Alkali metal complexes hydration energies

The mechanistic problems associated with the transport of cations (especially Na and K+) through membranes have continued to receive attention. The structures of macrocyclic alkali-metal complexes and the thermodynamics of their formation have been reviewed, as has the general question of the selectivity of the carriers towards the metals. The role of hydration energy has been considered and it has been suggested that, at least in the case of the enniatin ionophores, sandwich... 

In contrast to the alkali metals, the lanthanides do not form crown ether complexes readily in aqueous solution, due to the considerable hydration energy of the Ln + ion. These complexes are, however, readily synthesized by operating in non-aqueous solvents. Because many studies have been made with lanthanide nitrate complexes, coordination numbers are often high. Thus 12-coordination is found in La(N03)3(18-crown-6) (Eigure 4.4), 11 coordination in La(N03)3(15-crown-5), and 10 coordination is found in La(N03)3(12-crown-4). Other complexes isolated include Nd(18-crown-6)o.75(N03)3, which is in fact [ Nd(18-crown-6)(N03)2 +]3 psid(N03)6]. Other lanthanide salts complex with crown ethers small crowns like 12-crown-4 give 2 1 complexes with lanthanide perchlorates, though the 2 1 complexes are not obtained with lanthanide nitrates where the anion can... 

In its properties ammonium is very similar to potassium. At pH > 9.2 it joins OH and becomes hydroxide NH OH. Its salts are highly soluble ((NH S0 - 754 g-kg-i, NH HC03- 217 g-kg i, (NH )3PO - 203 g-k S NH H PO - 353 g-kg etc.). In complexations it takes part as central cation but these compounds have low stabihty constants. Much more active anionogenic ammonia. However, its complex formations with central cations are very weak and have no practical value. Still, ammonium is capable of actively adsorb due to relatively low hydration energy. That is why at cation exchange it can displace any alkali metal. It is especially actively adsorbed by montmorillonites as it is capable of penetrating in their inter-package space. 

The surface activity is considerably affected by the anion of the metal salt the work of adsorption increases with the decrease in hydration energy of anions (Fig. 9). Similar effects were observed when the transfer kinetics of alkali salts across the liquid membranes containing DBC was studied [121-123]. Evidently, this deviation from the linear dependence is due to the specific interaction of the macrocychc polyether with certain anions in the DBC MX complexes [124], which co-exists with the decreasing repulsion of low-hydration-energy anions from the boundaries between the low-permittivity media [66,67]. 

In dilute solutions, alkali metal ions rarely form complexes, but where these are formed, e.g. with and [EDTA]" (see Table 7.7), the normal order of stability constants is Li > Na > K > Rb > Cs. In contrast, when the aqueous ions are adsorbed on an ion-exchange resin, the order of the strength of adsorption is usually Li < Na+ < K+ < Rb+ < Cs. This suggests that the hydrated ions are adsorbed, since hydration energies decrease along this series and the total hydration interaction (i.e. primary hydration plus secondary interaction with more water molecules) is greatest for Li+. 

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