Microheterogeneity of solvent-swollen coal probed by proton spin diffusion

The phase-separated structure of solvent-swollen coal was characterized using its proton flip-flop spin diffusion properly. Five coals of different ranks were swollen by saturation with deuterated pyridine and were subjected to ¹H MR relaxation measurements. At least two distinct structural regions exist in the swollen coals based on the transverse relaxation characteristics as reported in the literature. The observed longitudinal relaxation was characterized by a single component, since spin diffusion, i.e., the transfer of ¹H magnetization from the mobile to the rigid phase is active in the swollen coals. The process of this transfer was monitored using a partially modified Goldman-Shen pulse sequence and analyzed with a simple model. It is shown that coal swells in a nonuniform fashion on a fine scale and that phase separation occurs on a scale ranging up to 20 nm even in good solvents such as pyridine.