We evaluated the effect of the molecular structures and amounts of silane-coupling agents - 3-glycidyloxy propyltrimethoxysilane (GPTMS), methacryloxypropyl trimethoxysilane (MPTMS), and vinyltrimethoxysilane (VTMS) - used to generate surface-modified silica on dispersibility according to surface interaction forces. The loading of coupling agents on the silica surface is positively correlated with the amount of coupling agent added. However, if this amount exceeds the single-layer adsorption calculated from the minimum coverage area of the silane-coupling agent, the extent of modification decreases. Additionally, the extent of modification tended to decrease as the coupling reaction temperature increased (40-100 degrees C). Repulsive force was observed for GPTMS-modified silica in water and MPTMS-and VTMS-modified silica in toluene. In contrast, clear repulsion was not observed for GPTMS in toluene and MPTMS- and VTMS-modified silica in water. With GPTMS-modified silica in water and VTMS-modified silica in toluene, the repulsive forces increase with the extent of modification, resulting in good dispersibility. Conversely, for VTMS-modified silica in water and GPTMS-modified silica in toluene, weakly repulsive and attractive forces were observed independent of the extent of modification, and flocculation and sedimentation were noted when the actual dispersibility was examined. These results indicate that the actual dispersibility can be evaluated from the surface interaction forces. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.