Adsorption of Poly(oxyethylene)nonylphenol Homologues on a Low-Ash Coal

The adsorption of poly(oxyethylene)nonylphenol homologues on a low-ash coal has been studied as functions of pH, temperature, and ethoxylation number. The free energies and the isosteric heats of adsorption have been calculated from the adsorption isotherms. It has been found that the adsorption is driven by the hydrophobic interaction between the alkyl-aryl group and the coal surface, with the poly(oxyethylene) group contributing little to the adsorption free energy. The results also show that the major portion of the adsorption free energy comes from entropy changes associated with the process. Based on this information, it is proposed that the nonionic surfactants studied in the present work adsorb on coal with the nonylphenol groups lying flat on the surface and the poly(oxyethylene) groups projecting toward the aqueous phase; however, the projection angle varies with the length of the poly(oxyethylene) group.