Alkali metal ions structure stabilizers

The insertion of the oxygen atoms widens the silicon lattice considerably. A relatively large void remains in each of the four vacant octants of the unit cell. In natural cristobalite they usually contain foreign ions (mainly alkali and alkaline earth metal ions) that probably stabilize the structure and allow the crystallization of this modification at temperatures far below the stability range of pure cristobalite. To conserve electrical neutrality, probably one Si atom per alkali metal ion is substituted by an A1 atom. The substitution of Si... 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

Alkali metal ions as structure stabilizers and other functions 562... 

Knowledge of structural details may help the interpretation of the peak selectivity exerted by 2.2.2-crypt. Figure 18 shows the structures of the alkali metal complexes of 2.2.2-crypt, obtained from X-ray diffraction studies on crystalline salts [39—421. First, it is clear that the low stability of the Li+ complex depends upon the circumstance that the smallest alkali metal ion is lost in the cavity and misses the interaction with an oxygen atom and with one tertiary amine group. The other... 

In the crown ethers (18) the interactions between the ligand and metal ion are considered to be more electrostatic in nature, rather than the covalent binding observed for the transition metal complexes of the aza, thia, and phospha macrocycles. The thermodynamic properties of these macrocycles have been extensively studied, with numerous reviews covering complexation, selectivity, and structural aspects, some with extensive tables of thermodynamic data. Considerable efforts have been made to correlate the interrelationship between cavity size of the macrocycles and stability of alkali and alkaline earth metal complexes. From X-ray and CPK models, cavity radii are determined as 0.86-0.92A for 15-crown-5 (64), 1.34-1.43 A for 18-crown-6 (65), and about 1.7 A for 21-crown-7 (66). For complex formation between the alkali metal ions and 18-crown-6, the maximum stability... 

The hexapeptide, cycb( L-Pro-Gly)3, has been shown to form complexes with a number of metal ions175 The compound exhibits ion selectivity for Li+ and Na+ over K+ and larger alkali metal ions. It also forms a Ca++ complex which has a stability constant in acetonitrile of stab = 1.1 x 10s M 1. With Mg++ three different complexes with cyclopeptide-cation stoichiometries of 2 1,1 1, and 1 2 are formed. Hypothetical structures of these complexes (Fig. 45) have been proposed which are reminiscent of the enniatin sandwich complexes. 

The closely related research on polyether chelates by Michal Szwarc and his co-workers led to a detailed determination of the structure and properties of carbanions in ion pairs and free ions. The fundamental principles which were developed and clarified in their numerous publications contribute to an understanding and interpretation of much of the polyamine chelate work as well. More recently the crown ether chelates, pioneered by Pederson and co-workers at the Dupont Laboratories, have given additional impetus to research on chelated alkali metal compounds. Crown ethers and amines are cyclic variations which can provide greater stability and specificity in complexation of cations, particularly the heavier alkali metal ions. 

Two additional developments in zeolite chemistry include the removal of tetrahedral aluminum atoms from the framework by complex-ing agents (53) and the increase in stability of a zeolite by essentially complete removal of the alkali metal cation (3). The latter process— ultrastabilization—is the center of some controversy. Two proposals for explaining the stability of these materials have been advanced one is based upon the removal of tetrahedral aluminum, which results in an increase in the Si/Al ratio of the framework, the formation of additional O—Si—O linkages, and a decrease in the unit cell constant. The other is based upon the complete removal of alkali metal ion, which may act as a catalyst in perturbing the structure at elevated temperatures. Although there may be merit to one or both proposals, probably neither is the sole explanation. 

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