A DETAILED MODEL OF LOCAL-STRUCTURE AND SILANOL HYDROGEN BANDING OF SILICA-GEL SURFACES

A refined and generalized version of a previously suggested model of t he silica surface, in which geminal silanols are situated on surface s egments similar to (100)-type faces of the beta-cristobalite structure and single silanols are situated on surface segments similar to corre sponding (111)-type faces, is supported by extensive spectroscopic dat a. In this model single silanols on the same (111)-type surface segmen t cannot form hydrogen bonds with each other. Whether or not adjacent geminal silanols on the same (100)-type surface segment can form hydro gen bonds with each other depends on the relative orientation of their hydroxyl groups. When two surface segments of either (100)- or (111)- type intersect convexly, hydroxyl groups cannot participate in hydroge n bonding across the intersection; but when two surface segments inter sect concavely, those silanols situated at the intersection can form h ydrogen bonds with their counterparts across the intersection. All the hydrogen-bonding silanols in this generalized beta-cristobalite model have a common feature: when any two silanols are hydrogen bonded ra e ach other, the two silicon atoms containing them are also situated on the same (100)-type surface segment. This idealized structure of the s urface of silica gel, which is clearly known from X-ray diffraction to be an amorphous material, may be distorted for various thermodynamic or kinetic reasons during its formation; therefore, a wide range of hy drogen-bonding strengths between lure hydroxyls is likely on a real si lica gel surface. The generalized beta-cristobalite surface model can also explain the reversible dehydroxylation and rehydroxylation proces ses on silica surfaces. Both single and geminal silanols participating in hydrogen bonding are most easily dehydroxylated under evacuation a t temperatures between 170 and 450 OC and form low-strain bicyclo[3.3. 0]octasiloxane rings. The mode of dehydroxylation on a silica surface undergoes a transformation between 450 and 650 degrees C, yielding hig hly strained trisiloxane rings for dehydroxylation at T greater than o r equal to 650 degrees C.