The medium effect for single ionic species

Abstract

The relation of the " medium effect " or " solvent activity coefficient " to the directly measurable real free energy of transfer is discussed. Measurements of the latter quantity for solutions in aqueous solvents containing methanol, ethanol, dioxan, acetone, ethylene glycol, formamide or dimethyl sulphoxide are reported and discussed. The surface composition of the solvent mixture is accessible and this information can be used to set reasonable limits on the possible value of the " medium effect ". Thus it can be shown that some assignments of single ion medium effects based on oversimplified models of solvation are inadequate. The assignment based on the neglect of liquid junction potentials between solvents is often surprisingly good.

Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, and Dimethyl Sulfoxide

“…18,[184][185][186][187][188][189][190][191][192][193] Unlike the solvation free energies obtained using the cluster pair or TATB approximations, the solvation free energies obtained from these types of experiments contain, in addition to the free energy required to couple the solute to the bulk solvent, a free energy term that depends on the surface potentials of the two phases that are brought into contact with one another. (As discussed in the first paragraph of the present article, the solvation free energies resulting from these types of experiments are usually referred to as real solvation free energies to distinguish them from absolute, or Gibbs, solvation free energies, which do not contain the contribution due to the potential of the phase.)…”

Real energies of ferricinium cation transfer from water to organo-aqueous solvents

“…Most probably, this is the reason for a discrepancy between their data and the data of Parsons and Rubin for the ethanol and acetone solutions, which increased with increasing concentration of solutions. Therefore, we use, predominantly, the data reported in [14]; if they are absent, the data of Parfenyuk et al are used taking into account that, for less volatile solvents (dimethyl formamide, isopropanol), a smaller error may be expected, especially at moderate concentra tions of organic solvent. In interpreting the data on the real transfer ener gies, the most complicated problem is the problem of the variation in the surface solvent potential.…”

“…Comparing the data on the chemical transfer ener gies [20,23] with real transfer energies determined in [14], we obtained a value of the change of surface potential drops in the mixtures of water with methanol and ethanol (Fig. 3).…”

“…The corresponding values estimated by another method in [6] are -280 and -350 mV. The latter number is, prob [14,20,23] for (1) methanol [20] and (2) ethanol [14,23].…”

“…Parsons and Rubin [14] were the first to indicate that the Born equation does not hold for ferricinium ion in the water-metha nol mixtures in a wide range of methanol concentra tions. They used their own data on real transfer ener gies of Cl -ion ( ) and the data of Alfenaar [15] on the potentials of Fc/Fc + couple against an equilibrium chloride electrode in the same solvent.…”