Physical and chemical properties of inactive surface hydrogen-bonded hydroxyl groups
The physical and chemical properties of surface hydrogen-bonded OH groups, formed by the hydrothermal reaction of silica hydrogels, xerogels, precipitated silicas (made with sodium silicate in the liquid phase at pH 11 and > 373 K) and aerosils, have been studied. In contrast to free OH groups, surface hydrogen-bonded OH groups, having an absorption band at 3500 cm–1, have low enthalpies of adsorption of water, methanol and methyl iodide, can be reversibly dehydroxylated and rehydroxylated by the adsorption of water vapour, and can be easily changed to OD groups by the adsorption of heavy water. These OH groups cannot be methoxylated with chlorotrimethylsilane or methanol. Cations such as Co2+ do not seem to be ion exchanged with the protons of these OH groups.