Selective Solvation of Ions in Mixed Solvents

The treatment of all those properties of electrolyte solutions, where selective solvation of ions in mixed solvents may play a major role, would result in an accumulation of data hard to follow up. Therefore, only those theoretical treatments of ion solvation have been mentioned in the following whose results have been used to ... 

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. 

Attempts have been made to treat selective ion solvation in mixed solvents as ligand exchange reactions [36 e]. We express ion X (cation or anion) existing in solvent A by XA and in solvent B by XB,-. Here, nf=ri if the molecular size of A is very different from that of B or if A is unidentate and B is bidentate (e.g. PC and DME in the solvation of Li+). Otherwise, it is usual that n=n. Then, the ligand exchange reaction in the mixture of A and B will proceed as follows with the increase in the concentration of B ... 

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. 

The Raman investigations on the structure of water-nonaqueous mixed solvents have been quite abundant from 1981 [41,312-325]. The effect of a cosolvent over the water hydrogen-bonded network as well as the formation of clusters and the hydrophobic hydration have been some of the matters studied. When an electrolyte is dissolved in a mixed solvent, some phenomena can be approached by means of Raman spectroscopy. In addition to the change in the extent of the ionic association promoted by the presence of a nonisodielectric cosolvent, which could be interesting for practical purposes [209], the preferential solvation can also be studied. Preferential or selective ion solvation occurs when the first solvation sphere around the cation and/or the anion has a different composition from that corresponding to the bulk mixed solvent [326-329]. 

In the electron-transfer system in the solvent mixtures with moderate polarities, the back electron-transfer process by the mixed-order kinetics has been observed, suggesting selective solvation of the radical ions, because of large fullerene molecules. 

Heats of solution of lithium perchlorate in 20, 40, 60, 80, 90, and 100 wt % acetronitrile-water mixtures at 298.16°K are reported. The heats of dilution were measured for lithium perchlorate in the mixed solvent containing 90 wt % CH CN. The heats of transfer (AHtr) of lithium perchlorate from water to aqueous acetonitrile were calculated. The results are discussed in terms of the structure of the solvent system and selective solvation properties of the lithium ion. 

The preferential solvation model allowed us to explain the positive and negative deviations from ideal behaviour and the synergetic effect exhibited for the solvato-chromic properties with the predominance of a species in the solvation sphere of the indicators. In the solvent systems comprised of [BF ]/[PF ] anions, the probes are preferentially solvated by the mixed solvent, while in the systems with [Cl]/ [Br] anions, the IL controls the solvation behaviour. These results are indicative of the involvement of ion-pair character of the IL. Additionally, it was possible to select binary mixtures with particular solvating properties varying not only in the solvent compositions but also the nature of the ionic or the molecular component of the mixture. 

The mixed solvent dimethylsulphoxide-ethanol (20/80% v/v) provides better solubility of salts than pure DMSO. Metal ions select ligands according to the series DMSO >H20 >EtOH. The solvated ion Cu2+ in the mixed solvent reacts with TMPyP( + 4) about five times more slowly toward the low ionic strength limit than does hydrated Cu2+ in aqueous solution (6). Whereas thiocyanate is oxidized by Cu2+ in water, Cu(NCS)+ is a well-defined complex in the mixed solvent. The stability constant has been determined to iC = 8 ( 0.2) M i (0.02 < 7c <0.05). Catalysis by NCS is quite remarkable, as shown in Fig. 23. 

Variable temperature studies for Br gave relaxation changes corresponding to the viscosity changes. The conclusion was, as in Ref. [51], that relaxation can be referred entirely to the ion-solvent interactions and, furthermore, that the more effective relaxation observed as the methanol content increases is due to the longer rotational correlation time of methanol compared to water. In other studies of halide ion quadrupole relaxation in mixed aqueous-alcohol solvent, as described in Section 5.1.5, the increase in relaxation rate on alcohol addition has been interpreted in an entirely different way. The data of Hall et al, [221] have recently been interpreted in terms of selective ion solvation by Neggia et al, [222] and the difference between alkali and halide ion relaxation in the presence of nonpolar groups was emphasized (cf. Section 5.1.5). 

Finally, let us briefly recollect the peculiarities associated with the use of mixed solvents. Firstly, it involves the existence of several forms of solvate and in particular, a selective ion solvation. A typical example of this kind was considered in section 4.6. Secondly, the composition of a mixed solvent in the nearelectrode layer differs from its bulk composition, thereby indicating the selective adsorption of one of the solvent components[428]. 

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