Thermodynamics of the adsorption of Tröger's base enantiomers from ethanol on cellulose triacetate

Abstract

The adsorption isotherms of the (−)- and (+)-enantiomers of Tröger's base on microcrystalline cellulose triacetate with ethanol as solvent were measured at 30, 40, 50 and 60°C using frontal analysis. The isotherms of the first eluted (−)-enantiomer can be described satisfactorily with the Langmuir equation at all temperatures. By contrast, the isotherms of the longer retained (+)-enantiomer exhibit a pronounced inflection point in the lower temperature range. This point nearly vanishes at 60°C where the isotherm is almost linear in the concentration range investigated. All these isotherms can be satisfactorily described by a quadratic isotherm equation. The standard thermodynamic functions were derived from these equilibrium data. The enthalpies and the isosteric heats of adsorption are not constant in the temperature region studied. The adsorption enthalpies of both enantiomers increase with increasing temperature. At constant temperature, the isosteric heat of adsorption of the (−)-enantiomer depends only slightly on the stationary phase concentration whereas that of the (+)-enantiomer increases strongly with increasing concentration.