Solvation number, cations

Covington and Lilley have tabulated the principal cation solvation number investigations to late 1969. 

The values of hj for different ions are between 0 and 15 (see Table 7.2). As a rule it is found that the solvation number will be larger the smaller the true (crystal) radius of the ion. Hence, the overall (effective) sizes of different hydrated ions tend to become similar. This is why different ions in solution have similar values of mobilities or diffusion coefficients. The solvation numbers of cations (which are relatively small) are usually higher than those of anions. Yet for large cations, of the type of N(C4H9)4, the hydration number is zero. 

A number of important issues involving the structure and dynamics of ionically conducting polymers have yet to receive thorough theoretical consideration. For example, in the case df multivalent cations, some systems exhibit cation transport whereas others do not, due to strong cation solvation. Therefore a term associated with ion-polymer dissociation must be important in systems which are on the borderline between these two extremes. This term is likely to be of the form ikHi, /2RT). 

We shall not treat a number of general problems of anionic polymerization such as autosolvation of the ion pairs, cation solvation with the electron-donor chain atoms, the role of the medium etc. Two problems attract our attention monomer activation during chain propagation and the direction of the epoxy ring opening. 

In ion solvation, the solvent molecules approach a cation with their negative charge and approach an anion with their positive charge (Fig. 2.1). Therefore, cation solvation is closely related to the electron pair donor capacity or Lewis basicity of solvents and tends to become stronger with the increase in donor number (DN). On the other hand, the anion solvation is closely related to the electron pair acceptability or Lewis acidity of solvents and tends to become stronger with the increase in acceptor number (AN). 

We can now discuss the solvation number. In systems such as the metha-nol-water-CaCl2 system shown in Figure 5, the hydration number is the greatest, that is, 11 at x3 = 0.020. If the hydration number of ions is calculated from the hydration entropy, Ca2+ is seven and Cl is two (3). If it is assumed that CaC is completely dissociated and both the cation and anion forms hydrate, the hydration number becomes 7 + 2X2 = 11, which agrees with the value obtained from the salt effect. 

Because NH3(1) has a much lower dielectric constant than water, it is a better solvent for organic compounds but generally a poorer one for ionic inorganic compounds. Exceptions occur when complexing by NH3 is superior to that by water. Thus Agl is exceedingly insoluble in water but NH3(1) at 25°C dissolves 207 g/100 cm3. Primary solvation numbers of cations in NH3(1) appear similar to those in H20 (e.g., 5.0 0.2 and 6.0 0.5 for Mg2+ and Al3+, respectively), but there may be some exceptions. Thus Ag+ appears to be primarily linearly 2-coordinate in H20 but tetrahedrally coordinated as [Ag(NH3)4]+ in NH3(1). It has also been suggested that [Zn(NH3)4]2+ may be the principal species in NH3(1) as compared to [Zn(H20)6]2+ in H20. 

If the lifetime of solvent molecules in the solvation shell of a cation is longer than 0.1 msec, the solvation number n of the cation follows directly from the area of the NMR line for the coordinated molecules. In pure solvents, n is equal to 6 for Al , Ga , Mg " " and equal to 4 for Be In mbced solvents the mean num-... 

The interaction of ions with solvent molecules suggests a more detailed picture in which during electrolysis the cations are transporting nj+ solvent molecules into the cathode compartment and the anions nj solvent molecules out of that region into the opposite direction. The residual molecules of the solvent, which remain unaffected by the ion movement, are regarded as free , n = ni+, ni are total solvation numbers of the ions which differ from those in Chapter III. The transference numbers t[ referred to the free solvent (index ) are called true transference numbers The diffusion current density referred to the velocity v j of the free solvent results from Eq. (51) ... 

To obtain individual ionic values, one has to make an assumption. One takes a large ion (e.g., larger than T) and assumes its primary solvation number to be zero," so that if the total solvation number for a series of salts involving this big anion is known, the individual hydration numbers of the cations can be obtained. Of course, once the hydration number for the various cations is determined by this artifice, each cation can be paired with an anion (this time including smaller anions, which may have significant hydration numbers). The total solvation numbers are determined and then, since the cation s solvation number is known, that for the anion can be obtained. 

One of the challenges of solvation studies consists in separating effects among the ions of a salt (e.g., those due to the anion and those due to the cation) and this difficulty, that of determining the individual solvation heats (see Section 2.15), invades most methods devoted to the determination of individual ionic properties (Fig. 2.46). When it comes to the solvation number of an ion, an unambiguous determination is even more difficult because not all workers in the field understand the importance of distinguishing the coordination number (the nearest-neighbor first-layer number) from... 

Apart from neutron diffraction, what other method distinguishes between the static or equilibrium coordination number and the dynamic solvation number, the number of solvent molecules that travel with an ion when it moves One method is to obtain the sum of the solvation numbers for both cation and anion by using a compressibility approach, assuming that the compressibility of the primary solvation shell is small or negligible, then using the vibration potential approach of Debye to obtain the difference in mass of the two solvated ions. From these two measurements it is possible to get the individual ionic solvation numbers with some degree of reliability. 

H2O molecules in the hydration spheres with the bnlk H2O solvent varies by many orders of magnitude depending on the metal ion. This, combined with the use of many different experimental techniqnes, has resulted in discrepancies in the literatnre, particnlarly for metal ions with a weakly bonnd, rapidly exchanging hydration sphere. The Na+ cation, for example, has had qnoted values for the primary solvation number ranging from 2 to 13, although a value close to six is generally accepted." ... 

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