Solvation of ions. XXVII. Comparison of methods to calculate single ion free energies of transfer in mixed solvents

Abstract

Free energies of transfer of ions from water to mixtures of water with acetonitrile (AN), with dimethylformamide (DMF), with dimethylsulfoxide (DMSO), and with ethylene glycol have been determined using both the tetraphenylarsonium tetraphenylboride [TATB] and the negligible liquid junction potential [E _j ] assumptions. By making use of ΔG _tr (Ag⁺)[TATB]=12 kJ-mol⁻¹ for transfer from DMSO to AN and by assuming negligible liquid junction potential in the cell

$${\text{Ag|AgNO}}_{\text{3}} {\text{(0}}{\text{.01}}M{\text{),S}}\parallel {\text{Et}}_{\text{4}} {\text{NPic(0}}{\text{.1}}M{\text{),AN}}\parallel {\text{AgNO}}_{\text{3}} {\text{(0}}{\text{.01}}M{\text{),AN|Ag}}$$

single ion free energies of transfer of silver ion ΔG _tr (Ag⁺)[E _j ] from DMSO to 35 pure and mixed solvents show a standard deviation of only 2 kJ-mol⁻¹ when compared with ΔG _tr (Ag⁺) calculated from the TATB assumption that ΔG _tr (Ph ₄ As⁺)=ΔG _tr (Ph ₄ B⁻). The ferrocene assumption [Fc] also gives acceptable agreement with ΔG _tr (Ag⁺)[TATB] provided that the solvents are not highly aqueous. Other cells with other junctions give less acceptable agreement between the E _j and TATB assumptions. It is essential that the salt bridge is always tetraethylammonium picrate in AN, if the E _j assumption is assumed. Because of the ease of making potentiometric measurements compared with the difficulty of measurements required for the TATB assumption, the negligible liquid junction potential method in the cell shown is recommended for estimating transfer free energies of single ions. The ferrocene assumption is acceptable only for non-structured aprotic solvents.