SOLID-STATE NMR-STUDY ON THE CONFORMATION AND MOBILITY OF N-OCTADECYLCHAINS IN A SILANE COUPLING AGENT ATTACHED TO THE SURFACE OF COLLOIDAL SILICA

The surface of colloidal silica of 22 nm and 100 nm in diameter has be en modified with a silane coupling agent, n-octadecyltriethoxysilane. The solid state C-13 NMR spectrum shows surprisingly that the covalent ly bonded n-octadecyl chains on the silica surface adopt largely exten ded all-trans conformation. By increasing the temperature to 70 degree s C, the trans conformations observed at room temperature can be conve rted to a disordered state in which the trans and gauche conformers ar e under rapid exchange (mobile disorder). On cooling to ambient temper ature, the trans conformation cannot be recovered and a distribution o f chain conformations can be observed, indicating that the disorder ha s been frozen-in (rigid disorder). Subsequent treatment of the surface modified silica with a second reagent, such as trimethylsilylchloride or ethanol reacts with the remaining surface hydroxy groups and cause s the n-octadecyl chains to collapse to the mobile, disordered state, The C-13 spin-lattice relaxation times for carbon atoms in the all-tra ns conformation are longer than those in the mobile, disordered chains , indicating that there is more restriction of motion in the trans oct adecyl chains. Instead of crystallization of the octadecyl chains on t he silica surface, other factors, such as static interactions, might b e responsible for the existence of trans conformation in the octadecyl chains.