Ionic enthalpies of transfer

Alot of information about the free energies of transfer of single ions between pure solvents has been accumulated. Less numerous are determinations in mixed solvents, and the ionic enthalpies of transfer and entropies of transfer as function of mole fraction are known as an exception only. In Table 1 ions and solvent mixtures are listed for which free energies of transfer and some other thermodynamic quantities have been determined. 

Fig. 2.12.5. (a) Ionic enthalpies of transfer from dimethylsulphoxide to propylene carbonate (from ref. E107 by permission of the authors). 

DIMETHYLFORMAMIDE-DIMETHYLSULFOXIDE F. Rallo and F. Rodante, Gaz. Chim. Ital., 102,56 (1972). Ionic enthalpy of transfer of picrate ion between DMSO and DMF. 

Friedman and co-workers have measured heats of solution of numerous electrolytes in PC at 25" C, primarily to obtain ionic solvation enthalpies, and enthalpies of transfer between this and other solvents. Because many of the alkali metal halides dissolve insufficiently fast for accurate heat measurements, most of the data are for uni-univalent electrolytes containing complex ions. The only additional heat data in this solvent, reported by Choux and Benoit, are for Ph4AsI. All measurements were made in dilute solution and most were corrected to infinite dilution using the Debye-Hiickel equation. In general, enthalpies of solution of electrolytes in PC follow the same trend as in DMSO that is, the alkali metal salts exhibit more negative heats than in water while the reverse is generally true for salts with complex cations. 

Methods for obtaining individual contributions to the enthalpy of solvation were discussed above. For the most part, identical procedures have been used to obtain the individual ionic contributions to the standard enthalpy of transfer. Appendix 2.11.15 lists these individual A/f°(ion) values. These data are based on the following methods ... 

Estimates of the heat of solvation of various ionic species in DMSO as compared to water have been made, and can be expressed as enthalpies of transfer. Some data are given below. Discuss their significance. 

Dissociation rates were obtained at high acidity where kn is fast and were corrected for ionic strength using a Debye-Huckel equation. The results show that the cryptate selectivity results mainly from kh and that the transition state for the reaction has little interaction between the metal and the cryptand to differentiate between metals. Rates kt increase with increasing cavity size. The thermodynamics of cryptand formation in water and methanol have also been used to calculate the free energy and enthalpy of transfer of the free ions between the two media. 

A much more commonly used method for splitting electrolyte enthalpies of transfer into the individual ionic contributions is the TATB method, according to which, in analogy with its use for the Gibbs energies, (Ph As", W ->S) = (BPh , W S), practiced by Arnett and McKelvey and by Friedman [38,39]. 

The dependence of the ionic standard molar enthalpies of transfer on the properties of the solvents and the ions are expressed in a manner similar to Equation 4.22a. For small cations, the operative expression for values at 25°C is [36] ... 

TABLE 4.3 Ionic Standard Partial Molar Enthalpies of Transfer from Water into Nonaqneons Solvents, W->S)/kJ-mol at 25°C ... 

These several assumptions do not lead to the same conclusions. For example, transfer activity coefficients obtained by the tetraphenylarsonium tetraphenyl borate assumption differ in water and polar aprotic solvents by up to 3 log units from those based on the ferrocene assumption. From data compiled by Kratochvil and Yeager on limiting ionic conductivities in many organic solvents, it is clear that no reference salt can serve for a valid comparison of all solvents. For example, the tetraphenylarsonium and tetraphenyl borate ions have limiting conductivities of 55.8 and 58.3 in acetonitrile. Krishnan and Friedman concluded that the solvation enthalpy of... 

Non-aqueous Solvation.—Structural radii and electron-cloud radii, together with lattice enthalpies and enthalpies of solvation of ionic crystals, have been reviewed.84 The free energies of transfer, AGtr(K+), of potassium ions from water to 14 non-aqueous solvents have been reported, and they were derived from measurements in an electrochemical cell assumed to have a negligible liquid-junction potential. The essentially electrostatic nature of its solvation allows K+ to be used as a model for non-specific solvent-ion interactions. A... 

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