Dynamic Nuclear Polarization by Asphaltene Radicals: Effects of Colloid Size and Solvent Diffusion

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Abstract
Dynamic proton polarization in colloidal asphalt solutions arises from electron—proton dipolar coupling. At 75 G, the dipolar interaction is modulated by colloid-tumbling motions and solvent-molecular diffusion. The NMR signal enhancement declines much more slowly with increasing magnetic field than would be predicted from a single correlation time appropriate to colloid tumbling. The observed decline suggests a dual-motion model in which radical tumbling controls the low-frequency transitions, and solvent diffusion controls the high-frequency transitions. The low-frequency transitions dominate proton relaxation. The enhancement and proton relaxation rate are observed to vary by more than an order of magnitude in a series of monoalkylbenzene solutions with a viscosity range of five to one. Enhancement declines with colloid flocculation in less-effective solvents; relaxation rate increases. In all cases, the individual low- and high-frequency transition rates are in accord with the dual-motion model.