Diffusion of cyclohexane in native and surface-modified mesoporous glasses

Diffusion of cyclohexane in mesoporous silica materials with different degrees of surface silanization has been probed by means of pulsed field gradient nuclear magnetic resonance. The self-diffusion coefficients have been measured at various pore fillings from about 10% of one monolayer coverage to complete pore saturation by the capillary-condensed phase. It is found that the surface modification, namely grafting of dimethyloctadecylmethoxysilane molecules to the silica surface, reduces diffusivities of guest molecules as compared to the native sample. The contribution of the Knudsen molecular diffusion to the measured diffusivity has been assessed using the model of fast molecular exchange between the adsorbed phase on the pore walls and the molecules in the gaseous phase in the pore interior. The diffusivity data were correlated with the degree of the surface modification, with the latter being probed by measuring H-1 and C-13 spectra using magic angle spinning (MAS) solid state NMR, nitrogen adsorption and thermogravimetry.