Solvents, mixed aqueous preferential solvation

Since anions are much less solvated in dipolar aprotic solvents (23) than in water, the hydrogen ion will be more highly solvated in the mixed solvent because it is preferentially solvated by monoglyme in the monoglyme-water mixtures rather than in the pure aqueous medium. The selective solvation is an important factor in an understanding of solute-solvent interactions in mixed solvent systems. Unfortunately, the detailed compositions of the primary solvation shell and the secondary mode of solvation (ion-dipole interaction) in mixed solvents are not yet clearly understood. 

So far the KBIs have been calculated for numerous binary systems, and the results were used to examine the solution behavior with regard to (1) local composition, (2) various models for phase equilibrium, (3) preferential solvation, and others One should also mention the use of the KB theory for supercritical fluids and mixtures containing supercritical components and for biochemical issues such as the behavior of a protein in aqueous mixed solvents. ... 

The present paper is devoted to the derivation of a relation between the preferential solvation of a protein in a binary aqueous solution and its solubility. The preferential binding parameter, which is a measure of the preferential solvation (or preferential hydration) is expressed in terms of the derivative of the protein activity coefficient with respect to the water mole fraction, the partial molar volume of protein at infinite dilution and some characteristics of the protein-free mixed solvent. This expression is used as the starting point in the derivation of a relationship between the preferential binding parameter and the solubility of a protein in a binary aqueous solution. 

Many characteristics of a protein in aqueous solvents are connected to its preferential solvation (or preferential hydration). The protein stability is a well-known example. Indeed, the addition of certain compounds (such as urea) can cause protein denaturation, whereas the addition of other cosolvents, such as glycerol, sucrose, etc. can stahihze at high concentrations the protein stiucture and preserve its en2ymatic activity [4-7]. The analysis of literature data shows that as a rule Ffor the former and r23 " <0 for the latter compounds. Recently, the authors of the present paper showed how the excess (or deficit) number of water (or cosolvent) molecules in the vicinity of a protein molecule can be calculated in terms of F2 the molar volume of the protein at infinite dilution and the properties of the protein-free mixed solvent [8]. The protein solubility in an aqueous mixed solvent is another important quantity which can be connected to the preferential solvation (or hydration) [9-13] and can help to understand the protein behavior [9-17]. 

The aim of the present paper is to establish a relation between (1) the preferential solvation (or hydration) of a protein and (2) the protein solubility in an aqueous mixed solvent. The obtained relation will be used to predict the protein solubility in an aqueous solvent in terms of the preferential binding parameter. 

In an extension of these ideas, Franks and Reid have examined the ionic entropies in mixed water-methanol solutions (see Appendix 2.4.42) and in 20 % aqueous dioxan, and have observed that the entropies of the ions for each of these systems can be expressed by an equation having the same form as eqn. 2.11.36. Similar to the pure solvent systems, the entropy of a given ion has no correlation with the solvent dielectric constant, nor is there a linear correlation of the entropy with solvent composition. Instead, the entropies reach a maximum in the vicinity of 40 mol per cent methanol. The authors explain this in terms of the solvent having the highest degree of structure near this composition. As in the pure non-aqueous solvents, the relative magnitude of the effect of ions on the solvent structure is the same for all ions, both negative and positive. This observation led the authors to conclude that there is no evidence for preferential solvation in these mixed solvent systems. 

The behaviour of acids in miscible aqueous organic mixtures is highly dependent upon the mole fraction of each solvent. In mixed-solvent systems, the solvent component with the greatest affinity for the solute will provide preferential solvation. Thus, when reducing the mole fraction of the preferred solvent, the properties of solutes display a significantly smaller than expected change until the preferred solvent drops below a certain mole fraction, typically below 20-40%, whereupon more dramatic changes in solute properties occur. ... 

Marcus Y (2001) Preferential solvation in mixed solvents. 10. Completely miscible aqueous cosolvent binary mixtures at 298.15 K. Monatsh Chem 132 1387-1411 Marcus Y (2002a) Solvent mixtures. Dekker, New York... 

Marcus Y (2002b) Preferential solvation in mixed solvents. 11. Eight additional completely miscible aqueous co-solvent binary mixtures and the relationship between the volume-corrected preferential solvation parameters and the structures of the co-solvents. Phys Chem Chem Phys 4 4462-4471... 

Marcus Y (2003) Preferential Solvation in Mixed Solvents. 12. Aqueous glycols. J Mol Liquids 107 109-126... 

Preferential Solvation of Ions in Aqueous Mixed Solvents... 

FIG U RE 3.2 Volume-corrected preferential solvation parameters ( ) for water-water interactions and Sr s (A) for water-solute interactions in the first solvation shell of aqueous mixtures with solutes identified in the label of the abscissa. (From Y. Marcus, 2001, Preferential Solvation in Mixed Solvents, Part 10, Completely Miscible Aqueous Co-Solvent Binary Mixtures at 298.15 K, Monatshefte fur Chemie, 132, 1387, by permission of the publisher, Springer.)... 

From Y. Marcus, 2001, Preferential Solvation in Mixed Solvents, Part 10, Completely Miscible Aqueous Co-Solvent Binary Mixtures at 298.15K, Mormtshefte fur Chemk, 132, 1387, unless otherwise noted. 

Other studies of the preferential solvation for which information can be derived from KBIs in ternary systems have also been made. The system -heptane -i- ethanol + 1-propanol at 313 K (Zielkiewicz 1995a) showed that ethanol and 1-propanol mix in a random manner in the presence of -heptane with no preferential solvation between these two solvents. The same author studied the solvation of N,N-dimethylformamide (C) in mixtures of water (A) and each of methanol, ethanol, and 1-propanol (B) at 313 K (Zielkiewicz 1995b). At Xc > 0.8 this component was solvated equally by A and B, but at Xc < 0.15 it was preferentially hydrated, that is, solvated by A, except when x > 0.8, where the solvation of C by A and B was random. A,A-dimethylformamide (C) featured also in the studies (Ruckenstein and Shulgin 2001a) of it in aqueous (A) methanol (B). The KBIs in the system n-hexane + 1-hexanol + methyl benzoate were studied at 298 K (Aparicio et al. 2005). They calculated the excess (or deficit) number of molecules of, say. A, around molecules of B in pseudobinary systems at constant mole fraction of C from... 

Marcus, Y. 1999. Preferential solvation in mixed solvents. Part 8. Aqueous methanol liom sub-ambient to elevated temperatures. Physical Chemistry Chemical Physics. 1, 2975. 

A great deal of experimental work has been carried out on acid-base equilibria in mixed solvents, especially mixtures of water with organic solvents. The presence of two solvent species introduces a number of complications. In the first place, there are now a number of different acidic and basic species derived from the solvent. Thus in aqueous alcohol we have as acids H2O, EtOH, H30, and EtOHj, and as bases H2O, EtOH, OH, and EtO". In the second place, the composition of the solvent can now vary in the neighbourhood of an ion (and to a smaller extent near an uncharged molecule) by a preferential solvation effect, so that the macroscopic properties of the solvent will be even less relevant than they are with pure solvents. For these reasons the problem of mixed solvents will not he discussed here. 

It is, of course, natural from many points of view that aqueous solutions have been in the foreground for studies of electrolyte solutions, while studies of halide ion quadrupole relaxation in non-aque-ous solvents are quite few. However, studies of non-aqueous and mixed solvent systems are in certain respects highly relevant. For example, in order to test relaxation theories the possibility of making marked changes in solvent dipole moment, molecular size, dielectric constant, solvation number etc. should be very helpful. Also, the elucidation of certain general aspects of interactions and particle distributions in electrolyte solutions may be more easily achieved for non-aqueous systems. One such point is ion-pair formation, which for simple salts is not of great importance in water. Finally, of course, the quadrupole relaxation method may, as for aqueous solutions, be applied to more special problems such as ion solvation, complex formation etc. In studies of preferential solvation phenomena disorder effects in the first sphere may in certain cases be expected to lead to dramatic changes in the quadrupole relaxation rate. 

Solvation of f-element cations in mixed aqueous solvents has been studied using the fluorescence technique. With Eu(III) ion as a probe (Tanaka et al. 1988, Lis and Choppin 1991), in water-acetone, water-acetonitrile and water-1,4-dioxane, the first solvation sphere is occupied exclusively by water molecules even at bulk-phase mole fractions of water as low as 0.1 (fig. 6, curve I). However, in solvents with more basic donors such as DMSO (DMSO is dimethylsulphoxideX the metal ion is preferentially solvated by DMSO even for very low mole fractions of DMSO in the solvent (curve II, fig. 6). Less regular behavior can be expected for systems in which (a) strong interactions exist between the components of the solvent mixture or between the coordinated... 

The standard partial molar volumes of electrolytes in mixed solvents can be modeled, as can those in neat solvents, in terms of the sum of the intrinsic volumes of the ions and their electrostriction. It is assumed that the intrinsic volumes, that is, the volumes of the ions proper and including the voids between ions and solvent molecules, are solvent independent, so that they do not depend on the natures of the solvents near the ions. Then, if no preferential solvation of the ions by the components of the solvent mixture takes place, the electrostriction can be calculated according to Marcus [32] as for neat solvents (Section 4.3.2.5), with the relevant properties of the solvents prorated according to the composition of the mixture. This appeared to be the case for the ions Li+, Na", K+, CIO ", AsE , and CFjSOj in mixtures of PC with MeCN, in which V (P,PC+MeCN) is linear with the composition over nearly the entire composition range. This is the case also for Me NBr in W+DMSO, as shown in Figure 6.1. Similarly, in aqueous methanol mixtures, smooth curves result for the ions Li", Na ", K+, Cs", CF, Br", and I" like those shown in Figure 6.1 for NaBr and KBr. However, when preferential solvation occurs, the... 

Solvolysis in Mixed Solvents.— The n.m.r. method for probing the solvation shell of complexes in mixed solvents, developed with particular reference to aquation of [Cr(NCS)6] in aqueous acetonitrile, has proved useful in investigating details of the mechanism of solvolysis of [Cr(ox)3] , this time in aqueous dimethyl sulphoxide. There is preferential solvation by the... 

Group 1. Alkali Metals. NMR spectroscopy has been used to examine the preferential solvation of Li" in non-aqueous mixed solvents. Salt effects on the self-association of ethanol in water have been studied. The eoneentration dependence of " N, Na, and Cs chemical shifts in MNO3-HNO3 has... 

Non-aqueous Solvents.—Mention has already been made of the evidence for an associative (/a) mechanism for the exchange of DMSO with the [Cr(DMSO) ] + ion. Both the entropy and volume of activation are large and negative. A recent study of the exchange process by n.m.r. in DMSO-MeNOj mixed solvents (nitro-methane is an inert, non-co-ordinating diluent which is known to have very little rate effect) shows that the exchange rate is approximately constant above 0.2 mole fraction of DMSO, but drops off sharply below this concentration. On the other hand the fraction of DMSO molecules in the solvation shell of the [Cr(DMSO) J + ion decreases immediately with the decrease in the DMSO mole fraction. This difference in concentration effects is postulated to arise from a unique outer-sphere solvation site which preferentially binds DMSO molecules and preferentially participates in the exchange reaction. 

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