COMPONENTS OF THE SURFACE FREE-ENERGY OF LOW-RANK COALS IN THE PRESENCE OF N-ALKANES

Measurements of contact angles for water, glycerol, formamide and diio domethane 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 o f the surface free energy of coals and coal precovered with n-alkane f ilms were calculated. Also the total surface free energy was determine d from the so-called equation of state. Using the values of the compon ents of surface free energy or total free energy of the coal, the spre ading of the n-alkanes at the coal/water interface and the free energy of interaction between coal particles in the water phase were determi ned. On the basis of these results we have stated that, on the coal su rface in air, a very stable, n-alkane film, resulting from a wetting p rocess by spreading is formed, which first of all reduces the apolar c omponent of the surface free energy of the coal and can partially bloc k the polar component. However, it was found that n-alkanes cannot com pletely 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 pres ence of an n-alkane film on a coal surface decreased the attractive in teractions between the coal particles in the water phase, owing to Lif shitz-van der Waals forces and increased interactions resulting from a cid-base forces in comparison to 'pure' coal.