The wettability and surface structure of precipitated silica surface loaded trimethylsilyl groups were investigated. By measuring water and nitrogen adsorptions, it was found that the non-treated sample had micro pores. The pores of diameter less than 0.8nm were not modified, while the pores of diameter from 0.8 to 0.9nm were modified by trimethylsillyl groups and closed by them. From these results, it was assumed that the former pores would become sites for water vapor as surface silanol groups after modification, and the latter pores would be independent of water adsorption. The character of the bare silica surface disappeared above a surface conversion rate of 33percent. These results are deduced from a neopentane adsorption experiment. Furthermore, the water a adsorption capacity near the saturated pressure, as an index of micro-wettability, did not reach theoretical monolayer capacity above the same conversion rate (33percent). It was also indicated that the preferential dispersion property as an index of macro-wettability drastically changed at a conversion rate of about 33percent. From these results, it was demonstrated that the wettability is closely related to the surface structure interfering with the adsorption of water.
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