Water adsorption kinetics and contact angles of silica particles

Water adsorption was used to characterise the wettability of non-porous col loidal silica spheres with varying degrees of dehydration. In particular, w ater adsorption kinetics at saturated vapour pressure were correlated with advancing water contact angles determined by capillary penetration and the surface coverage of hydroxyl groups determined by diffuse reflectance infra red spectroscopy. Water uptake was found to be controlled by (I) the hydrox ylation state of the silica particle surface and (2) the rate of water cond ensation to form multilayers. The processes that control water adsorption k inetics were fitted with first-order rate equations, thus enabling the conc entration and reactivity of surface hydroxyl groups to be estimated. A Cass ie approach was used to estimate the contact angle from water adsorption an d infrared data; these were compared with contact angles determined by liqu id penetration. A good correlation was observed between the hydroxylation-s tate of silica and the contact angle. An improved understanding of the inte rplay between surface chemistry, water adsorption and particle wettability has resulted. (C) 2001 Elsevier Science B.V. All rights reserved.