Ion Solvation in Neat Solvents

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... 

Table 5-21 shows that the addition of even small proportions of EPD solvents affeets the reaetion rate markedly. The rate acceleration thus obtained is produced by a specific solvation of sodium ion, which tends to dissociate the high-molecular mass ion-pair aggregate of the sodio-malonic ester that exists in benzene solution (degree of aggregation n is equal to 40... 50 in benzene). This indicates that the kinetically active species is a lower aggregate of the free carbanion. Further evidence for a specific cation solvation is derived from the six-fold rate difference observed in tetrahydrofuran (fir = 7.6) and 1,2-dimethoxyethane (fir = 7.2), despite the fact that these two solvents possess nearly equal relative permittivities. The latter solvent is able to solvate sodium ions in the manner shown in Eq. (5-127). Especially noteworthy is the high reactivity exhibited on the addition of dicyclohexyl[18]crown-6. In benzene solution containing only 0.036 mol/L of this crown ether, the alkylation rate is already equal to that observed in neat 1,2-dimethoxyethane [351]. 

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. 

In microemulsions RTE s feel a restricted environment in the core of the microemulsions. Thus, we have observed slower solvation time of C-153 in microemulsions compared to neat KITLs, bnt the retardation in the solvent relaxation time is much less compared to that of neat RTILs. With an increase in the [Bmim] [PFg] content the size of microemulsions increases. The small change in time constants of solvent relaxation with an increase in R is due to an increase in the size of the microemulsions. With an increase in size, the free motion of ions of RTILs in the core of the microemulsions has increased, and this leads to change in time constants of solvent relaxation. 

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 physical and chemical properties of (binary) solvent mixtures, necessary for understanding the solvation of ions in the mixtures, are dealt with in Sections 3.4.1 and 3.4.2, respectively. The preferences of ions for certain solvents over others are described in terms of their standard molar Gibbs energies of transfer from a source solvent (water has generally been arbitrarily selected as this source) to neat target solvents in Section 4.3.2.I. These sections should be consulted to complement the present discussions regarding ions in mixed solvents. 

Clustering of a solnte by a small number of solvent molecules allows one more variable for testing our ideas about how properties scale with the number of molecules in the solvation shell.For example, fast ionization of a neat cluster generates an ion in a non-equilibrium environment and is a way to explore the dynamics of ion solvation. Here we consider another aspect of the caging dynamics in a series of experiments a dihalogen ion, solvated by n CO2 molecules, is photoexcited above its dissociation limit. The quantum yield of atomic and molecular ions is determined as a function of n... 

Phenyllithium is known to form, in equilibrium with the monomer, a dimer 25 (Scheme 1-21) held together by electron-deficient partial bonds. However, phenyllithium can also adopt the structure of a lithium lithiate ion pair 26 (Scheme 1-21). What makes the difference The solvent plays a capital role. As long as the solvation forces remain moderate as in diethyl ether, the doubly carbon-lithium-carbon bridged dimer 25 is energetically most favorable. In a 2 1 mixture of cylohexane and diethyl ether, phenyllithium even assembles a tetrameric cluster. " " in neat THF, the four-centered dimer 25 is found in equilibrium with the monomer. " However, in the presence of the powerful electron-donor hexamethylphosphoric triamide (HMPA), it is the ion pair 26 that coexists with the monomer. Analogous lithiate complexes have been identified with... 

It may appear from the above that only nonpolar liquids yield nonmolecular solvent anions upon the ionization. Perhaps this is misleading Most polar liquids studied by radiation chemists are aliphatic alcohols and water, and these liquids yield solvated electrons rather than radical ions. Although there has been sporadic interest in other polar liquids (e.g., neat acetone), the current state of knowledge of such systems does not allow one to reach any conclusion as to the nature of the reducing species observed therein (although, see Sec. 4). 

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