THE CHANGE IN THE WATER-VAPOR AFFINITY OF FINE SILICA PARTICLES LOADED WITH TRIMETHYLSILYL GROUPS

Microscopic and macroscopic wettabilities of fine silica powder surfac es loaded with trimethylsilyl groups (TMS) were studied. The modificat ion was carried out by the chemical reaction of hexamethyldisilazane ( HMDS) molecules with surface silanols and the TMS coverage was determi ned by elemental analysis of carbon. The microscopic wettability was i nvestigated by water vapor adsorption and IR spectroscopic methods. Th e macroscopic wettability was examined by various preferential dispers ion tests. The results are summarized as follows. (1) HMDS molecules p referentially reacted with free type surface silanols. (2) The macrosc opic wettability drastically changed at a TMS coverage of about 40%. ( 3) The effect of steric hindrance appeared clearly at a TMS coverage a bove 40%. Hence, a part of the residual surface silanols is shielded b y TMS and water molecules will not access these sites. (4)In the case of TMS coverage below about 40%, cooperative adsorption takes place at the first stage of adsorption. Then by the occurrence of multilayer a dsorption, a continuous two-dimensional water layer is formed at the l atter stage of adsorption. On the Ether hand, in the case of coverage above about 40%, the adsorbed amount was about 65% of the theoretical monolayer capacity in spite of a high relative pressure at about P/P-0 = 1. Therefore, such a continuous two-dimensional water layer is not formed on account of the steric hindrance of the TMS and multilayer ad sorption will not occur in this case. The surface property is estimate d to be hydrophobic. (5) The results of preferential dispersion tests as the macroscopic wettability are in agreement with the results of wa ter vapor affinity as the microscopic wettability.