Hydrates cations and anions

Using the Uhlig formula [Eq. (4)] the values of AG]/ (z-indep) have been obtained from ionic radii, as shown in Table 4. For the hydrated cations and anions, their hydrated radii (r ) have been employed for r in Eq. (4). Table 4 also shows the values of AGf/° " (z-... 

In the following recent applications of the new ab initio simulation technique will be demonstrated, which would have posed serious difficulties to conventional QM/MM MD schemes, which need analytical solute-solvent interaction potentials and where some artifacts as outlined in the previous chapter would certainly cause errors in the results. These applications will be grouped to hydrated cations and anions, in another section also hydrated neutral molecules forming hydrogen bonds to the solvent water and hydrolysis processes will be discussed. In all cases structural and dynamical data of the solutions will be presented. 

In the presence of electrolyte solutions, hydrated cations and anions participate in the formation of contacts and determine their strength. The addition of electrolytes can either increase the structuring of water layers or, vice versa, weaken this process (positive and negative hydration). Some electrolytes strengthen water structure, the others weaken it. In the first case, the viscosity of water increases in the second case it decreases. The first group consists of polyvalent electrolytes and the electrolytes, such as halides of alkaline elements, while the second group - of some electrolytes, such as KI. 

The precipitation of salts according to Rule I is accompanied by a large favorable entropy change, as the strongly hydrated cations and anions release numerous waters of hydration. In contrast, the dissolution of salts according to Rule II is accompanied by very little entropy change, since one ion is a stmctme breaker, while the other is a structure maker the dissolution occurs because the ions are mismatched... 

In this section we consider the equilibria associated with solids dissolving in water to form aqueous solutions. When an ionic solid dissolves in water, we typically assume that it dissociates into separate hydrated cations and anions. For example, when calcium fluoride dissolves in water, we typically represent the situation as follows ... 

This suggests that ions in water are unlikely to be bare, and hydrated cations and anions are expected (see Figure 13.9). The size of the ion is, therefore, expected to be larger than the crystallographic radius. However, it may well be that the experimental determination of the size of an ion will depend on the method used to determine it. For instance can it be assumed that estimates of the size of an ion from equilibrium properties such as the dependence of the mean ionic activity coefficient on ionic strength will be the same as the size of the moving entity in conductance studies ... 

Thus, cation water clusters favour internal structures in contrast to the surface strucmres favoured by anionic water clusters. This critical difference in the structural preferences of hydrated cation and anion clusters provides important cues for the design of cation- and anion-specific ionophores and receptors. Indeed, we note that most cation receptors have spherical structures, while almost all anion receptors do not have compact spherical structures but have a vacant space around the anion binding site without full coordination (which might be exceptional for the F ion with strong electronegativity for which the excess electron is strongly bound to F due to its small ion radius). However, as the temperature increases, the hydration structure tends to be more spherical due to entropy effects. 

Reliable, though relative, information about the size of ions in aqueous media can be obtained from data on the electrophoretic mobility of these ions [160], as the velocity of their movement in an electric field is direcdy proportional to their charge and inversely proportional to their hydrodynamic radius. According to these estimations (Table 12.3), the size of hydrated cations and anions decreases according to the following series ... 

Fig. 2.3 Crystal structures of intermetallic clathrates represented as packing of cage polyhedrons 20-atom dodecahedron blue), 24-atom tetrakaidecahedron (green), 26-atom pentakai-decahedron (red) and 28-atom hexakaidecahedron (yellow). The example compositions for different clathrate families (hydrate, cationic, and anionic) are given for comparison...

The three properties of water most significant in the solvation of metal ions and complexes are its high dipole moment (1.84 D), its dielectric constant (78.3 at 25.0 C), and its propensity toward hydrogen bonding. The high dielectric constant of H2O decreases the need for close association of cations with anions in aqueous solutions. As a result, free hydrated cations and anions diffuse independently through the solution, their movements... 

The bonds formed between the ions and water molecules are known as ion-dipole interactions. Energy is released during the hydration process and hence hydration is an exothermic process. The hydrated ions are no longer attracted to the oppositely charged ions so they enter the water. Eventually, provided water is present in significant excess, the lattice is completely broken down and all the ions are hydrated (see Chapter 15). The hydrated cations and anions are not attracted to each other owing to the presence of the water molecules. 

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