Water adsorption kinetics and contact angles of silica particles

Water adsorption was used to characterise the wettability of non-porous colloidal silica spheres with varying degrees of dehydration. In particular, water 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 infrared spectroscopy. Water uptake was found to be controlled by (1) the hydroxylation state of the silica particle surface and (2) the rate of water condensation to form multilayers. The processes that control water adsorption kinetics were fitted with first-order rate equations, thus enabling the concentration and reactivity of surface hydroxyl groups to be estimated. A Cassie approach was used to estimate the contact angle from water adsorption and infrared data; these were compared with contact angles determined by liquid penetration. A good correlation was observed between the hydroxylation-state of silica and the contact angle. An improved understanding of the interplay between surface chemistry, water adsorption and particle wettability has resulted.