Solvents for Ions

The accuracy of these methods is tested by finding the mean absolute error between the computed and experimental free energies of solvation. The SM4 method does well for neutral molecules in alkane solvents with a mean absolute error of 0.3 kcal/mol. For neutral molecules, the SM5 methods do very well with mean absolute errors in the 0.3 to 0.6 kcal/mol range, depending on the method and solvent. For ions, the SMI method seems to be most accurate with... 

These arguments are similar to those employed in the derivation of the Butler-Volmer equation for electron-transfer reactions in Chapter 5. However, here the reaction coordinate corresponds to the motion of the ion, while for electron transfer it describes the reorganization of the solvent. For ion transfer the Gibbs energy curves are less symmetric, and the transfer coefficient need not be close to 1/2 it may also vary somewhat with temperature since the structure of the solution changes. 

The fact that X for both salts lies in the range 0.20-0.25 shows that water in the membrane is a less effective solvent for ions than is bulk water i.e. the low-dielectric-constant matrix polymer lies well within the range of the electrostatic fields around the ions. Our value of Xg, the molar distribution coefficient of sodium chloride between polymer and solution, is in good agreement with values obtained by direct measurement (1,5,10, 11,12). This is further evidence in favour of our theories and assumptions. 

Water is an excellent solvent for ions and for substances that contain polarized bonds (see p.20). Substances of this type are referred to as polar or hydrophilic ( water-loving ). In contrast, substances that consist mainly of hydrocarbon structures dissolve only poorly in water. Such substances are said to be apolar or hydrophobic. 

In the 1950s, the push-pull mechanism whereby a proton is donated by one centre and received at another was considered as a major contributor to enzyme catalysis and to give rise to third-order terms in non-enzymic reactions. Third-order terms were observed in organic solvents but in water they are of minor importance because of the solvating power of this solvent for ions and polar centres. 

Nitrobenzene — Aromatic nitro compound (C6H5NO2, Mw 123.1 gmol-1, density 1.2gem-3 at 20 °C, dielectric constant er = 35.6, solubility in water 1.9 gL-1 at 20 °C). Nitrobenzene is a standard solvent for ion-transfer experiments using the 4-electrode technique (see -> ion-transfer at liquid-liquid interfaces), and also for ion-transfer experiments with immobilized droplets (-> droplets, electrochemistry of immobilized ). 

Equation 5.22a is just the Kirkwood expression (3.7a), and thus it is obvious that at infinite dilution Cg = e, the dielectric constant of the pure solvent. For ion-dipole mixtures, (5.21b) remains valid but it is shown in Appendix D that at finite ion concentration (5.21a) is replaced by... 

Table 2.11.10 shows the large differences in AGJ (H obtained from the two equations for the transfer from water to several mixed solvents. For ion transfer from water to acetic acid (HAc)-water mixtures, eqn. 2.11.32 does not yield a linear relation and it was suggested that if the data were available, eqn. 2.11.33 would indeed be linear. Approximate results for these systems, which were obtained from eqn. 2.11.32, are given in Appendix 2.11.9. The use of eqn. 2.11.32 for the transfer of ions from water to DMSO has not led to any reasonable results and it was concluded that this relation must be used with care. In another study ... 

The more recent treatment by Krishnan and Friedman shows that for this system the simple ions and tetraalkylammonium ions form two distinct straight lines (see Fig. 2.12.5a). The important thing to notice is that if one accepts the view that PC is the ideal solvent for ions, then the water-like behaviour in terms of enthalpies of solvation of the other solvents decreases in the order... 

Contrary to expectations from the open tetrahedral hydrogen-bonded structure of ice, liquid water is a close-packed rather than an open liquid. It shares this property with 1,2-ethanediol, glycerol, and formamide, among common solvents, see Table 1.5. These four liquids have (1 — Vx/V) <0.1, whCTeas most otho" common solvents for ions have larger values of this quantity. Liquids that have large fractions of free volume, i.e., are open , are quite compressible, and there exists a moderate positive correlation of (1 — Vx /1 ) with the isothermal compressibility. Water shares a low isothermal compressibility, kj = 0.457 GPa at 25 °C, with the above named low-openness solvents for which kj < 0.5 GPa , whereas for other common solvents the values range from 0.524 (dimethylsulfoxide) to 1.706 (n-hexane) (Marcus 1998). 

It is well known that water is a good solvent for ions and polar molecules. On the other hand, it is a poor solvent for nonpolar molecules such as hydrocarbons. Perhaps surprisingly, the insolubility of hydrocarbons and other nonpolar compounds is not due to a positive enthalpy effect. The enthalpy of mixing hydrocarbons with water is foimd to be either very small or negative. Therefore, the positive value for AG must arise from a negative entropy change. This effect, called the hydrophobic effect, assumes importance in many processes, including solubilization and the adsorption of compounds at interfaces. 

Dielectric constant (Section 6.13C) A measure of a solvents ability to insrrlate opposite charges from each other. The dielectric constant of a solvent roughly measures its polarity. Solvents with high dielectric constants are better solvents for ions than are solvents with low dielectric constants. 

A large number of studies have been performed on the interaction of small molecules with various zeolite frameworks, focusing especially on the diffusion of methane202,239-256 nd water " - (the latter is usually present in zeolites and is important as a solvent for ion exchange), but also on benzene and toluene, o° 3-267 xylenes, nitroaniline, higher hydrocarbons up to icl CF4, CCI4, and SFg.271-273 discussed by Demontis... 

Summarizing, it is concluded that the uniqueness of water (liquid) is characterized by the strong association of its molecules, which gives rise to an extraordinary high boiling temperature and heat capacity. In spite of that association, liquid water molecules have normal mobilities as is reflected in the viscosity and the relaxation time. Furthermore, water is a good solvent for ions and polar components, whereas apolar substances are poorly soluble in it, if soluble at all. 

In view of the requirements noted here, hydrocarbons are unsuitable as solvents for ions, although aromatic hydrocarbons do interact with electrolytes that themselves have aromatic or large aliphatic groups attached to an ionized atom or group... 

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