Stabilization factor, complex cation exchange

In addition to specific carrier features, a number of external factors may also have marked effects on transport rates. The nature of the membrane phase (in particular for liquid or supported liquid [6.10b] membranes) influences the distribution equilibria as well as the stability and selectivity of the complex in the membrane and the cation exchange rates at the interfaces. The nature of the coextracted anion affects transport via a (cationic complex-anion) pair (Fig. 10) simply by modifying the amount of salt extracted into the membrane this amount decreases with higher hydration energy and lower lipophilicity of the anion (for example, chloride compared with picrate). The concentration of salt in the aqueous phase will, of course, affect the amount extracted into the membrane and therefore the transport rates (for illustrations of these effects see for instance [6.1]). 

Because of the complex equilibria involved (Figure 18a), transport rates depend on the delicate balance of many factors including complexation properties and lipophilicity of the carrier, the cation and anion being transported, and the nature of the membrane species itself. Importantly, transport rates are not directly proportional to the cation-carrier complex stability but present a maximum as a function of Ks. If the complex stability is too low, insufficient cation will be complexed at the initial interface and, similarly, if the complex stability is too high, insufficient cation will be released at the opposite interface. A compromise between thermodynamics (stability) and kinetics (exchange rates) of complexation is involved. 

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