NMR line width, relaxation and diffusion studies of cyclohexane confined in porous silica

The rotational and translational dynamics of cyclohexane confined within silica pores of 4-50 nm in diameter, have been studied as a function of temperature by measuring H-1 NMR line widths, self-diffusion coefficients, spin-spin relaxation times, and spin-lattice relaxation times, and the results are compared with values obtained for the bulk material. The confinement in the pores gives rise to substantial changes in the molecular dynamics and phase behaviour. The line shape and T-2 measurements revealed a two-phase system, consisting of a liquid-like component at the surface and a plastic phase in the centre of the pore. The highly mobile surface layer is observable far below the reduced transition temperature of the confined cyclohexane. A high diffusion rate of the adsorbed cyclohexane is observed over a wide temperature range - even well below the region of the depressed freezing point. The diffusion coefficient of the molecules in the surface layer is three orders of magnitude larger than in the plastic phase of bulk cyclohexane. (C) 1999 Elsevier Science B.V. All rights reserved.