A Detailed Model of Local Structure and Silanol Hydrogen Bonding of Silica Gel Surfaces

Abstract

A refined and generalized version of a previously suggested model of the silica surface, in which geminal silanols are situated on surface segments similar to (100)-type faces of the β-cristobalite structure and single silanols are situated on surface segments similar to corresponding (111)-type faces, is supported by extensive spectroscopic data. In this model single silanols on the same (111)-type surface segment cannot form hydrogen bonds with each other. Whether or not adjacent geminal silanols on the same (100)-type surface segment can form hydrogen 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 hydrogen bonding across the intersection; but when two surface segments intersect concavely, those silanols situated at the intersection can form hydrogen bonds with their counterparts across the intersection. All the hydrogen-bonding silanols in this generalized β-cristobalite model have a common feature: when any two silanols are hydrogen bonded to each other, the two silicon atoms containing them are also situated on the same (100)-type surface segment. This idealized structure of the surface 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 hydrogen-bonding strengths between two hydroxyls is likely on a real silica gel surface. The generalized β-cristobalite surface model can also explain the reversible dehydroxylation and rehydroxylation processes on silica surfaces. Both single and geminal silanols participating in hydrogen bonding are most easily dehydroxylated under evacuation at temperatures between 170 and 450 °C 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 °C, yielding highly strained trisiloxane rings for dehydroxylation at T ≥ 650 °C.