Solvation of ions in pure and mixed solvents
The problem of the influence of the solvent on activities of ions is discussed. Some proposals for the splitting of thermodynamic functions for electrolytes in solution into single ion values are reviewed. The effect of different solvation of ions on phenomena such as transfer of solvent in electrolysis, selective solvation in mixed solvent, phase boundary potential, surface tension and critical solution point in non-miscible solvents, and on solubilities of electrolytes in mixed solvents, may qualitatively and semiquantitatively be determined from the knowledge of single ion solvation behaviour. The most direct information on selective ion solvation in mixed solvent is provided from NMR measurements on, for example, Al3 ions in a mixture of two aprotic solvents. I. THERMODYNAMICS OF IONIC SOLVATION The standard free enthalpy of solvation of an electrolyte MX in a solvent L is defined by the free enthalpy of the process (Figure 1) M(gas) + X(gas) Z M(L) + X(L) (1) with the fugacities of the ions in the gasphase and the activities of the ions in solution being unity. This free enthalpy may be obtained with a Haber Born cycle and turns out to be of the order of from —150 kcal/mole (1-1 electrolytes) to more than — 1000 kcal/mole (e.g. for Al3 + salts). The uncertainty involved in the calculation of these energies amounts to at least 2 kcal/mole. However, the difference of free enthalpies of solvation of a given electrolyte in two different solvents, which we shall call the free enthalpy of transfer, is a smaller quantity ()LIGt 20 kcal/mole) and its experimental precision is much better, since data not precisely known, especially electron affinities of anions needed in the Haber Born cycle, cancel out in AGt. Process (1) is never realized in practice and is only of theoretical importance. Of much greater interest for the practical chemist is the process M(L1) + X(L1) M(L) + X(L2) (2) ¶ This review is not intended to be exhaustive and only covers work done in this laboratory 327