Inaccessible hydroxyl groups on silica are accessible in supercritical CO2

Ben McCool; Carl P Tripp

doi:10.1021/jp050192q

Inaccessible hydroxyl groups on silica are accessible in supercritical CO2

J Phys Chem B. 2005 May 12;109(18):8914-9. doi: 10.1021/jp050192q.

Authors

Ben McCool¹, Carl P Tripp

Affiliation

¹ Laboratory for Surface Science and Technology and Department of Chemistry, University of Maine, Orono, Maine 04469, USA.

PMID: 16852060
DOI: 10.1021/jp050192q

Abstract

The three main types of hydroxyl groups on a silica surface are classified as isolated, hydrogen bonded, and inaccessible. The isolated and hydrogen bonded groups are the most important as these readily exchange with D(2)O and thus are exposed to reactant molecules. However, it has generally been accepted that the inaccessible groups do not participate in surface reactions as only a small fraction of these groups exchange with D(2)O. It is shown that the inaccessible hydroxyl groups on nonporous fumed silica and mesoporous MCM-48 silica powders and films fully exchange with D(2)O and are reactive with octadecylydimethylchlorosilane when supercritical CO(2) is used as the solvent. Furthermore, it is found that the CO(2) penetrating the regions containing the inaccessible groups is not removed by simple evacuation but rather slowly diffuses from the silica over periods of months.