Solvation coordination number

Coordination and solvation numbers refiect the simple idea that the solvation of ions or molecules consists of a coordination of solute and solvent molecules. The coordination number is defined as the number of solvent molecules in the first coordination sphere of an ion in solution [103], This first coordination sphere is composed only of solvent molecules in contact with or in bonding distance of the ion such that no other solvent molecules are interposed between them and the ion. This kind of solvation is sometimes termed primary or chemical solvation. Coordination numbers, determined by different experimental techniques [103], range in water from approx. 4 for Be to approx. 9 for Th" , although the majority of the values are close to 6 e.g. for AP ). 

Many X-ray diffraction studies of electrolyte solutions have been carried out in aqueous solutions [Gl, 4, 5]. Values of the most probable distance, between the oxygen atom in water and a number of monoatomic ions are summarized in table 5.1. In the case of the cations, this distance reflects the radius of the cation plus the effective radius of the water molecule measured in the direction of the lone pairs on oxygen. In the case of alkali metals, the effective radius of water increases from 122 pm for Li" " to 131 pm for Cs when the Shannon and Prewitt radii are assumed for the cations (see section 3.2), the average value being 127 pm. This result can be attributed to the observation that the coordination number for water molecules around an alkali metal or alkaline metal earth cation changes with cation size and electrolyte concentration. In the case of the Li" " ion, this number decreases from six in very dilute solutions to four in concentrated solutions [5]. Because of the electrostatic character of the interaction between the cation and water molecules, these molecules exchange rapidly with other water molecules in their vicinity. For this reason, the solvation coordination number should be considered as an average. 

The coordination number is the number of solvent molecules in the primary solvation shell. This quantity can be estimated (for ions) by conductance measurements and by... 

The definition of solvent exchange rates has sometimes led to misunderstandings in the literature. In this review kjs 1 (or fc2lsolvent]), sometimes also referred to as keJ s 1, is the rate constant for the exchange of a particular coordinated solvent molecule in the first coordination sphere (for example, solvent molecule number 2, if the solvent molecules are numbered from 1 to n, where n is the coordination number for the solvated metal ion, [MS ]m+). Thus, the equation for solvent exchange may be written ... 

Clementi, E. Determination of Liquid Water Structure, Coordination Numbers for Ions and Solvation for Biological Molecules, in Lecture Notes in Chemistry, Vol. 2, Springer-Verlag, Berlin, Heidelberg, New York, 1976. 

Besides water, other solvent molecules have been investigated to a small extent in connection with anion solvation. We mention here two examples of solvation by formic acid, concerning the ions Cl- 230> and CIO4 235). In the first case coordination numbers up to four were considered. For the one to one complexes the energy of interaction is larger for Cl- than for CIO4. 

Although the potential energy functions can be made to reproduce thermodynamic solvation data quite well, they are not without problems. In some cases, the structure of the ion solvation shell, and in particular the coordination number, deviates from experimental data. The marked sensitivity of calculated thermodynamic data for ion pairs on the potential parameters is also a problem. Attempts to alleviate these problems by introducing polarizable ion-water potentials (which take into account the induced dipole on the water caused by the ion strong electric field) have been made, and this is still an active area of research. 

The molecular structure of liquids is best analyzed using the concept of RDF. This is of particular importance in solute-solvent structures as it defines the solvation shells around the solute molecule. Therefore, we analyzed the solvation of the anion F using the RDF between the anion and the oxygen of the water molecules, as shown in Fig. 2. At least three solvation shells are well defined. The integration of these peaks defines the coordination number, or the number of water molecules in each solvation shell. The first shell that ends at 3.15 A with a maximum at 2.65 A has, on average, 6.6 molecules of water. The second shell,... 

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