Wettability and the adhesion force between two silica particles were measured as a function of relative humidity. The wettability of silica particles was controlled by rehydroxylation and modification by hexamethyldisilazane and evaluated by a preferential dispersion test, heat of immersion, and water adsorption. The adhesion force between two particles was measured by atomic force microscopy using a "colloidal technique". The wettability varied from hydrophilic to hydrophobic at trimethylsilyl density ) 1.0 nm -2 as modified groups shielded the residual silanol groups and hindered the formation of a capillary bridge between the two particles. This "hindrance effect" is thought to result in the absence of a critical increase in adhesion force at high relative humidity and also the reduction of surface energy at low humidity where hydrogen bonding dominated the adhesion force.

The adhesion force between nonporous and porous silica particles was measured by AFM as a function of relative humidity. Various porous samples were prepared by hydrothermal treatments in different conditions. Geometric structure was evaluated by gas adsorption, and the mechanism of rehydroxylation that occurred in hydrothermal treatments was investigated by silanol density, IR spectra, and immersional heat. The adhesion force was measured by AFM, and the result was compared with the geometric structure and the rehydroxylation process. As a result, we proved that capillary condensation occurred in small spaces between two particles contacting each other after water filled the pores and capillary force appeared to contribute to a steep increase of adhesion force. The adhesion force under low relative pressure was attributed to hydrogen bonding force and reflected both the contact area and hydrophilicity of the surface.

The relationship between wettability and surface geometric structure of modified silica particles and glass plates was investigated. Surface modification was performed by the autoclave method with 1-dodecanol to control the surface wettability. The preferential dispersion test proved that wettability varied at a surface modification ratio of 20%, which coincided with the changing point of the geometric structure of modifier chains determined by the adsorption method. The geometric structure was also evaluated by atomic force microscopy (AFM), and we could obtain the hexagonal packing of chains of the modifiers both in water and air at high surface modification ratio. Imaging of the surface of the nanosized particles on a glass plate was accomplished in water by taking advantage of the hydrophobic attractive force, which was proved by adhesion force measurements.

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