Components of the surface free energy of low rank coals in the presence of n-alkanes

Abstract

Measurements of contact angles for water, glycerol, formamide and diiodomethane on the surfaces of four low rank coals (31.1, 31.2, 32.1 and 32.2) covered with n-alkanes (n-hexane, n-undecane and n-hexadecane) were made. Using the approaches of van Oss et al., the geometric mean of the interfacial free energy, and Young's equation, the components of the surface free energy of coals and coal precovered with n-alkane films were calculated. Also the total surface free energy was determined from the so-called equation of state. Using the values of the components of surface free energy or total free energy of the coal, the spreading of the n-alkanes at the coal/water interface and the free energy of interaction between coal particles in the water phase were determined. On the basis of these results we have stated that, on the coal surface in air, a very stable, n-alkane film, resulting from a wetting process by spreading is formed, which first of all reduces the apolar component of the surface free energy of the coal and can partially block the polar component. However, it was found that n-alkanes cannot completely spread over a coal surface immersed in water, and the contact angle depends on the type of n-alkane. It was also found that the presence of an n-alkane film on a coal surface decreased the attractive interactions between the coal particles in the water phase, owing to Lifshitz-van der Waals forces and increased interactions resulting from acid-base forces in comparison to ‘pure’ coal.