Wettability of fine silica powder surfaces modified with several normal alcohols

The chain length effects of chemical modification with various normal alcohols on the wettability of silica have been studied. The modifications were carried out by chemical reaction of alcohol molecules with surface silanols. The amount of silanols on sample surface was obtained by the Grignard method, and the amount of modifiers was determined by thermal analysis. As a result, surfaces were designed to have various concentrations of normal alkoxyl groups. The wettability of these well-defined samples for water were investigated by water vapor adsorption, heat of immersion and various preferential dispersion tests. The results are summarized as follows: (1) the wettability of samples with modifiers of carbon number above eight markedly changes at the modification ratio of about 20%; (2) the samples chemisorbed alcohol with less than eight carbon atoms, the modification ratios of above 20% are required to give hydrophilic surfaces. Because the bulkiness effect of modifiers on wettability decreases with reduction of chain length; (3) in the cases of hydrophobic samples, cooperative water adsorption takes place at the first stage of adsorption. With the occurrence of multilayer adsorption, a continuous two dimensional water layer is formed at the latter stage of adsorption. In the cases of the hydrophobic samples, the adsorbed amount was less than theoretical monolayer capacity in spite of high relative pressure at about P/P-0 = 1. Therefore, such a continuous two dimensional water layer is not formed as a result of the steric hindrance of the modifiers. In this case, multilayer adsorption dose not occur. The surface property is estimated to be hydrophobic; (4) the results of preferential dispersion tests as the evalution of macroscopic wettability are in agreement with the results of water vapor affinity and heat of immersion as the nanoscopic wettability. (C) 1999 Elsevier Science B.V. All rights reserved.