PROBING HYDROGEN-BONDING AND THE LOCAL ENVIRONMENT OF SILANOLS ON SILICA SURFACES VIA NUCLEAR-SPIN CROSS-POLARIZATION DYNAMICS

By studying H-1-->Si-29 cross-polarization dynamics of two untreated a nd two ''dry'' silica gel samples (one evacuated at 25 degrees C and o ne evacuated at 200 degrees C), we find that all the surface silanols on the two untreated silicas are hydrogen bonded, either to the hydrox yl groups of adjacent silanol(s) or to water molecule(s). About 46% an d 47% of the geminal silanols and 53% and 58% of the single silanols t hat were hydrogen bonded only to water in the two untreated silicas be come non-hydrogen bonded on the two ''dry'' silica surfaces, but the r emainder of the silanols of the untreated silicas (i.e., those hydroge n bonded to other silanols) remain hydrogen bonded to other silanols u pon drying. The ratio of the number of hydrogen-bonding single silanol s to the number of hydrogen-bonding geminal silanols is 17-to-1 for a Fisher silica surface evacuated at 25 degrees C and 16-to-1 for a Bake r silica surface evacuated at 200 degrees C. These results can be expl ained in terms of a generalized silica Surface model based on the beta -cristobalite crystal structure.