COAL MACROMOLECULAR NETWORK STRUCTURE-ANALYSIS - SOLVENT SWELLING THERMODYNAMICS AND ITS IMPLICATIONS

The thermodynamics of the coal swelling process is considered in light of various correlations of swellability with solvent properties. It h as been found that the swellability of coals in specifically interacti ng solvents is a strong function of the electron-donating capacity of the solvents, as measured by two very different scales. The two scales are seen to be closely related. The swelling also correlates with the heat of immersion of coals in solvents, and thus it appears that it i s the enthalpy of interaction of specifically interacting solvents wit h surface functionalities in coal that mainly determines swelling beha vior. There is generally a particular number of specific interaction s ites in the coal that determine the maximum extent of swelling, though there is the possibility of nonspecific interactions contributing to further swelling, beyond this ''titration end point''. Variation in sw elling of a North Dakota lignite with specifically interacting solvent activity appears to suggest that the process is entropically driven, beyond a very early stage when interaction of ''free sites'' in the co al dominates. The dissociation of noncovalent cross-links is an import ant feature of the swelling process.