Pore-size dependence of the self-diffusion of hexane in silica gels

Quasielastic neutron scattering has been used to investigate the diffusion of hexane molecules adsorbed in mesoporous silica gels with pore diameters of 20, 40, and 60 Angstrom, respectively. Within the temperature range 180 Kless than or equal toTless than or equal to240 K molecular reorientations, which are best described by a tumbling rod, and translational diffusion of the molecular center of mass could be observed. In each investigated sample two different environments were identified for the adsorbed molecules: the vicinity of the pore walls and the center of the pores. The hindering potentials for both translation and rotation are found to be considerably stronger for the molecules on the pore walls. The fraction of this molecule type decreases with increasing pore size. The self-diffusion coefficients derived from our data range from D-trans=0.6 to 2.4x10(-6) cm(2) s(-1). The activation energies of the translational diffusion are within the range 139 meVless than or equal toE(a)less than or equal to302 meV. They decrease with increasing pore size with a clear tendency towards the activation energy of liquid (bulk) hexane. (C) 2002 American Institute of Physics.