TRANSLATIONAL DIFFUSION OF LIQUIDS AT SURFACES OF MICROPOROUS MATERIALS - THEORETICAL-ANALYSIS OF FIELD-CYCLING MAGNETIC-RELAXATION MEASUREMENTS

We present a theory of nuclear-spin relaxation appropriate to the care of a. mobile liquid dipolar spin diffusing in a quasi-two-dimensional model porous system in the presence of rare paramagnetic impurities f ixed at the surface of the pores. This theory predicts that the H-1 sp in-lattice relaxation rate will be linear in two parts when plotted as a function of the logarithm of the magnetic-field strength and the sl opes Of these distinct linear regions should be in the ratio 10:3. The theory predicts also a typical pore size dependence for such a rate. The theory is tested at several temperatures using acetone, acetonitri le, dimethylformamide, and dimethylsulfoxide on microporous chromatogr aphic glass bends that have paramagnetic ion impurities at the level o f 40 ppm. H-1 spin-lattice relaxation rates are recorded over magnetic -held strengths corresponding to H-1 Larmor frequencies between 0.01 a nd 30 MHz using a field-switched magnetic relaxation dispersion spectr ometer. The data support the theory quantitatively. The diffusion cons tant D(sic) for the proton-bearing molecule perpendicular to the norma l of the pore surface is found to be nearly a factor of 10 smeller tha n in the bulk solvents. It is characterized by a small activation ener gy similar to those in the bulk solvent. These results demonstrate tha t magnetic relaxation dispersion at low magnetic-held strengths in hig h-surface-area heterogeneous systems may be quantitatively understood in terms of the parameters of the spatial confinement and the local tr anslational dynamics.