Cecil F. Wells

Ionic solvation in water + co-solvent mixtures: Part 19. Free energies of transfer of single ions from water into mixtures of water with polyhydroxy compounds: an examination of the assumptions used in determining ΔGt⊖ (i)

Abstract The assumptions underlying the spectrophotometric solvent-sorting method for determining Δ G t XXX ( i ) for individual ions from water into water-rich water + co-solvent mixtures have been critically examined. Additions of the hydrophilic co-solvents glucose, mannitol, sorbitol and inositol to water support the original view, derived from the effect of additions of glycerol, that the main assumption holds at least for x 2 ≲ 0.25 and for some with x 2 ≲ 0.35. Errors in criticisms of this method are specified and the extent of the applicability of the TATB method for producing Δ G t XXX ( i ) is examined. Free energies of transfer for H + , Cl - , Na + , K + and OH - have been calculated from E XXX data and vapour pressures for these hydrophilic co-solvents where available. These are compared with values obtained using additions of hydrophobic alcohols and aprotic cosolvents.

Ionic solvation in water + co-solvent mixtures

Abstract The method developed previously for deriving free energies of transfer of the charge only on large single ions from water into water-rich water + co-solvent mixtures Δ G t XXX ( i ) e has now been applied to water + ethanol mixtures. This derivation depends only on the experimental p K values of a large acidic positive ion producing a neutral base or of a large neutral acid producing a negative ion and on the total free energy of transfer of the proton, Δ G t XXX (H + ). Water + ethanol has been chosen for this investigation as Δ G t XXX (H + ) for mole fractions of ethanol − 0.28–0.40 is largely independent of the assumptions used in either the spectrophotometric solvent sorting method or in the reference ion method using Δ G t XXX (Ph 4 As + ) = Δ G t XXX (BPh 4 − for calculating Δ G t XXX (H + ). The significance of the variation of Δ G t XXX ( i ) e with ionic size, the sign of the charge and the extent of the distribution of the charge over the bulk of the ion in this composition range x 2 ~ 0.28–0.40 is discussed.

Ionic solvation in water + co-solvent mixtures: Part 17. The “neutral” component of free energies of transfer of single ions from water into water + ethanol mixtures

Abstract The influence of the contribution to the total free energy of transfer of an ion i ΔG⊖t(i) arising from the transfer of the neutral bulk of the ion ΔG⊖t(i)n is assessed. Values for ΔG⊖t(i)n for a range of inorganic and organic molecules, in the transfer water → water + ethanol mixtures, are calculated from the solubilities of the gaseous molecules in general, and of solid molecules in one case. ΔG⊖t(i)n is then compared with ΔG⊖t(i) previously determined for ions. By using the basic relationship of scaled particle theory, after eliminating the contribution from the free energy of cavity formation by restricting consideration to the transfer to water-rich mixtures only, experimental values for the specific effect of changes in solvent composition on the interaction of the neutral bulk of i with the solvent are obtained. It is shown that this latter contribution to ΔG⊖t(i) is small for simple inorganic ions, leaving the free energy of transfer of the charge alone ΔG⊖t(i)e dominant in determining ΔG⊖t(i). For more complex ions containing organic groups, the contribution of ΔG⊖t(i)n to ΔG⊖t(i) begins to compare with ΔG⊖t(i)e, depending on the size of the organic groups.

Ionic solvation in water + co-solvent mixtures. Part 8. Total free energies of transfer and free energies of transfer with the “neutral” component removed of single ions from water into water + acetone

Abstract The acid dissociation constants of a wide range of acids in water+acetone mixtures have been combined with values for the free energy of transfer of the proton. Δ G 0 t (H + to calculate values for the free energy of transfer of ions which derive only from the charge on the ion. Δ G 0 t ( i ) c . As the values of Δ G 0 t (H + ) have been revised, revised values for the total free energies of transfer of cations and anions, Δ G 0 t (M + ) and Δ G o t (X - ), are given. New data for Δ G o t (MX n ) is also split into values for Δ G 0 t (M n + ) (where n =1 and 2) and Δ G 0 t (X − ). These free energies of transfer, both total and those deriving from the charge alone, are compared with similar free energies in other mixtures water+co-solvent. Values for Δ G o t ( i ) c do not conform to a Born-type relationship and show the importance of structural effects in the solvent even when only the transfer of the charge is involved.

Ionic solvation in water–Co-solvent mixtures. Part 18.—Free energies of transfer of single ions from water into water–2-methoxyethanol mixtures

ΔG°t(H+), the free energy of transfer of the proton from water into water–co-solvent mixutres, has been determined experimentally for the co-solvent 2-methoxyethanol keeping the conditions water-rich. Specific criticisms of the method used are discussed and rejected . ΔG°t(X–) for simple anions has been calculated from ΔG°t(HX) leading to the calculation of ΔG°t(M+) from ΔG°t(MX). ΔG°t(i)e, the free energy of transfer of the charge alone, has been determined for organic cations and anions using ΔG°t(H+) found here, combined with values for the pK of acids and bases in water–2-methoxyethanol. ΔGintt, the difference in the partial molar free energy of interaction of a molecule with the solvent in transferring the molecule from water into the mixture has been determined for two electrically neutral organic bases. The values for ΔG°t(i), ΔG°t(i)e and ΔGintt(i) are compared among themselves and with values found in mixtures of water with other co-solvents.

Ionic solvation in water–cosolvent mixtures. Part 14.—Free energies of transfer of single ions from water into water–ethylene carbonate and water–propylene carbonate mixtures

The spectrophotometric solvent-sorting method for determining the free energies of transfer for individual ions, ΔG°t(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 ΔG°t(i) values found, viz.ΔG°t(i) for i= a cation is negative and ΔG°t(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 ΔG°t(i)e, the free energy of transfer of the charge alone, are found for some large organic anions.

Kinetics of the solvolysis of trans-chloroazidobis(1,2-diaminoethane)cobalt(III) ions in water and in water + propan-2-ol mixtures

Although previous kinetic results on the solvolysis of trans-[Coen2N3Cl]+ in water + co-solvent mixtures suggest that plots of log (rate constant) against the reciprocal of the dielectric constant are linear and independent of the co-solvent used, such a plot for propan-2-ol as co-solvent shows considerable curvature. The conclusion that might be drawn from the earlier results that the influence of solvent structure on the initial and transition states is about the same does not apply here and the application of a free-energy cycle to the process initial state → transition state in water and in the mixed solvent shows that the effect of solvent structure dominates the complex in the transition state. Enthalpies and entropies of activation for the solvolysis are correlated with changes in the physical properties of the mixtures and are compared with those for the cis-complex.

The stability of the proton in water + co-solvent mixtures

Abstract The Hammett acidity function H 0 gives an indication of the ability of a neutral molecule to be protonated and can be used to compare the relative stabilities of protons in solvents consisting of neutral molecules. The rise in H 0 when an organic, oxygen-containing base is introduced into water containing a mineral acid, accompanied by a decrease in conductivity, conforms to the existence of a solvent-sorting equilibrium around the solvated proton together with competing equilibria involving the protonation of the Hammett-indicator base by two different proton solvates. The proton solvate containing a co-solvent molecule is more stable than that without it, and the ionic mobility of the proton involved with the former is less than that in the latter. Free energies of transfer of the proton from water into the mixture Δ G t ⊖ (H + ) calculated using this solvent-sorting method and the reference ion method (Δ G t ⊖ (Ph 4 As + ) = Δ G t ⊖ (BPh 4 − ) are compared with the rises in H 0 in water-rich media: those using the latter method are shown to be inconsistent with the rise in H 0 for some alcohols. From the extent of the conversion of the aqua-proton into the proton solvate containing the co-solvent, the factors influencing the proton-co-solvent solvate are discussed for a range of solvent compositions. The principal effect over the whole range of water-rich compositions is found to be the ability of substituent groups to supply or withdraw electron density from the basic site in the co-solvent molecule.

Kinetics of the solvolysis of the trans-dichlorobis(1,2-diaminoethane)cobalt(III) ion in water + t-butyl alcohol mixtures

The kinetics of the solvolysis of the 1,6-[Coen2Cl2]+ 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-[Coen2Cl2]+ 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.

Kinetics of the solvolysis of [Co(Rpy)4Cl4]+ with R = 4-t-Bu, 3-Me or 3-Et in mixtures of urea with water

SummaryThe kinetics of the solvolysis of complex ions trans-[Co(Rpy)4Cl2]+, with R = 4-t-Bu, 3-Me and 3-Et, have been investigated in mixtures formed by adding urea to water, which enhances the dielectric constant and decreases solvent structure. Differential effects of the changes in solvent structure on the initial and transition states are found to be important factors controlling changes in the rate constant with solvent composition. The variation of the enthalpy and the entropy of activation with solvent composition are contrasted with their variations found for the solvolysis of [Co(Rpy)4Cl2]+ in mixtures where solvent structure is enhanced by additions of a co-solvent to water. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the Co3+ cation in the transition state is more stable than the Co3+ cation in the initial state in the water + urea mixtures.