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| - A Method for Counting the Hydrogen-Bond Cross-Linksin Coal
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| - When small amounts of a good hydrogen-bond acceptor such as pyridine orTHF are added toa nonpolar swelling solvent such as chlorobenzene or toluene, there isa very rapid increase incoal swelling with increasing acceptor concentration which presentlyalmost levels off. Whenswelling is plotted against pyridine concentration, the result is whatappears to be a titrationcurve. This curve permits an estimation of the number ofhydrogen-bond cross-links in the coal.We believe that this behavior is due to the selective associationbetween the hydrogen-bondacceptor (e.g., pyridine) and hydroxyl groups which are cross-linksbetween macromolecular chainsin the coal. The selectivity of the acceptor for cross-linkinghydroxyls over other hydroxyls (morefavorable free energy for association of pyridine with thecross-linking hydroxyl) is due to themuch more favorable entropy change which occurs when one of thesecross-links is disrupted byformation of a new hydrogen bond to pyridine. This disruptionfrees a portion of the coal toadopt many more possible conformations lending to the favorable entropychange. This modelleads directly to the prediction that this titration curve will beindependent of the hydrogen-bond acceptor, a prediction which is verified by the identical curvesobserved using pyridine orTHF in chlorobenzene with Illinois No. 6 coal. Changing thenonpolar solvent does not result ina change in the number of hydrogen bonds in agreement with this model.About 1/3 of the hydroxylgroups in Illinois No. 6 coal form network-active hydrogen bonds(cross-links) and there are about1.7 hydrogen bonds per 100 carbon atoms. In the higher rankPittsburth No. 8 coal there areapproximately 0.3 cross-linking hydrogen bonds per 100 carbonatoms.
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