THE EFFECT OF THERMAL-TREATMENT OF SILICA-GEL ON ITS SURFACE FREE-ENERGY COMPONENTS

By applying the wicking method the values of Lifshitz-van der Waals, g amma(s)(LW), as well as acid-base: electron-donor, gamma(s)(-), and el ectron-acceptor, gamma(s)(+), components of surface free energy were d etermined for thermally pretreated silica gel samples. A slight increa se in gamma(s)(LW) (from 42.0 to 47.7 mJ m(-2)) was observed for 400 d egrees C pretreatment, followed by a decrease (to 45.0 mJ m(-2)) for t he samples treated at higher temperatures (600 and 800 degrees C). The elimination of surface silanols (i.e. main adsorption sites) resulted in essential changes of acid-base component values. The electron-dono r (Lewis base) properties of the silica surface were found to dominate . The interactions had weakened with increasing temperature of pretrea tment. The gamma(s)(-) value decreased from 58.4 to 13.8 mJ m(-2), and this was accompanied by a small increase in electron-acceptor compone nt (from 0.2 to 3.1 mJ m(-2)). Because the silanol groups on the silic a surface are commonly considered as electron-acceptor adsorption cent ers, the observed changes in the acid-base interactions may be interpr eted in terms of adsorption of water molecules, which form hydrogen bo nds with oxygen atoms of surface silanols. Such a model of water molec ule adsorption is in agreement with the literature data. It is interes ting that the decrease in gamma(s)(-) is practically linear with decre ase in the number of -OH groups between 200 and 800 degrees C. Taking into account the minimum value of the hydrogen bonding energy (10 kJ m ol(-1)) and the determined values of acid-base components, the number of surface electron-donor and electron-acceptor silanols was estimated . Good agreement was found with the literature data. (C) 1998 Elsevier Science B.V.