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 ΔGot(Xx–) are calculated from values for ΔGot(HxX) using the experimentally determined values for ΔGot(H+), and the derived values for ΔGot(Xx–) with x= 1.0 are used to calculate values for ΔGot(M+). Values for ΔGot(Xx–) are positive and values for ΔGot(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 ΔGot(i) is much reduced, compared with their influence when structure-forming cosolvents are used: for example, ΔGot(BPh–4) is positive with added urea and negative with other cosolvents. Moreover, values for ΔGot(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.