Microheterogeneity of solvent-swollen coal probed by proton spin diffusion

The phase-separated structure of solvent-swollen coal was characterized usi ng its proton flipflop spin diffusion property. Five coals of different ran ks were swollen by saturation with deuterated pyridine and were subjected t o H-1 NMR relaxation measurements. At least two distinct structural regions exist in the swollen coals based on the transverse relaxation characterist ics as reported in the literature. The observed longitudinal relaxation was characterized by a single component, since spin diffusion, i.e., the trans fer of H-1 magnetization from the mobile to the rigid phase is active in th e swollen coals. The process of this transfer was monitored using a partial ly modified Goldman-Shen pulse sequence and analyzed with a simple model. I t is shown that coal swells in a nonuniform fashion on a fine scale and tha t phase separation occurs on a scale ranging up to 20 nm even in good solve nts such as pyridine.