Polarizable ions

Cul) is not due to point defects but to partial occupation of crystallographic sites. The defective structure is sometimes called structural disorder to distinguish it from point defects. There are a large number of vacant sites for the cations to move into. Thus, ionic conductivity is enabled without use of aliovalent dopants. A common feature of both compounds is that they are composed of extremely polarizable ions. This means that the electron cloud surrounding the ions is easily distorted. This makes the passage of a cation past an anion easier. Due to their high ionic conductivity, silver and copper ion conductors can be used as solid electrolytes in solid-state batteries. 

Known as the thermite reaction, this process is so strongly exothermic that the iron is produced in the molten state. In this case, the replacement Fe3+ by Al3+ is very favorable because Al3+ is a smaller, harder, less polarizable ion, so this reaction is in agreement with the hard-soft interaction principle (see Chapter 9). 

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. 

Now A8/A6 differs little from unity, and therefore n must have a large value (about 35) to satisfy the equation. Since n is actually much smaller than 35 for all kinds of ions, the CsCl lattice cannot be stable. The introduction of non-rigid ions therefore has a serious effect on Goldschmidt s treatment. In KF, where the ions are of equal size, a CsCl structure is no longer to be expected. It is the van der Waals forces, described in Section 46, tending towards high coordination numbers, which lead to the formation of CsCl lattices, but only in the compounds CsCl, CsBr and Csl which are composed of strongly polarizable ions. 

The purely hydrogen-bonded structures range from the acids to the polyhydroxyl compounds such as carbohydrates. In contrast the alkali h3rdroxides are not hydrogen bonded to any extent, but resemble more closely ionic compounds with polarizable ions. Even in alkaline earth hydroxides, except those of beryllium, no true hydrogen bonds are formed, and it is only in the hydr oxides of the third group, which are of amphoteric character, that the hydrogen bonds reassert themselves [I]. 

The conventional viewpoint, which assumes that the ionic atmosphere is spherically symmetric, does not take account of the inevitable effects of ionic polarization. From an analysis of the general solution (19), however, it is evident that the ionic atmosphere must be spherically symmetric for nonpolarizable ions, and the DH model is therefore adequate. (Moreover, in very dilute solution polarization effects are negligibly small, and it does not matter whether we choose a polarizable or unpolarizable sphere for our model.) But once we have made the realistic step of conferring a real size on an ion, the ion becomes to some extent polarizable, and the ionic cloud is expected to be nonspherical in any solution of appreciable concentration. Accordingly, we base our treatment on this central hypothesis, that the time-average picture of the ionic solution is best represented with a polarizable ion surrounded by a nonspherical atmosphere. In order to obtain a value for the potential from the general solution of the LPBE we must first consider the boundary conditions at the surface of the central ion. 

For charging up a central polarizable ion and its ionic cloud we thus derive... 

However, the dispersion interactions (between ions and the whole system) generate repulsive forces between the water/air interface and the highly polarizable ions (Cl-, Br, I") and attractive forces between the interface and the less polarizable ions (Na+, Li+, K+).3 Some recent experimental results5 also challenged the traditional Langmuir picture of a surface layer depleted of ions they revealed, however, the opposite, namely that the more polarizable anions are positively adsorbed on the interface.5 The conclusion of Hu et al.5 was supported by the numerical simulations of Jungwirth and Tobias,6 which demonstrated that the polarizability of halogen anions (Cl , Br-, I ) is directly related to their propensity for the surface. The less-polarizable ions (Na+ and F ), however, preferred the bulk water.6 These results are opposite to the predictions of the ion dispersion theories.3... 

An interesting example of a specific ion effect in microemulsions is a strong increase in reactivity found for large, polarizable anions such as iodide. The tendency for such ions to interact with, and accumulate at, the interface can be taken advantage of for preparative purposes. The increased concentration of such ions in the interfacial zone, where the reaction takes place, will lead to an increase in reaction rate. Expressed differently, the reactivity of iodide and other highly polarizable ions [62, 63] will be very high in such systems. The microemulsions need not be based on cationic surfactants that would drive the anions to the interface by electrostatic attraction. Also microemulsions based on nonionic surfactants display the effect because large, polarizable anions interact... 

This conclusion is further strengthened considerably by the theoretical calculation of CBE originally performed by Pearson and Gray (102) and later on somewhat modified by Pearson and Mawby (8). Values of CBE are calculated according to three models, viz. the hard sphere model, the polarizable ion model and the localized molecular orbital model. Only the last one, treating the bonds as covalent, is able to account in a satisfactory way for the values found experimentally for such halides as HgCl2 and CdCl2. For LiCl and NaCl, on the other hand, an acceptable fit with the experimental values is obtained already by the hard sphere model, which certainly indicates a predominantly electrostatic interaction. 

The complex ions, in which the central ion has not the inert gas configuration, are very numerous these are especially formed with readily polarizable ions, such as Cl, (Br, I), CN, CNS and S. In these cases the polarization as well as the Van der Waals-London energy can contribute to the heat of formation of the complex. The following are examples K4CdCl6, K2[Hg(CN)4], Ks[Ag(SCN)J, KFeS2 etc. 

According to Schott, et al., H and all bivalent and trivalent cations form complexes with the ether linkages of nonionic surfactants, increasing their solubility (5). Among the anions, only large, polarizable ions like iodide and thiocyanate break the... 

The idea of polarizable ions also explains the shape of certain simple molecules. If we ideally picture the HgO molecule as an O "-ion with two H-nuclei, no dipole is induced in the 0-ion if the H-nuclei are on opposite sides of the O-ion and in such a position that the centres of the three ions lie in a straight line. If an angle is formed, however, a dipole is always produced in the O-ion thus if the O-ion has a sufficiently high value of a the equilibrium configuration is always a bent one. The pyramidal form of NH3 may be explained by similar considerations. This conception is too... 

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