Solvent Structure around Ions

Ionic forces are large at a small distance between two ions, with the charges Z e. The attractive energy E(R) at a distance R between two ions with charges Zj and Z2 is given by Coulomb s law (atomic units)  

The interatomic distance R is of the order of 3 A or 6B. The repulsion energy at this distance if the charge is 1 (for example, Li+, Na+, or K+) is thus 1/6 H in vacuum and, since 1 H is equal to 27.2 eV, the attractive potential energy is about 5 eV or 500 kJ/ mol. This is an enormously large number. In the next section, we will calculate the enthalpy gain, the Born energy, when ions are placed in polarizable solvents. 

Some data are shown in Table 6.1. Of course the larger the metal ion, the larger the radius of the hydration shell. An important corollary is that the attraction of a water layer around K+ (as measured by the free energy of hydration AG ) is considerably smaller than a similar layer around a smaller alkali ion. Thus, potassium ions throw off their accompanying water molecules much more easily than sodium ions, if this is necessary to enter a channel of a cell membrane in living matter. Potassium penetrates the cell wall more easily in an ionic channel, since it may pass the channel without a load of water molecules around it. 

Some Experimental Data Taken from the Simulation of I. Benjamin 

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It was shown earlier that the acceptor number can be treated as a measure of disorder of the solvent structure around the ion, reflecting the changes of caused by the solvent-solvent interaction. The contact... 

The potential of mean force due to the solvent structure around the reactants and equilibrium electrolyte screening can also be included (Chap. 2). Chapter 9, Sect. 4 details the theory of (dynamic) hydro-dynamic repulsion and its application to dilute electrolyte solutions. Not only can coulomb interactions be considered, but also the multipolar interactions, charge-dipole and charge-induced dipole, but these are reserved until Chap. 6—8, and in Chaps. 6 and 7 the problems of germinate radical or ion pair recombination (of species formed by photolysis or high-energy radiolysis) are considered. 

Removal of the inconsistency in the manner Ninham, Bostrom and co-workers have described in a series of papers goes part of the way to a resolution, but only at the level of the primitive model. More is involved once the molecular nature of the solvent is taken into account. Ionic polarisability is combined at the same level with electrostatics to determine local induced water structure around ions. That is a key determinant of hydration, and of so-called ion specific Gurney potentials of interactions between ions due to the overlap of these solute-induced solvent profiles so too for ionic adsorption at interfaces. The notions involved here embrace quantitatively the conventional ideas of cosmotropic, chaotropic, hard and soft ions. Some insights into this matter can be obtained via the alternative approach of computer simulation techniques of Jungwirth etal. But the insights are hamstrung so far by a pragmatic restriction that limits... 

Thus, ionic solvation is associated with a substantial rearrangement of solvent structure its primary structure is broken where the ion is located, and its molecules undergo reorganization (reorientation) within a certain volume around the ion. 

Most authors have accounted for the mutual influence of ions and solvent molecules only by assuming a firmly bound solvent sheath around the ion. The structure of the bulk solvent and the influence of electrolyte concentration on this structure are not taken into consideration. 

Studies on structure and dynamics of liquids have recently been extended to solvate structure of ions in non-aqueous solutions, and to the structure of complexes with relatively complicated ligands. We can also handle special problems like hydrophobic solvation is. Diffraction studies have been performed on new solvents as e.g. trifluoroethanol [23] and tetramethyl urea [26], and on solvent mixtures [27-30]. More recently the preferential solvation of ions has been subjected by an XD investigation in MgCh-water-methMol ternary systems [31], and the solvation structure around the cations proved to undergo the change of solvent molecules proportionally to the relative concentration of the two solvents. 

From these data it is suspected that the molecules of the solvate structure of lithium ion might be largely effected by the solvent molecules. Since the solubility of some lithium salts is relatively high in MN-dimethyl formamide (DMF), concentrated solutions can also be examined. In a previous study the solvate structure of lithium has been described in an 1.5 mol dm LiNCS solution in DMF [38]. A new XD measurement has been carried out for a LiCl solution of the same concentration. Table 1 hows the structural parameters for the lithium solvates in both solutions. The structural parameters were determined by a least-squares fitting method (LSQ). After the subtraction of the contributions ascribed to the intramolecular stmcture of the DMF molecules and to the assumed structure around the anions from the total structure function of the solution, the resulted difference curve was approximated by calculated model curves. The result is shown in Figure 1. 

The properties of the ions and the solvent which are ignored are similar to those ignored in the Debye-Hiickel treatment. These are very important properties at the microscopic level, but it would be a thankless task to try to incorporate them into the treatment used in the 1957 equation. Furthermore, Stokes Law is used in the equations describing the movement of the ions. This law applies to the motion of a macroscopic sphere through a structureless continuous medium. But the ions are microscopic species and the solvent is not structureless and use of Stokes Law is approximate in the extreme. Likewise, the equations describing the motion also involve the viscosity which is a macroscopic property of the solvent and does not include any of the important microscopic details of the solvent structure. The macroscopic relative permittivity also appears in the equation. This is certainly not valid in the vicinity of an ion because the intense electrical field due to an ion will cause dielectric saturation of the solvent immediately around the ion. In addition, alteration of the solvent stmcture by the ion is an important feature of electrolyte solutions (see Section 13.16). However, solvation is ignored. As in the Debye-Hiickel treatment the physical meaning of the distance of closest approach, i.e. a is also open to debate. 

Structure breaker ions bind water molecules around them sufficiently to cause a mismatch between the structure of the bound water and that of unmodified water typical of the pure solvent. But the order induced is not great enough to outweigh the disorder created in the misfit region, and this will occur with the less strongly polarising ions such as Rb, ... 

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