Using nonresonant second harmonic generation spectroscopy, we have monitored the change in surface charge density of the silica/water interface over a broad pH range in the presence of different alkali chlorides. Planar silica is known to possess two types of surface sites with pKa values of ∼4 and ∼9, which are attributed to different solvation environments of the silanols. We report that varying the alkali chloride electrolyte significantly changes the effective acid dissociation constant (pKa(eff)) for the less acidic silanol groups, with the silica/NaClaq and silica/CsClaq interfaces exhibiting the lowest and highest pKa(eff) values of 8.3(1) and 10.8(1), respectively. Additionally, the relative populations of the two silanol groups are also very sensitive to the electrolyte identity. The greatest percentage of acidic silanol groups was 60(2)% for the silica/LiClaq interface in contrast to the lowest value of 20(2)% for the silica/NaClaq interface. We attribute these changes in the bimodal behavior to the influence of alkali ions on the interfacial water structure and its corresponding effect on surface acidity.

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 ...

Specific ion effects (SIEs) are known to influence the acid/base behavior of silica and the interfacial structure of water, yet evidence of the effect of pH on SIEs is lacking. Here broadband vibrational sum frequency generation (SFG) spectroscopy was used to study SIEs on the water structure at the electrical double layer (EDL) of silica as a function of pH and monovalent cation identity from pH 2-12 at 0.5 M salt concentration. SFG results indicate a direct Hofmeister series of cation adsorption at pH 8 (Li < Na < K < Cs), with an inversion in this series occurring at pH > 10. In addition, an inversion in SFG intensity trends also occurred at pH < 6, which was attributed to contributions from asymmetric cation hydration and EDL overcharging. The highly pH-dependent SIEs for silica/water have implications for EDL models that often assume pH-independent parameters.

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.