Ionic coordinate solvation energies

Similar observations hold for solubility. Predominandy ionic halides tend to dissolve in polar, coordinating solvents of high dielectric constant, the precise solubility being dictated by the balance between lattice energies and solvation energies of the ions, on the one hand, and on entropy changes involved in dissolution of the crystal lattice, solvation of the ions and modification of the solvent structure, on the other [AG(cryst->-saturated soln) = 0 = A/7 -TA5]. For a given cation (e.g. K, Ca +) solubility in water typically follows the sequence... 

C—X, Cf, X- and C+ fX (see Fig. 2), the solvation energy increasing the driving force of these dissociations. It is possible that a coordination catalyst is not active in the C—X state but only in one or other of the ionized states. Such behavior blurs the distinction between ionic and coordination polymerization. 

In 1954 Weiss32 used Bernal and Fowler s simplified solvation model,16 with an Inner Sphere of ionic coordination, i.e., a small spherical double layer around the ion of charge ze, followed by a sharp discontinuity at radius q, the edge of the Outer Sphere or Dielectric Continuum. He used a simple electrostatic argument to determine the energy to remove an electron at optical frequency from the Inner Sphere ... 

Dynamics In light of the encouraging results for absolute solvation energies and equilibria, applications of continuum solvation models to the dynamics of organic reactions also are expected to be very fruitful. Ionic reactions (e.g., the classical S[s 2 mechanism) may proceed in qualitatively different ways in solution and in the gas phase, and continuum solvation models provide a convenient and economical way to map out solvation energy changes as a function of the reaction coordinate. 

Most cations are strongly solvated, since their radii are small, and the free energy of solvation is approximately proportional to z2/r +, where ze0 is the cation charge in coulombs and r+ its ionic radius. The result of this is that even if the charge on the electrode is negative, there is usually little tendency for these cations to shed their water molecules and adsorb directly on the metal surface. Thus, the distance of closest approach of cations is determined by the radius of the inner solvent coordination sphere, and if the metal surface itself constitutes a plane, then the cation nuclei, at the distance of closest approach, will also constitute a plane termed the outer Helmholtz plane (OH P). 

Table 2. Ionic Radii, Hydration Numbers, Free Energies of Solvation, Surface Charge Densities, and Coordination Geometry of Alkali and Alkaline Earth Cations of Interest...

The first term on the right hand side refers to the bare ion and disappears because we are engaged in differences of free energies. The second term refers to the coordination model of ion-solvent interaction in the primary solvation shell and the third term takes into account long range interactions. The last contribution may be approximated by the electrostatic interaction of a charged species with the solvent. The radius of the charged species is equal to that of the solvated ion e.g., ionic radius + diameter of the solvent molecules in the primary solvation shell). 

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