Iso-solvation point

A convenient measure of the degree of selective solvation is the bulk solvent composition at which both solvents of a binary mixture participate equally in the contact solvation shell. This is the solvent composition at which the NMR chemical shifts lie midway between the values for the two pure solvents. This composition has been called the equisolvation or iso-solvation point (usually expressed in mole fractions of one solvent) [125]. According to Fig. 2-10, this point describes the bulk solvent composition at which both solvents A and B participate equally in the solvation shell of the cation or the anion, respectively. 

A useful probe of the immediate chemical environment of solute ions is the NMR chemical shift of alkali metal ions obtained in binary solvent mixtures [111, 126, 295]. These measurements are based on the assumption that the chemical shift of the solute cation is determined in an additive fashion by the solvent molecules comprising the first solvation shell. For example (cf. Fig. 2-11), the iso-solvation point of Na in dimethyl sulfoxide/acetone mixtures occurs at xxO.21 cmol/mol dimethyl sulfoxide, indicating the higher solvating ability of this solvent relative to acetone. As shown schematically in Fig. 2-11, the preferential solvation of Na by dimethyl sulfoxide displaces its chemical shift towards <5dmso and a deviation from the straight line is observed. 

TABLE 6.5 The Iso-solvation Points Equaling the Local Mole Fractions for Sodium Ions in Binary Mixtures of Solvents A and B°... 

---