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 cyclohexane has been studied between 296 and 346 K. The results can be successfully explained by a two-step model involving a Langmuir adsorption of first a layer of molecules oriented parallel to the surface and next, as the concentration of the adsorbate in the liquid phase increases, a gradual replacement of this layer with a monolayer of molecules oriented perpendicular to the surface. The maximum adsorption of benzoic 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 the theoretical value of 7.4 mu mol/m(2). The Langmuir constant for the second step, in which parallel adsorbed molecules are replaced by perpendicular 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 associated with this part of the adsorption process are 37.9 kJ/mol and 0.191 kJ/mol K, respectively. In the same temperature interval the Langmuir constant for the first step of the adsorption process involving parallely 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.