Speciation of Silanol Groups on Commercial Precipitated Silicas via IR Spectroscopy

The surface chemistry of amorphous silica is governed by the concentration, environment, and accessibility of silanol groups, yet quantitative comparison between closely related silica samples remains challenging due to surface heterogeneity and overlapping spectroscopic signatures. In this study, infrared spectroscopy is combined with H-D isotopic exchange and size-selective molecular probes to speciate isolated, internal, and associated silanol populations across a series of silica samples with similar composition but differing surface characteristics. A dilution effect in surface silanol density with increasing specific surface area was observed, and notable batch-to-batch variability further highlights the sensitivity of silanol speciation to subtle differences in material preparation. Nominally similar silicas can exhibit pronounced differences in silanol distribution and accessibility, including measurable batch-to-batch variability that is not reflected by BET surface area or total hydroxyl content. Internal and associated silanols represent a substantial fraction of the total hydroxyl population and display variable accessibility to probe molecules. These findings suggest that global surface descriptors alone are insufficient for describing silica surface chemistry and that explicit consideration of silanol speciation and accessibility is required for reliable interpretation of spectroscopic and chemical measurements.