Abstract
Measurement of the contact angle of water, glycerol, formamide, methylene iodide, α-bromonaphthalene, ethylene glycol and its aqueous solutions was carried out on the surface of Polish coal of rank 31.1, 31.2, 32.1, 32.2, 33, 34 and 35. On the basis of these results, the critical surface tension of wetting of Polish coal of various ranks was determined by Zisman's method, and the dispersion and nondispersion components of the surface free energy of coals were calculated using a modified Young equation. The results presented show that the surface free energy of the coals studied originates not only from dispersion but also from nondispersion intermolecular interactions, and the average energy is a little higher than the average critical surface tension of wetting of coal. It was also shown that low coal ranks (31 and 32) are more heterogeneous than higher coal ranks (33, 34 and 35), but no close relationship between the average surface free energy and coal ranks was found.
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