Ionophores, ligands and reactands

The reversibility of a host-guest reaction is known to follow a thermodynamic description of the equilibration process. This thermodynamic description distinguishes the enthalpic and the entropic term. Electrostatic interactions between hard ions and a ligand have been shown to be mainly entropy-stabilized which means that the predominant conformer of the ion-ligand complex is more stable than the predominant conformer of the free solubilized ligand. On the other hand, in reversible chemical reactions based on covalent interactions, much higher potential energies are involved. These processes are assumed to be enthalpy-driven. Subsequently, the selectivity of both mechanisms, that involving ligands and that involving rectands, must be governed by fundamentally different rules. Electrostatic interactions are sensitive to the lipophilicity or polarity of a medium, whereas for reactands, it is primarily the partition equilibrium of the target analyte that is ruled by the solvent. The reactivity is less influenced by the solvent. Some aspects in the development of ion-selective electrodes and their thermodynamics are resumed. Analogous to sensors based on ligands or ionophores, the reversible chemical recognition of alcohols and amines by reactands and chromoreactands is described and compared to ion-selective complexation. (C) 1999 Elsevier Science B.V. All rights reserved.