Interactions of Silica Surfaces

Adhesion, friction, and colloidal forces in air and aqueous salt solutions have been measured between various silica surfaces prepared by depositing amorphous but highly smooth silica films on mica. The results show four interesting and interrelated phenomena: (i) The adhesion of silica surfaces in air increases slowly with contact time, especially in humid air where the "contacting" surfaces become separated by an ∼20-Å-thick layer of hydrated silica or silica gel. (ii) The friction of two silica surfaces exhibits large sticking or "stiction" spikes, whose magnitude increases in the presence of water and when the surfaces are kept in contact longer before sliding. (iii) The non-DLVO repulsion commonly seen at short range (<40 Å) between silica surfaces immersed in aqueous solutions is monotonically repulsive, with no oscillatory component, and is quite unlike theoretical expectations and previous measurements of forces due to solvent structure. (iv) Dynamic contact angle measurements reveal time-dependent effects which cannot be due to a fixed surface chemical heterogeneity or roughness. The results indicate that silica surfaces undergo slow structural and chemical changes during interactions with water and with each other. More specifically, we propose that the unusual interfacial and colloidal properties of silica are due, not to hydration effects, but to the presence of an ∼10-Åthick gel-like layer of protruding silanol and silicilic acid groups that grow on the surfaces in the presence of water. These protruding groups react chemically (sinter) with similar groups located on an opposing surface and give rise to the unusual time-dependent adhesion, friction, and non-DLVO forces observed. Concerning the effects on colloidal interactions, the surface gel-layer effectively shifts the OHP outward and adds a monotonic short-range polymer-like steric repulsion to the DLVO interaction. The mechanism proposed here is quite different from the commonly accepted one, in which modified water structure at the silica surface is believed to give rise to a repulsive "hydration force." The proposed mechanism in terms of a surface layer of silica gel is consistent with the known surface chemistry of silica and accounts for the results reported here and also for many other unusual surface and colloidal properties of silica.