SELF-DIFFUSION IN FLUIDS IN POROUS-GLASS - CONFINEMENT BY PORES AND LIQUID ADSORPTION LAYERS

Diffusion coefficients of 10 different polar and nonpolar liquids fill ed in porous glasses with mean pore diameters of 4 or 30 nm were deter mined with the aid of the NMR field-gradient technique, In the time sc ale of these experiments (0.3 to 500 ms) diffusion coefficients were f ound to be time independent, Within the experimental error, no influen ce of the polarity of the adsorbate can be stated, The diffusion coeff icients of all investigated fluids in glass with 4 and 30 nm pores wer e reduced by factors of 0.17 and 0.63, respectively, relative to the b ulk values, This relatively weak reduction can be explained by conside ring the known porosities of the adsorbents, The second objective of t his study was to examine the diffusion behaviour below the melting poi nt of adsorbates in porous glass, Fluids confined in pores do not free ze at the bulk freezing temperatures, In this respect, two phases must be distinguished, A maximal two monolayer thick film adsorbed on the inner surfaces does not crystallize at all, whereas the ''free'' fract ion of the fluid in the pores freezes at reduced temperatures accordin g to the Gibbs-Thompson relation, The nonfrozen surface layers form a network in which self-diffusion can be investigated, Experiments have been carried out with cyclohexane, A reduction factor of 0.06 was foun d relative to the extrapolated values of the entirely unfrozen fluid i n porous glass with a mean pore diameter of 30 nm, It is, thus, demons trated that molecules in adsorption layers virtually retain their tran slational degrees of freedom along the surfaces, The lowering of the d iffusivity is mainly due to the geometric restriction rather than to t he interaction with the surface, Copyright (C) 1996 Elsevier Science I nc.