Anomalous surface diffusion of water compared to aprotic liquids in nanopores

H-1 nuclear magnetic relaxation dispersion experiments show remarkable diff erences between water and acetone in contact with microporous glass surface s containing trace paramagnetic impurities. Analyzed with surface relaxatio n theory on a model porous system, the data obtained for water show that pr oton surface diffusion limited by chemical exchange with the bulk phase per mits long-range effectively one-dimensional exploration along the pores. Th is magnetic-field dependence coupled with the anomalous temperature depende nce of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore size s and on a large variety of frequencies and temperatures. supports this int erpretation. The analysis demonstrates that acetone diffuses more slowly, w hich increases the apparent confinement and results in a two-dimensional mo del for the molecular dynamics close to surface relaxation sinks. Surface-e nhanced water proton diffusion, however, permits the proton to explore a gr eater spatial extent of the pore, which results in an apparent one-dimensio nal model for the diffusive:motions of the water that dominate nuclear spin relaxation. [S1063-651X(99)04209-9].