Influence of the Hydrogen Bonding Environment on Vibrational Coupling in the Electrical Double Layer at the Silica/Aqueous Interface

Vibrational spectroscopy is a powerful tool for determining the local hydrogen-bonding environment. However, vibrational coupling present in H2O can make it difficult to relate vibrational spectra to a molecular description of the system. While numerous bulk studies have shed light on this phenomenon, the influence of both intra- and intermolecular vibrational coupling on the resulting electrical double layer spectra at buried interfaces remains largely unexplored. By utilizing the combination of vibrational sum frequency generation (vSFG), electrokinetic measurements, and the maximum entropy method on isotopically diluted water (HOD) at the silica/aqueous interface, we reveal the influence of vibrational coupling on the Stern and diffuse layer spectra as the surface charge density is varied. By comparing our HOD spectra with the corresponding H2O spectra from pH 10 to 2, we find that the diffuse layer spectra of H2O are dominated by both intra- and intermolecular coupling leading to significant differences in the H2O and HOD spectra. In contrast, the spectral response of HOD and H2O in the Stern layer as a function of pH is similar, providing strong evidence that the O-H oscillators in the Stern layer are evolving in a similar manner for both the H2O and HOD systems. However, we observe differences in the frequency centers at low pH that are less significant at higher pH suggesting that intermolecular coupling in the Stern layer is evolving as the surface charge density is varied.