Fluorine in silica gels.

Fluorine has a remarkable and diverse action in silica sols, gels and gel-derived glasses. Introduced in ionic form, it causes more rapid gelation, it reduces surface area and water adsorption of dry gels, and it helps to eliminate bubble formation during sintering and reheating of gel-derived glasses. The mechanisms for these actions of the fluoride ion are reviewed, with a more detailed discussion of the rate of gelation. Several previously held views of the way fluoride accelerates gel formation are compared with a new suggestion that it is caused by an electrostatic attraction between the proton of the OH of silicic acid and F(-) present as Si-F groups.