Bimodal or Trimodal? The Influence of Starting pH on Site Identity and Distribution at the Low Salt Aqueous/Silica Interface

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 similar to 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.