The use of the spectrophotometric solvent sorting method for determining the free energy of transfer of the proton, AG,"(H+), from water into waterxosolvent mixtures has been extended to mixtures formed by adding structure breaking urea to water. Values for AG;(XZ-) are calculated from values for AG,"(H,X) using the experimentally determined values for AG,"(H+), and the derived values for AG:(XZ-) with x = 1.0 are used to calculate values for AG:(M+). Values for AG,"(XZ-) are positive and values for AG:(M+) are negative at mole fractions of urea x < 0.15, as found for mixtures of water with structure-forming cosolvents (excluding BPh,).However, in water-urea mixtures, the influence of structure-forming hydrocarbon ligands in controlling values for AG,"(i) is much reduced, compared with their influence when structureforming cosolvents are used: for example, AG:(BPh;) is positive with added urea and negative with other cosolvents. Moreover, values for AG,"(i) for i = Ph,As+ and BPh;, 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.

The kinetics of the solvolysis of the 1,6-[Coen2C1,]+ ion in mixtures of water with t-butyl alcohol have been investigated for concentrations of the alcohol up to a mole fraction of 0.16 for a range of temperatures. Values for the enthalpy and entropy of activation show an extremum at the same mole fraction where the relative partial molar volume of t-butyl alcohol shows a minimum; both also show an inflection at the concentration of t-butyl alcohol where the mixtures have a maximum in their ultrasonic absorption. In general, plots of log(rate constant) against the reciprocal of dielectric constant are curves for the solvolysis of 1 ,6-[Coen2C1,]+ in water +cosolvent mixtures, and the application of a free-energy cycle to this solvolysis in water + t-butyl alcohol mixtures shows that changes in solvent structure have a greater effect on the pentacoordinated cobalt ion in the transition state than on the hexacoordinated ion in the initial state. These results are compared with those for the solvolysis of other complex cations in water + cosolvent mixtures, particularly where the cosolvent is t-butyl alcohol.

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The spectrophotometric solvent-sorting method for determining the free energies of transfer for individual ions, AG:(i), from water into a mixture of water and cosolvent has now been applied to cosolvents which increase the dielectric constant and enhance the solvent structure when added in small concentrations to water. The spread of AGY(i) values found, uiz. AG:(i) for i = a cation is negative and AG;(i) for i = an anion is positive, is similar to those found in mixtures where the cosolvent decreases the dielectric constant and enhances the solvent structure or increases the dielectric constant and decreases the solvent structure. Values for AG:(i)e, the free energy of transfer of the charge alone, are found for some large organic anions.