Ionic moment, halide ions

The results for the adsorption of the halide ions on the paraffin surface seem to indicate that valence alone cannot explain ionic adsorption. However, an explicit form for the adsorption isotherms that takes into consideration factors such as the polarizability and the hydration energy of the ions is not easy to visualize at the present moment. 

The complexation ability of anions is shown in Ref. [81], to rise together with the increase of their ionic moments and polarization. In the halide-ion sequence their formal ionic moments (rx, nm"1) decrease from F to I-, whereas the polarization (a, A3) rises in the same sequence, as shown in Fig. 1.3.6. The concurrence of the actions of these factors results in the non-monotonic character of the complexation ability changes in the F-Cl-Br-I sequence. 

Fig. 1.3.6. Dependence of the formal ionic moment (I) and the polarization (a) of halide ions vs...

The changes of solubility in halide melts may be explained by changes of complexation abilities of halide ions. In general, the complexation ability increases together with the formal ionic moment, r, A , and the polarization, a, A, of anions. In the sequence F >CT>Br >1 the formal ionic moments decrease and polarization increases as in Figure 21.3.2. The superimposition of these factors leads to the extremum in the bonding energy in the halide complexes, therefore, its minimum for chloride or bromide complexes should be expected. The studies show that this leads to minimal solubilities in molten bromides. 

Figure 21.3.2. The formal ionic moments (I, r"., A ) and polarizations (a) of halide-ions.

The dissolution of ionic species (Fig. 11.3) occurs through the ion-dipole forces described in Section 10.2. Each positive ion in solution is surrounded by water molecules oriented with the negative end of their dipole moments toward the positive ion. Each S04 anion in solution is surrounded by water molecules oriented with the positive end of their dipole moments toward the anion. When a halide such as KCl is dissolved, the anion forms a hydrogen bond with one of the H atoms in a water molecule that places the atoms O —H—Cl nearly in a straight line as described in Section 10.2. 

---