NMR LINEWIDTH, RELAXATION AND DIFFUSION STUDIES OF HEXAMETHYLDISILANECONFINED IN POROUS SILICA

The rotational and translational dynamics of hexamethyldisilane confin ed within silica pores of diameter 4, 6 and 20 nm were studied as a fu nction of temperature by measuring H-1 and C-13 NMR linewidths, self-d iffusion coefficients, spin-spin relaxation times and spin-lattice rel axation times, and the results were compared with values obtained for the bulk material. The confinement in the pores gives rise to substant ial changes in the molecular dynamics and phase behaviour. The linesha pe, T-2 and self-diffusion measurements revealed a two-phase system co nsisting of a liquid-like component at the surface and a crystalline s olid in the centre of the pore. The highly mobile surface layer is obs ervable far below the reduced transition temperature of the confined h examethyldisilane. A high diffusion rate of the adsorbed hexamethyldis ilane is observed over a wide temperature range, even well below the r egion of the depressed transition point. The diffusion coefficient of the molecules in the surface layer is three orders of magnitude larger than in the plastic phase of bulk hexamethyldisilane. (C) 1998 John W iley & Sons, Ltd.