Silylation of clay mineral surfaces

Silylation of clay mineral surfaces has attracted much attention because silylated products exhibit properties suitable for many applications in materials science and environmental engineering. Successful silylation strongly depends on the reactivity of clay mineral surfaces (e.g., density of surface hydroxyl), characteristics of silane (e.g., number of functional group and configuration of silane) and the reaction conditions (e.g., polarity of solvent and reaction temperature). For non-swelling clay minerals such as kaolinite, pre-intercalation with small polar molecules is an indispensable step in the silylation of interlayer surfaces. The temperature of the grafting reaction has a significant influence on silane intercalation, and the displacement of pre-intercalated molecules, as well as on the structure of the silylated products. On the other hand, silane is readily intercalated into swelling clay minerals such as montmorillonite. The broken edges of 2:1 type clay minerals are the most reactive sites for grafting. The polarity of the solvents used is another important factor controlling the extent of grafting, and the basal spacing of the silylated products. Grafting silane during the course of clay mineral synthesis (defined as in situ silylation in this review) has proved to be an efficient way of silylating clay mineral surfaces.