Thermodynamics of anion exchange on a chloride-intercalated zinc-aluminum layered double hydroxide:: a microcalorimetric study

The thermodynamic properties of anion exchange on a chloride-intercalated zinc-aluminium layered double hydroxide have been studied at 298.15 K. The heats for total exchange of Cl- for F-, Br-, I-, OH-, NO3-, and SO42- have been determined by microcalorimetry and the standard molar enthalpies of these exchange reactions have been estimated. For the processes involving OH- and NO3-, the exchange isotherms have been determined using potentiometry in the former case and capillary ion analysis in the latter. The standard molar Gibbs free energy and entropy changes upon exchange of Cl- for OH- and exchange of Cl- for NO3- have been calculated as a function of the fraction of exchanged Cl-. These exchange processes appear to be entropy-driven, but the selectivity is controlled by the enthalpic contribution.