The electric double layer governs the processes of all charged surfaces in aqueous solutions; however, elucidating the structure of the water molecules is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation spectroscopy, heterodyned second harmonic generation, and streaming potential measurements. We find that the Stern layer water molecules and diffuse layer water molecules respond differently to pH changes: unlike the diffuse layer, whose water molecules remain net-oriented in one direction, water molecules in the Stern layer flip their net orientation as the solution pH is reduced from basic to acidic. We obtain an experimental estimate of the non-Gouy−Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the observed changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy−Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

Vibrational spectroscopy is a powerful tool for determining the local hydrogen-bonding environment. However, vibrational coupling present in H2O makes 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 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 pH is varied. For the Stern layer spectra, we observe differences in the frequency centers at pH 2 that are less significant at higher pH, signifying the presence of intermolecular coupling that can be related to the double-donor hydrogen-bonded structure of water. Furthermore, the differences in the evolution of the Stern layer of H2O and HOD suggest that the presence of intramolecular coupling in the former may distort the spectral response. Moreover, we observe that the evolution of HOD closely matches the pK a of the out-of-plane silanols predicted by previous molecular dynamic simulations.

The electric double layer governs the processes of all charged surfaces in aqueous solutions, however elucidating the structure of the water molecules is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation and heterodyned second harmonic generation techniques and streaming potential measurements. We find that the Stern layer water molecules and diffuse layer water molecules respond differently to pH changes: unlike the diffuse layer, whose water molecules remain net-oriented in one direction, water molecules in the Stern layer flip their net orientation as the solution pH is reduced from basic to acidic. We obtain an experimental estimate of the non-Gouy-Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions. We quantify how these contributions result in a considerable influence on the vibrational lineshapes. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

The electric double layer governs the processes of all charged surfaces in aqueous solutions, however elucidating the structure of the water molecules is challenging for even the most advanced spectroscopic techniques. Here, we present the individual Stern layer and diffuse layer OH stretching spectra at the silica/water interface in the presence of NaCl over a wide pH range using a combination of vibrational sum frequency generation and heterodyned second harmonic generation techniques and streaming potential measurements. We find that the Stern layer water molecules and diffuse layer water molecules respond differently to pH changes: unlike the diffuse layer, whose water molecules remain net-oriented in one direction, water molecules in the Stern layer flip their net orientation as the solution pH is reduced from basic to acidic. We obtain an experimental estimate of the non-Gouy-Chapman (Stern) potential contribution to the total potential drop across the insulator/electrolyte interface and discuss it in the context of dipolar, quadrupolar, and higher order potential contributions that vary with the observed changes in the net orientation of water in the Stern layer. Our findings show that a purely Gouy-Chapman (Stern) view is insufficient to accurately describe the electrical double layer of aqueous interfaces.

Isolating the contribution of silica in second harmonic generation (SHG) studies at the silica/water interface remains a challenge. Herein, we compare SHG intensities with previously measured zeta potentials and vibrational sum frequency generation (SFG) intensities to deconvolute the silica contribution in the SHG measurements. Under conditions that promote overcharging, the zeta potential and the SFG measurements follow a similar trend; however, SHG yields the opposite behavior. The results can only be rationalized by considering a significant pH-dependent increase in the silica contribution. Using a simplistic, yet physically motivated model, we demonstrate that silica can interfere either constructively or destructively with water. By computing the hyperpolarizabilities of neutral and deprotonated silica clusters with density functional theory [CAM-B3LYP/6-31+G­(d,p)], we reveal that one potential source of this pH-dependent response of silica is a change in the hyperpolarizability upon the deprotonation of surface sites, suggesting that SHG is directly sensitive to surface charging. The direct sensitivity of SHG to the surface charge density of the substrate suggests that SHG would be a powerful tool in studying other mineral oxides such as alumina and titania.