Treatments Based on Stepwise Solvent Replacements

3 Treatments Based on Stepwise Solvent Replacements The stepwise replacement of one solvent in a binary mixture of A+B around and ion P, Equations 6.6a and 6.6b, has been treated along the Unes used in solution coordination chemistry. The equilibrium quotient for the replacement of j molecules of A by j molecule of B can be represented in terms of the volume fractions (]) of the solvent components according to Cox et al. [75] as  

The values of these quotients are obtained from spectroscopic measurements (e.g., NMR) or electrochanically by methods employed in solution coordination chemistry. They lead to the standard molar Gibbs energy of transfer of P from the source solvent A to the mixture  

An alternative treatment is specifically related by Covington and Newman [76] to NMR chemical shift data for specific ions P, extrapolated to infinite dilution, in binary solvent mixtures A+B. It is assumed that the chemical shift 5 is linear with the fractional solvent composition in the solvation shell of the ion  

The expression for the standard molar Gibbs energy of transfer is similar to Equation 6.24, but in terms of the mole fractions rather than the volume fractions used by Cox et al. [75]. However, A G (I, A - A + B) has an additional term that is related to the electrostatic effect of the transfer from the source solvent with permittivity to the solvent mixture with its different permittivity (unless transfer is between isodielectric solvents). 

In a subsequent development by Covington and Newman [77], a relationship with the Kirkwood-Buff approach was established, vaUd for infinite dilution of the ion P. The preferential solvation parameter = x (,) - pertaining to the solvation shell of the ion is  

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