Isosolvation point

Fig. 2-11. NMR chemical shift of Na as a function of the mole fraction of dimethyl sulfoxide (DMSO) in a binary mixture of DMSO and acetone (according to [295]). Straight line ideal case without preferential solvation, primary solvation shell of the same composition as the bulk solvent mixture. Curved line real case with preferential solvation of Na by DMSO and isosolvation point at xoMso/(cmol mol ) 0.21, that is, the mole fraction of the bulk solvent for which the solvated ion chemical shift is the average of the shifts obtained in the pure solvents (A<5 = <5dmsO - Acetone)-...

NMR spectroscopy has been used in the study of the preferential solvation of ions in two-component mixtures by Frankel et al. [248]. They assumed that the midpoint of the chemical shift between two pure solvents, when studying the resonance frequency of the ion in question, points to the isosolvation point at which the inner solvation shell of the studied ion is equally populated by molecules of both solvents. If at this point the mole fraction of one of the two solvents in the bulk is low, its solvating properties are high, and conversely. 

Table 10. Isosolvation points of Na(I) in binary solvent mixtures [250],...

Solvent added of NMF Mole fraction to NMF at isosolvation point, x. Donor number of solvent added to NMF 38.8 Xj 0.74... 

In Table 10 we give, as an illustration, such isosolvation points for sodium(I) in the binary mixtures of 7V-methylformamide (NMF) with various solvents determined by Popov and coworkers [250]. The last column shows the normalized values of the parameter which, in the ideal case, should be close to the donor numbers of the applied solvents. By comparing pairs of values from the last two columns of Thble 10, we can see that although these values are not identical, the trend of the change of the (38.8 Xi/0.74) parameter for the applied solvents is similar (with the exception of pyridine (Py) and ethanol) to that of the donor numbers. This order of solvating power, found for Na(I), may differ to some extent for other cations. This study [250] also shows that the earlier proposed DN value of 18.1 for water is too low. 

A measure of selective solvation of single ions is the isosolvation point, that is, the solvent composition at which the mean numbers of molecules of the two solvent components bound to an ion are the same. Rahimi and Popov (1976) found by Ag+ NMR that the isosolvation point for Ag+ in water-acetonitrile was at 0.075 mol fraction of acetonitrile. This means that solvation by this component is preferred by about 6.3 kJ mol over water. 

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