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Adsorption of benzyl alcohol from carbon tetrachloride solutions onto alumina and kaolinite is investigated by ultraviolet spectroscopy for analysis of alcohol content. The adsorption isotherms are produced and adsorption mechanisms are discussed by taking into account the self-association of the alcohol molecules. Both isotherms show well-defined plateau sections corresponding to an alcohol packing density of about 0.54 nm2 per molecule. This area is sufficient for the molecules to be adsorbed in a vertical orientation, but with the benzene ring lying along the plane of the interface. If this is the situation, the formation of multilayers as observed above an alcohol concentration of 0.04 mol/kg may occur by stacking of the benzene rings. The stability of such a configuration is achieved by sharing the π-electrons. However, self-association of alcohol molecules to form open chain and cyclic oligomers may affect the formation of multilayers on the minerals. Although the open chain aggregates do have unsaturated valencies for forming hydrogen bonds with the mineral surface, this is not the case for cyclic alcohol aggregates. The open chain oligomers may therefore compete with free alcohol monomers for available sites on the mineral. Thus, data show that the situation caused by self-association of benzyl alcohol should be taken into consideration when adsorption mechanisms are discussed. |
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