Solvation of electrolytes, preferential

Thermodynamics of Preferential Solvation of Electrolytes in Binary Solvent Mixtures... 

Preferential Solvation of Some Electrolytes by Water and Diethyl Ether in Sulfolane... 

HP he study of the behavior of electrolytes in mixed solvents is currently arousing considerable interest because of its practical and fundamental implications (1). Among the simpler binary solvent mixtures, those where water is one component are obviously of primary importance. We have recently compared the effects of small quantities of water on the thermodynamic properties of selected 1 1 electrolytes in sulfolane, acetonitrile, propylene carbonate, and dimethylsulfoxide (DMSO). These four compounds belong to the dipolar aprotic (DPA) class of solvents that has received a great deal of attention (2) because of their wide use as media for physical separations and chemical and electrochemical reactions. We interpreted our vapor pressure, calorimetry, and NMR results in terms of preferential solvation of small cations and anions by water and obtained... 

Clearly, the conditions above hold because the system, though open with respect to D, is not open with respect to A and C individually. The KB theory applies for any three-component system of W, A, and C without any restriction on the concentrations of A and C, i.e., when the system is open to each of its components. If this happens for an electrolyte solution, clearly the conservation of the total charge in the system will not hold, and fluctuations in A and C would lead to fluctuations in the net charge of the system. One should not interpret equations (8.49), (8.51), or (8.53), as implying anything on the preferential solvation of W, A, or C. The reason is that the condition of the conservation of the total number of A and C must impose a long-range behavior on the various pair correlation functions. This is similar to a two-component system of A and B in a closed system, where we have (see section 4.2) the conservation relations... 

When hexamethylbenzene was oxidised in acetonitrile containing acetic acid, perchlorate electrolytes favoured the acetamidation reaction, whereas fluoroborate led to relatively higher acetoxylation yields. This fact was interpreted in terms of preferential solvation of fluoroborate by acetic acid. 

Mixtures of water with cosolvents are often used as solvents for electrolytes and ions so that those properties of the mixtures that are related to the (possibly preferential) solvation of the ions by the components of such mixtures need to be known. As for the neat solvents dealt with in the previous sections of this chapter, the discussion concerning those solvents marked as miscible with water in Table 3.10 involves physical and chemical properties, to be dealt with in turn. Much of the information below is adapted from the book by Marcus [56]. 

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... 

The consideration of an individual ion in the function D has the usual problems of thermodynamic quantities pertaining to individual ions connected with it. When the method is applied to the transfer of a symmetrical electrolyte, the result would indicate the same preferential solvation of the cation and of the anion (counter to experience, according to which heterosolvation prevails). This is, because according to Equation 6.21, the Kirkwood-Buff integrals extend to infinity and so comprise both the cation and the anion even at infinite dilution. The application of the TATB... 

Very little work has been done in this area. Even electrolyte transport has not been well characterized for multicomponent electrolyte systems. Multicomponent electrochemical transport theory [36] has not been applied to transport in lithium-ion electrolytes, even though these electrolytes consist of a blend of solvents. It is easy to imagine that ions are preferentially solvated and ion transport causes changes in solvent composition near the electrodes. Still, even the most sophisticated mathematical models [37] model transport as a binary salt. 

In studies on solvent effects involving variation in the composition of two component mixtures, similar types of outer-sphere interactions yield preferential solvation wherein the solvent composition of the outer-sphere may differ markedly from the bulk solvent composition. Supporting electrolyte species and buffer components may also participate in outer-sphere interactions thereby changing the apparent nature (charge, bulk, lability) of the reacting solvated metal ion or metal complex as perceived by a reacting ligand in the bulk solvent. 

If cation and anion are preferentially solvated by the same solvent component (homo-selective solvation) the phase separation temperature of the solvent mixture is shifted to higher temperatures. On the other hand, with heteroselective solvation of an electrolyte, when one ion is preferentially solvated by one solvent component and the counterion by the second component, the upper critical solution temperature decreases by adding that electrolyte. 

Preferential solvation is not restricted to ions of electrolytes dissolved in multi-component solvent systems. Even for dipolar nonelectrolyte solutes the composition of the solvation shell can deviate from that of the bulk solvent mixture, as shown for P-disulfones [255] and A-methylthiourea [256]. 

After defining the local composition and preferential solvation, we turn to discuss these quantities in more detail first, in three-component systems and later in two-component systems. This order of systems is not accidental. The concept of PS was first defined and studied only in three-component systems a solute s diluted in a two-component solvent. It is only in such systems that the concept of PS could have been defined within the traditional approach to solvation. However, with the new concept of solvation, as defined in section 7.2, one can define and study the PS in the entire range of compositions of two-component systems. In the last section of this chapter, we present a few representative examples of systems for which a complete local characterization is available. These examples should convince the reader that local characterization of mixture is not only equivalent to its global characterization, but also offers an alternative and more informative view of the mixture in terms of the local properties around each species in the mixture. We also present here a brief discussion of two difficult but important systems electrolyte and protein solutions. It is hoped that these brief comments will encourage newcomers into the field to further study these topics of vital importance. 

All the definitions of the local composition and preferential solvation are applicable for any mixture, including electrolyte solutions. Suppose for simplicity we have a solution of an electrolyte D in water W. Viewing this system as a mixture of a two-component system, we can apply all the equations of the previous section. 

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