The influence of concentration on specific ion effects at the silica/water interface

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.