NMR-Studies of the Diffusion of Cyclohexane in Porous Silica

Self diffusion measurements of cyclohexane adsorbed in porous silica glass with a range of pore diameters are reported. The diffusion measurements were made by Nuclear Magnetic Resonance (nmr) using the spin-echo pulsed field-gradient method (pgse). No deviation from a Gaussian diffusion process and no evidence of bounded motion was found. However, a different behaviour to that reported in the literature for similar samples which were water saturated was observed for the relative reduction of the diffusion constant, D(eff)/D(o). The relation D(eff)/D(o)=e(-b/d) was found to describe the diffusion of the cyclohexane and this supports a semi-empirical model of molecular motion in confined geometry which can be characterised by two physical parameters, namely the pore diameter, d, and the ''effective'' permeability, 1/b.