Characterization of Structural and Interfacial Properties of Solvent-Swollen Coals by Inverse Liquid Chromatography Technique

Structural and interfacial properties of coals (C = 65–91 wt %) were investigated by means of inverse liquid chromatography, the stationary phase of which was coal particles preswollen in solvents. The carrier phase was acetonitrile, methanol, or n-hexane. The molecular probes used were alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs), n-alkanes, and aliphatic alcohols. A pulse of probe compound was repeatedly injected into the column, and the retention volume was measured in the temperature range of 20–50 °C. When alkylbenzenes and PAHs were injected, the retention volume became larger than that of the carrier solvent, showing that their elution was controlled by the adsorption onto the coal surface. In most cases, the capacity ratio (the increment of the elution time of the probe relative to the elution time of the carrier solvent) increased with increasing molecular volume of probe compound for each group of alkylbenzenes or PAHs. The capacity ratio of the PAHs was much larger than that of alkylbenzenes when their molecular volumes were equal, which indicates that the aromatic ring of a probe molecule was recognized by the coal more selectively than were alkyl chains. The π-π interaction was important for high-rank coals and the OH-π interaction for low-rank coals.