Preferential solvation free energy

In the case of regular solutions, the dependence of x /x on Xs/xs is linear [244] and the slope of this function depends on the extent of preferential solvation. However, the change of the free energy of solvation, represented by the free energy of transfer, when the solvent composition changes from 8, to 82 is not always monotonic, as shown in Fig. 9. 

Proton NMR studies of N-methyl formamide (NMF) and NMA at high dilution in deuterated solvents have shown that the level of cis isomer of NMF is 8% in water, 10.3% in chloroform, 8.8% in benzene, and 9.2% in cyclohexane, while the level of cis-NMA (a model for the secondary peptide bond) is 1.5% in water and does not change very much in nonpolar solvents [18]. Ab initio molecular calculations suggest that the small difference in dipole moments in cis and trans forms explain the relative insensitivity of amides to solvent change, unlike esters [22,41], This may be explained by nearly identical free energies of solvation for the two isomers [18]. The energy difference between cis and trans isomers in aqueous solution (AG° = 2.5 kcal mol-1) accounts for the preferential trans conformation adopted by most peptide bonds. Similar results were obtained with nonproline tertiary amides [22]. 

Comparison with Experiment. DEUTERIUM OXIDE AND WATER MIXTURES. For a solute showing no preferential solvation (K = 1, AGps0, = 0), then dp = xp from Equation 65, and a plot of 5 against xp is a straight line. Such behavior has been found for the fluoride ion in D2O-H2O mixtures (24). The free energy of transfer from Equation 67 for an ion which shows no preferential solvation will be given by... 

Dioxane and Water. Grunwald and co-workers (GBK) (7) used a vapor pressure method to obtain the differential of the free energy of transfer of a solute with respect to solvent mole fraction at 50 wt% dioxane. On the basis of what has now become known as the large-ion assumption (8), they separated cation and anion effects by equating the free energies of transfer for tetraphenylborate and tetraphenylphosphonium ions. They concluded that Na+ was preferentially solvated by dioxane, a surprising result then, but less unexpected now that complexes of the alkali metals with polyethers have been discovered (dioxane... 

If we adopt some reasonable extrathermodynamic assumption (37) in order to get the individual free energy of transfer of the ions, we find that AG°t(—) of the bromide ion is positive from water to acetone and that AG°t (+) of the n-Bu4N+ is negative over the whole solvent concentration range. In terms of preferential solvation models we would say that Br is solvated by the water molecules and n-Bu4N+ by the acetone molecules. 

Generally, in mixtures of protic solvents with aprotic solvents and especially in mixtures of two aprotic solvents the dependence of AG° j on composition is fixed by the transfer free energy of ion i between both pure solvent components. For cations quite often the sign of d (AG° +)/dx2 is opposite to that of AG +(x2 = 1) 46-49) ag +(x2 = 1) < 0. That means, AG + decreases on addition of component 2, because the cation is preferentially solvated >y solvent component 2. If, on the other hand, AG + (x2 = 1) > 0, the solvent 2 changes AG + only drastically when the mixture is composed mainly out of solvent component 2 that men as, d (AG +)/ dx2 > 0 In the case of anions, these relations only hold sometimes X... 

Solvation is a critical issue in carbohydrate modeling. Both implicit and explicit solvation strategies have been employed. While both a semiempirical quantum mechanical continuum water modeP and a free energy simulation with explicit water have predicted similar values for the free energy difference between the p- and a-anomers of 5 (AGp ), the former approach suggested that the solution free energy (—0.5 kcal/mol) was dominated by gas phase effects, whereas the latter simulation indicated that preferential solvation of the P-anomer was in part responsible for the value (-0.3 kcal/mol) of AGp > . 

Solvent activity coefficients allow the construction of diagrams such as Fig. 6.2.1 which show the relative free energies of ions in a range of solvents. However, they do not permit us to forecast which solvent is fractionated into the solvation sphere in a mixed solvent environment, since in addition to the ion-solvent interaction we must also consider the solvent-solvent interactions. The preferential solvation of ions such as [Cr(NCS)e] by CH3CN in CH3CN-H2O solvent mixtures is perhaps best regarded as a rejection of CH3CN by the water structure, rather than a predominance of any ion-solvent interactions. 

An attempt to introduce a quantitative measure of preferential solvation of ions concluded that it occurs primarily due to the difference in the Gibbs energies of solvation of the two solvents under study. Solvent solvent interaction is another important factor controlling preferential solvation. This is illustrated by the solvation of Co" in mixtures of TMU and water. TMU is a much stronger donor solvent than water and, thus, would be expected to preferentially solvate with cations over water. However in H2O TMU mixtures with a larger portion of water, the Co" ion preferentially solvated with water. The results can be explained in terms of strong TMU-H2O intermolecular interactions in the bulk, which result in the disappearance of free TMU. In mixtures containing TMU as a major portion, Co" is preferentially solvated with TMU, while spectro-photometric evidence shows that Co" forms the tetra-solvated [Co(TMU)4] " " in neat TMU. 

It should be borne in mind that components of a mixed solvent may be nonuniformly distributed between the gel and the solution. For a swollen gel, the composition of the mixed solvent inside a gel is usually the same as in the external solution, whereas the collapsed gel is enriched with the thermodynamically good solvent component as compared to the external solution. The results of a theoretical analysis showed that such a redistribution of the solvent components between collapsed gel and solution is more pronounced in a system with a higher value of the parameter of interaction xas between the solvent components. This is related to the fart that an increase in the xab value reflects the growing tendency to phase separation in the solvent, whereby preferential solvation of the thermodynamically good solvent component in the gel becomes energetically more favorable leading to free energy variation in the same direction as that upon the phase separation. 

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