FIELD-CYCLING NMR RELAXOMETRY OF MOLECULES UNDERGOING LEVY WALKS AT THE SURFACE OF FINE PARTICLES AND POROUS-GLASS

Slow surface dynamics of solvents confined in the pore space of porous glass and fineparticles was studied by proton field-cycling relaxomet ry, Molecular reorientations mediated by translational displacements a re shown to be an important low-frequency T-1-relaxation mechanism in porous media at low nuclear magnetic resonance (NMR) frequencies in th e strong adsorption limit. This is revealed by the pronounced T-1-disp ersion in this case while a flat one is observed for weak adsorption. The following nanoporous materials were examined in this respect: fine particles of ZnO, TiO2, porous glass, Silanized surfaces lead to a som ewhat steeper T-1-dispersion of non-polar cyclohexane than the native, i.e. polar, surfaces. On the other hand, there is no strong influence of the surface groups on the T-1(omega) slope of (polar) acetone, The results are discussed with reorientations mediated by translational d isplacements enhanced by Levy walks on the surface. (C) 1998 Elsevier Science Inc.