ADSORPTION OF BENZOIC-ACID ON SYNTHETIC CALCITE DISPERSED IN CYCLOHEXANE AS A FUNCTION OF TEMPERATURE

The adsorption of benzoic acid on synthetic calcite dispersed in cyclo hexane has been studied between 296 and 346 K. The results can be succ essfully explained by a two-step model involving a Langmuir adsorption of first a layer of molecules oriented parallel to the surface and ne xt, as the concentration of the adsorbate in the liquid phase increase s, a gradual replacement of this layer with a monolayer of molecules o riented perpendicular to the surface. The maximum adsorption of benzoi c acid in the perpendicular oriented monolayer has been determined to be 7.3(45) mu mol/m(2), independent of the temperature, and close to t he theoretical value of 7.4 mu mol/m(2). The Langmuir constant for the second step, in which parallel adsorbed molecules are replaced by per pendicular adsorbed ones, varies from 2 x 10(3) mu mol at 296 K to 16 x 10(3) L/mol at 346 K, and the enthalpy and entropy changes associate d with this part of the adsorption process are 37.9 kJ/mol and 0.191 k J/mol K, respectively. In the same temperature interval the Langmuir c onstant for the first step of the adsorption process involving paralle ly adsorbed benzoic acid molecules is estimated to be fairly constant, and within a factor of 2 from the best fit value of 33 x 10(3) L/mol. (C) 1998 Academic Press.