Probing Hydrogen Bonding and the Local Environment of Silanols on Silica Surfaces via Nuclear Spin Cross Polarization Dynamics

By studying 1 H→ 29 Si cross-polarization dynamics of two untreated and two “dry” silica gel samples (one evacuated at 25 °C and one evacuated at 200 °C), we find that all the surface silanols on the two untreated silicas are hydrogen bonded, either to the hydroxyl groups of adjacent silanol(s) or to water molecule(s). About 46% and 47% of the geminal silanols and 53% and 58% of the single silanols that were hydrogen bonded only to water in the two untreated silicas become non-hydrogen bonded on the two “dry” silica surfaces, but the remainder of the silanols of the untreated silicas (i.e., those hydrogen bonded to other silanols) remain hydrogen bonded to other silanols upon drying. The ratio of the number of hydrogen-bonding single silanols to the number of hydrogen-bonding geminal silanols is 17-to-1 for a Fisher silica surface evacuated at 25 °C and 16-to-1 for a Baker silica surface evacuated at 200 °C. These results can be explained in terms of a generalized silica surface model based on the β-cristobalite crystal structure.