Theory of the acid-base hydrogen bonding interactions, contact angles, and the hysteresis of wetting: application to coal and graphite surfaces

An understanding of the acid-base behavior of coal surfaces is needed, if the wetting of coal by water and other liquids, and the separation processes such as floatation and oil agglomeration of coal, are to be exploited. The modern theory of the acidic and basic parameters of surface free energy, γ⊕ and γ⊖, as well as the apolar component γLW, is reviewed, together with the equations for contact angles in terms of these properties. Contact angles of water, glycerol and methylene iodide on 14 different coals, and on graphite, have been measured. The advancing angle data and the retreating angle data lead to two decidedly different sets of the parameters, γLW, γ⊕ and γ⊖. γLW r, based on retreating angles, is 10 mJ/m2 higher than γLW a, based on advancing angles; and both are independent of % carbon and % oxygen, i.e. of coal rank. γ⊕ r lies between 1.2 and 2.4 mJ/m2, and γ⊕ a is close to 0.2 mJ/m2, independent of composition. γ⊖ r and γ⊖ a exhibit downtrends with increasing carbon content. For each of the three parameters, the data for graphite fit the trends found with the various coals. The surface structures that would account for these trends are briefly discussed. The data point to a two-phase surface structure for coal and for graphite.