Halide-Induced Cooperative Acid–Base Behavior at a Negatively Charged Interface

Using second harmonic generation and sum frequency generation spectroscopy, we monitor the influence of sodium and potassium halides on acid–base processes at the negatively charged silica/aqueous electrolyte interface. We find that the two types of acidic silanols at the surface are very sensitive to the presence of halides in the aqueous phase. As the halide size increases, the pH at which half the more acidic sites are deprotonated (pH_0.5) shifts to lower pH. Conversely, the pH_0.5 of the less acidic sites shifts to higher pH with increasing halide size. We also observe titration curves of increasing sharpness as the halide size increases, indicative of positive cooperativity. Using a simple cooperative model, we find that the cooperative unit for the dissociation of more acidic surface sites is ∼1, 2, and 3 for the chloride, bromide, and iodide electrolytes, respectively, which reveals that the larger anions promote deprotonation among the more acidic silanol groups. We also find that the fraction of more acidic sites, proportional to the relative surface charge density at neutral pH, increases from 20% to 86% as the sodium halide is varied from chloride to iodide. As the percentage of more acidic sites and the surface charge at neutral pH increases, the effective acidity of the less acidic sites decreases, indicating that greater surface charge density renders the remaining silanol groups more difficult to deprotonate. As the relative amount of less acidic sites increases, their deprotonation events exhibit negative, rather than positive, cooperativity revealing charge repulsion between neighboring silanol groups.