Ionic solvation in water–cosolvent mixtures. Part 11.—Free energies of transfer of single ions from water into water–urea mixtures

Abstract

The use of the spectrophotometric solvent sorting method for determining the free energy of transfer of the proton, ΔG°_t(H⁺), from water into water–cosolvent mixtures has been extended to mixtures formed by adding structure breaking urea to water. Values for ΔG^o_t(X^x–) are calculated from values for ΔG^o_t(H_xX) using the experimentally determined values for ΔG^o_t(H⁺), and the derived values for ΔG^o_t(X^x–) with x= 1.0 are used to calculate values for ΔG^o_t(M⁺). Values for ΔG^o_t(X^x–) are positive and values for ΔG^o_t(M⁺) are negative at mole fractions of urea x < 0.15, as found for mixtures of water with structure-forming cosolvents (excluding BPh^–₄). However, in water–urea mixtures, the influence of structure-forming hydrocarbon ligands in controlling values for ΔG^o_t(i) is much reduced, compared with their influence when structure-forming cosolvents are used: for example, ΔG^o_t(BPh^–₄) is positive with added urea and negative with other cosolvents. Moreover, values for ΔG^o_t(i) for i= Ph₄As⁺ and BPh^–₄, which are closely related in size and peripheral hydrocarbon structure, differ considerably in water—urea mixtures, showing the importance of the sign of the charge.