Vibrational sum frequency generation (SFG) spectroscopy was utilized to distinguish different populations of water molecules within the electric double layer (EDL) at the silica/water interface. By systematically varying the electrolyte concentration, surface deprotonation, and SFG polarization combinations, we provide evidence of two regions of water molecules that have distinct pH-dependent behavior when the Stern layer is present (with onset between 10 and 100 mM NaCl). For example, water molecules near the surface in the Stern layer can be probed by the pss polarization combination, while other polarization combinations (ssp and ppp) predominantly probe water molecules further from the surface in the diffuse part of the electrical double layer. For the water molecules adjacent to the surface within the Stern layer, upon increasing the pH from the point-of-zero charge of silica (pH ∼2) to higher values (pH ∼12), we observe an increase in alignment consistent with a more negative surface with increasing pH. In contrast, water molecules further from the surface appear to exhibit a net flip in orientation upon increasing the pH over the same range, which we attribute to the presence of the Stern layer and possible overcharging of the EDL at lower pH. The opposing pH-dependent behavior of water in these two regions sheds new light on our understanding of the water structure within the EDL at high salt concentrations when the Stern layer is present.

Second harmonic generation (SHG) is commonly employed to monitor processes at mineral oxide/liquid interfaces. Using SHG, we determine how the starting pH affects the acid−base chemistry of the silica/aqueous interface. We observe three different sites with pK a values of approximately 3.8, 5.2, and ∼9 (pK a -I, pK a -II, pK a -III, respectively), but the presence and relative abundance of these sites is very sensitive to the starting pH. For titrations initiated at pH 12, all three sites are observed, whereas only two sites are observed for titrations initiated at pH 2 or pH 7. Moreover, exposure to pH 2 facilitates the formation of pK a -II and pK a -III sites, while exposure to pH 7 results in pK a -I and pK a -III sites. Based on previous computational work, we assign these sites to three different hydrogen bonding environments at the interface including a hydrophobic site for the most acidic silanol corresponding to pK a -I. ■ INTRODUCTIONThe silica/water interface is one of the most environmentally and technologically relevant interfaces. The charged nature of this interface above pH 2, results in most processes at this interface involving electrostatic interactions. 1 These electrostatic interactions, which largely depend on the acid−base chemistry of silica, are critical to many geochemical, environmental and industrial processes. Consequently, to accurately predict both pollutant adsorption and transport 2,3 and interactions between analytes and glass substrates in biodiagnostics, 4 a complete picture of the silica interface is required. Furthermore, numerous geochemical studies on silicates have found that a layer of amorphous silica forms at the quartz/water 5 and silicate/water 6 interfaces, indicating that the reactivity of amorphous silica is relevant to a variety of geochemical systems. 5−7 Despite its importance, measuring the interfacial acid−base chemistry of silica over a wide pH range is challenging. Because silica is an insulator, techniques that measure the surface charge density through the conductivity of the material cannot be used. Potentiometric methods are amenable to silica, and consequently, there have been many studies that have looked at how the acid−base chemistry of silica colloids is perturbed by the addition of aqueous electrolytes. 8,9 More recently, X-ray photoelectron spectroscopy measurements have yielded the interfacial potential of colloidal silica. 10 However, one of the many difficulties in measuring colloidal silica is that it is unstable over a large pH range. 8 An alternative strategy involves utilizing planar silica and directly determining its acid−base chemistry with surface specific methods. Nonlinear optical techniques like second harmonic generation (SHG) and sum frequency generation (SFG) present unique advantages that include the ability to identify interfacial molecules based on their spectroscopic signatures, differentiate between molecules ordered at the interface versus those in the bulk solution, and study interfaces for a variety of materials including ...

Historically, different pH dependent behaviour at the mineral oxide/aqueous electrolyte interface has been observed by non-resonant second harmonic generation (SHG) and resonant sum frequency generation (SFG), despite a general understanding that both techniques are dominated by the response of water. Here we compare the two at the silica/aqueous interface at high salt concentration and as a function of pH to shed light on the origins of both measurements. From this comparison and SHG measurements at the silica/air interface, we conclude that SHG originates from the net order of water and the silica substrate, with the latter dominating the observed intensities below pH 6.5. In contrast, SFG is dominated by the higher SF activity, yet lower number density, of waters that contribute in the low wavenumber range, according to molecular dynamic simulations. Furthermore, spectral resolution in SFG of oppositely oriented water populations prevents cancellation of signal making it more difficult to relate SF intensity to the net order of water.

Second harmonic generation spectroscopy is a useful tool for monitoring changes in interfacial potential at buried insulator/liquid interfaces. Here we apply this technique to the silica/aqueous interface and monitor the changes in interfacial potential while varying the pH in the presence of different alkali halides at 0.1M concentration. Within the pH range explored, the bimodal distribution of acidic sites on planar silica is clearly observed, corresponding to two types of acidic SiOH groups. Comparing these data with previous work at 0.5M sheds light on whether the presence of the ions stabilizes the charged or neutral state of the surface sites. For the alkali chlorides, with the exception of NaCl, we observe that the presence of the alkali chlorides stabilize the less acidic site in the protonated (SiOH) rather than deprotonated (SiO(-)) form. This unusual influence of the cation is attributed to the combination of interactions at the interface between water, surface sites and the electrolyte. Overall, we observe that the influence of the alkali ion on the ratio of the two types of sites and their effective acid dissociation constants is minor at 0.1M, unlike that observed at 0.5M. In contrast, the influence of the anion on the cooperative dissociation of surface sites and their relative distribution is little affected upon decreasing the concentration, which indicates that these specific anion effects are prevalent in nature.

The molecular origin of overcharging at mineral oxide surfaces remains a cause of contention within the geochemistry, physics, and colloidal chemistry communities owing to competing "chemical" vs "physical" interpretations. Here, we combine vibrational sum frequency spectroscopy and streaming potential measurements to obtain molecular and macroscopic insights into the pH-dependent interactions of calcium ions with a fused silica surface. In 100 mM CaCl 2 electrolyte, we observe evidence of charge neutralization at pH~10.5, as deducted from a minimum in the interfacial water signal. Concurrently, adsorption of calcium hydroxide cations is inferred from the appearance of a spectral feature at ~3610 cm -1 . However, the interfacial water signal increases at higher pH, while adsorbed calcium hydroxide appears to remain constant, indicating that overcharging results from hydrated Ca 2+ ions present within the Stern layer. These findings suggest that both specific adsorption of hydrolyzed ions and ion-ion correlations of hydrated ions govern silica overcharging with increasing pH. File list (2) download file view on ChemRxiv Rashwan_Manuscript.pdf (1.77 MiB) download file view on ChemRxiv Rashwan_SupportingInformation.pdf (1.07 MiB)