the interfacial O−H stretch, which is an excellent reporter of its hydrogen bonding environment. My work, described in this dissertation, is focused on using steady-state and time-resolved vSFG spectroscopy to understand how (a) the type of mineral, (b) the crystalline faces of the mineral, (c) the surface charge of the mineral, and (d) the ionic strength of the bulk liquid affect the structure and the ultrafast dynamics of the mineral/water interface. The results obtained for alumina/water interface are compared with the more commonly studied silica/water interface. The Al2O3/H2O interface provides an ideal opportunity to study the behavior of water next to a positively charged, neutral, and negatively charged mineral surface since its point of zero charge (PZC) lies near the pH of neutral water (pH 6-8). In contrast, due to its low PZC (~pH 2), silica surface is usually neutral or negatively charged. Additionally, a-alumina is crystalline in structure and can be cut to expose different faces (e.g., 0001, 11 ̅02, 112 ̅0) which has been speculated to uniquely affect the ordering and the hydrogen bonding environment of its adjacent water molecules. Our vSFG spectra of the O-H stretch at the alumina/water interface revealed the presence of highly red-shifted 3000 cm-1 species, which is absent at the silica/water interface. With the help of DFT calculations, we assigned the red-shifted peak to the O-H stretch of strongly hydrogen bonded surface aluminol groups and/or interfacial water molecules that are hydrogen bonded to the mineral surface. The 3000 cm-1 species was present at both the Al2O3(0001) and the Al2O3(112 ̅0) surface, but was more prominent for the latter which resulted in the O-H stretch of the Al2O3(112 ̅0)/H2O interface to appear more red-shifted compared to the Al2O3(0001)/H2O interface. This observation provided us with an experimental evidence that the water next to the Al2O3(112 ̅0) surface is in a stronger hydrogen bonded environment than next to the Al2O3(0001) surface as predicted by Catalano’s X-ray reflectivity measurements. Additionally, IR pump - vSFG probe experiments were used to investigate the ultrafast vibrational dynamics of the O-H stretch at alumina/water interfaces. The vibrational dynamics at the Al2O3(112 ̅0)/H2O interface was observed to be fast (T1 = 100-130 fs) and not affected by the mineral surface charge (controlled by bulk pH) or the ionic strength (up to 0.5 M NaCl). In contrast, for the Al2O3(0001)/H2O interface, the vibrational dynamics was observed to be two times faster for the charged surface (T1 ~ 105 fs) compared to the neutral surface (T1 ~ 220 fs). This result provides further evidence that the water next to the Al2O3(112 ̅0) surface is more ordered and/or strongly hydrogen bonded compared to the water next to the Al2O3(0001) surface. The vibrational dynamics observed at the charged Al2O3(0001)/H2O interface (T1 ~105 fs) is faster than in bulk water and at the charged silica/water interface (T1 = ~200 fs). We speculate that the red-shifted 3000 cm-1 species present at the alumina/water interface plays a major role in the mechanism of vibrational relaxation since the 3000 cm-1 species is present at the alumina/water interface and not at the silica/water interface. The main mechanism of vibrational relaxation of the O-H stretch in bulk water is known to proceed via the Fermi-resonance coupling between the overtone of the water bend and the O-H stretch. The presence of red-shifted O-H species at the alumina/water interface could lead to better coupling between the O-H stretch and the water bend overtone and hence result in faster vibrational relaxation than in bulk water. Alternatively, a new pathway of vibrational relaxation via an ultrafast excited state proton transfer reaction could be operative for the alumina/water interface due to the presence of different types of O-H stretches (arising from aluminol groups and water molecules). Such a mechanism would not be possible at the silica interface due to the significantly lower density of surface bound hydroxyls. We are not able to determine the dominant mechanism for vibrational relaxation at the alumina/water interface with our current experiments and therefore, our future work will involve collaborations with theoretical groups in order to answer this question.