Solid state NMR study on the conformation and mobility of n-octadecyl chains 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 been modified with a silane coupling agent, n-octadecyltriethoxysilane. The solid state C-13 NMR spectrum shows surprisingly that the covalently bonded n-octadecyl chains on the silica surface adopt largely extended all-trans conformation. By increasing the temperature to 70 degrees C, the trans conformations observed at room temperature can be converted to a disordered state in which the trans and gauche conformers are under rapid exchange (mobile disorder). On cooling to ambient temperature, the trans conformation cannot be recovered and a distribution of chain conformations can be observed, indicating that the disorder has 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 causes the n-octadecyl chains to collapse to the mobile, disordered state, The C-13 spin-lattice relaxation times for carbon atoms in the all-trans conformation are longer than those in the mobile, disordered chains, indicating that there is more restriction of motion in the trans octadecyl chains. Instead of crystallization of the octadecyl chains on the silica surface, other factors, such as static interactions, might be responsible for the existence of trans conformation in the octadecyl chains.