Empirical Equation for Internal Mobility

Okada et al. found that the internal mobilities of LT in molten alkali nitrates are well expressed as a function of molar volume, independent of the kind of the second cation. This finding leads to a general empirical equation  

In such systems as (M, Mj (i/2))X (M, monovalent cation Mj, divalent cation X, common anion), the much stronger interaction of M2 with X leads to restricted internal mobility of Mi. This is called the tranquilization effect by M2 on the internal mobility of Mi. This effect is clear when Mj is a divalent or trivalent cation. However, it also occurs in binary alkali systems such as (Na, K)OH. The isotherms belong to type II (Fig. 2) % decreases with increasing concentration of Na. Since the ionic radius of OH-is as small as F , the Coulombic attraction of Na-OH is considerably stronger than that of K-OH. 

It is sometimes difficult to distinguish between the free space effect and the tranquilization effect the former is usually more pronounced at lower temperatures. However, if the two effects are superimposed, for example, possibly for at very low concentration in (Li, K)(CO,)(i/2), it is nearly impossible to deconvolute them. 

The tranquilization effect may be also explained in terms of the dynamic dissociation model (Fig. 4), where C, interacts more strongly with A than C does, when the separating motion of C from the reference ion A will be retarded. Thus, Ct plays the role of a tranquilizer ion. 

The effect of highly polarizable cations on transport properties has scarcely been studied. Since the nitrate melts of Ag and TL are stable and have high polarizabilities, as shown in Table 5, their internal mobilities in binary mixtures containing one or both of these cations have been measured frequently. The isotherms are shown for and m,., in Figs. 10 and 11, 

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