The wettabilities of fine silica powder surfaces loaded with trimethylsilyl groups (TMS) were studied microscopically and macroscopically. Modification was carried out by the chemical reaction of hexamethyldisilazane (HMDS) molecules with surface silanols, and the TMS coverage was determined by the elemental analysis of carbon. The microscopic wettability was investigated by water vapor adsorption and the IR spectroscopic method. The macroscopic wettability was examined by various preferential dispersion tests. The results are summarized as follows: (1) HMDS molecules preferentially reacted with free type surface silanols. (2) The macroscopic wettability drastically changed at a TMS coverage of about 40%. (3) The effect of steric hindrance appeared clearly at a TMS coverage above 40%. Hence, a portion of residual surface silanols is shielded by TMS, and water molecules cannot access these sites. (4) In the case of TMS coverage below about 40%, cooperative adsorption takes place durring the first stage of adsorption. Then, by the occurrence of multilayer adsorption, a continuous two dimensional water layer is formed during the latter stage of adsorption. On the other hand, for the samples whose coverages are above about 40%, the adsorbed amount was about 65% of theoretical monolayer capacity in spite of high relative pressure at about P/P0=1. Therefore, such a continuous two-dimensional water layer is not formed due to the steric hindrance of the TMS, and multilayer adsorption cannot occur in this case. The surface property is estimated to be hydrophobic. (5) The results of preferential dispersion tests on macroscopic wettability were in agreement with the results of the water vapor affinity of microscopic wettability.
