Surface interactions of dimethylsiloxy group-modified fumed silica

Fumed silica is widely used in industry as a rheological additive. Produced by a flame process, fumed silica consists of aggregates of hydrophilic primary particles. The thickening effect is related to particle-particle interactions leading to agglomerates and particle networks. These particle-particle interactions depend heavily on the liquid medium. Hydrophilic silica, whose surface is covered by silanol groups, shows greatest thickening in low polarity systems owing to strong hydrogen bonds. In polar media, wetting of the hydrophilic surface weakens particle interactions and breaks down thickening. In contrast, silylated hydrophobic silica is an effective rheological additive in high polar media. This effect is related to hydrophobic interactions between silica particles. Additionally, silylation of the silica surface by dimethylsiloxy groups results in greater thickening than silylation by trimethylsiloxy groups. Gas adsorption data reveal particle-particle interactions arising from interpenetration of the polymer-like silylation layer.