ADSORPTION OF CTAB AT LIGNITE-AQUEOUS SOLUTION INTERFACE

The adsorption of a cationic surfactant (cetyltrimethylammonium bromid e, CTAB) from aqueous solution onto the lignite was investigated. The samples used for the adsorption experiments were: fresh; demineralized ; demineralized then oxidized. The oxidation process was conducted by dry method (in air at 150 and 200 degrees C). The UV spectrophotometri c techniques were used for the analysis of CTAB in solution. The adsor ption isotherms were measured experimentally at 21 and 45 degrees C, a nd it was found that the adsorption capacities of all samples increase d with rising temperature, and the adsorption of CTAB decreased by dem ineralization and oxidation. The changes in the thermodynamic quantiti es, enthalpy and entropy, related adsorption process that occurred in some steps were calculated. Isosteric heats of adsorption and the corr esponding changes in entropy have found to be negative for all of the samples. From the abrupt difference between Delta H and Delta S for th e demineralized sample and those of fresh sample, it was concluded tha t the orientation of CTAB molecules on the surface of the demineralize d sample differs than that of fresh sample, and also that the adsorpti on of CTAB on the demineralized sample takes place with the hydrophobi c interactions in addition to the electrostatic interactions, while in the case of fresh sample, the predominant mechanism of adsorption is through electrostatic interactions. Also, the lower values of BH for b oth demineralized and oxidized samples in comparison with the deminera lized sample indicates that the oxidation process diminishes the contr ibution of the hydrophobic interactions to the adsorption of CTAB.