MEASUREMENT OF REAL FREE-ENERGIES OF TRANSFER OF INDIVIDUAL IONS FROM WATER TO OTHER SOLVENTS WITH THE JET CELL

Resolution of the activities of solutions of electrolytes into the individual ionic contributions cannot be carried out rigorously and requires the introduction of extrathermodynamic assumptions which have inherent uncertainties. The most commonly used approaches are basically similar in that they are based on the assumed solvent independence of the difference in the enthalpy or Gibbs energy of transfer of pairs of model solutes, e.g., tetraphenylarsonium and tetraphenylborate ions, or ferricinium ion and ferrocene. In this work we follow an alternative approach pioneered by Parsons involving measurement in the jet (Kenrick) cell of outer-potential differences between solutions of the same electrolyte in two solvents. These potential differences provide the 'real' free energies of transfer of individual ions which, in turn, differ from the usual Gibbs energies of transfer by the work required to transfer the ion through the dipolar layers at the two solvent-gas interfaces. One objective of this work was to improve the reliability of real free energy of transfer measurements, which are experimentally demanding, to within ca. +/- 0.5 kJ-mol-1 in order to match typical uncertainties in Gibbs transfer energies of electrolytes. This goal was met, in most instances, by careful evaluation of experimental parameters (particularly jet pressure). A major improvement over previous measurements was made by adding a supporting electrolyte which allowed stable potentials to be obtained at test electrolyte concentrations as low as 10(-4)M. Real free energy changes are reported for the transfer of silver ion from water to methanol, ethanol, acetonitrile, propylene carbonate and dimethyl sulfoxide, as well as for the transfer of chloride ion from water to methanol and ethanol. Reliable data of this kind may lead to improved understanding of either the properties of the surfaces of solvents or the interactions of model solutes with solvents, depending on which of the two fields develops most.