Mixed-cation systems

In an electrochemical study of the behavior of the counterions in the mixed cation systems sodium and silver, sodium and calcium, and silver and calcium containing the dodecyl sulfate anion, Corkill and Goodman [102] determined the CMC of sodium, calcium, and silver dodecyl sulfates in water and in solutions of sodium and silver nitrate. 

Much light is shed on questions like the above by results recently obtained with mixed cation systems of the type (Sm/Y)S, (Sm/Gd)S, etc. (32,31b, 7). Such systems have been much investigated, as they are found to lead to the ICF condition in SmS without resorting to pressure. The systems raise the interesting question of to what extent... 

Cancrinite crystallization from mixed cationic systems is an example, certainly not generalizable, of the determinant role played by the cations in framework silicate crystallization. In the present case the specificity is even double, because either the small cation (Li+) or the large one (Cs+,T1+, or Rb+) are essential for the synthesis and a role for each of them has been hypotized. The completion of the structural analysis, now in progress, may confirm the hypotheses, explaining also some still unclear points. 

The similarities in the Na cation distributions between the (NH, Na), (Ca,Na) and (La,Na) mixed cation systems in hydrated Y zeolites can be best demonstrated by comparing the sodium-23 MASNMR spectra at cation exchange levels where the NMR lines from the Na cations in the supercages and smaller cages are best resolved. For the (NH, Na), (Ca,Na) and (La,Na) cation systems these occur at exchange levels of 41, 45, and 52%, respectively. 

The location of extra-framework species can be more difficult to determine than framework atom positions, and is best performed in conjunction with accurate chemical analysis. The usual approach is via difference Fourier syntheses, in which comparison of observed reflection intensities and those predicted on the basis of the nearly complete structural model enables the residual electron density (in the case of X-ray diffraction) to be located. This can then be interpreted in terms of the species present and their expected distances from the framework. Extra-framework charge-balancing cations may occupy sites at less than unit occupancy, that is they may be either present or absent from that site in any given unit cell, and the situation is further complicated in mixed cation systems, where different cations may partition between different sets of extra-framework sites. 

It is worth noting that in such mixed systems the lattice constant a is a linear function of concentration, whereas in both mixed-anion and mixed-cation systems (Figure 113) c is slightly curved. 

As already indicated, ion exchange resins are osmotic systems which swell owing to solvent being drawn into the resin. Where mixed solvent systems are used the possibility of preferential osmosis occurs and it has been shown that strongly acid cation and strongly basic anion resin phases tend to be predominantly aqueous with the ambient solution predominantly organic. This effect (preferential water sorption by the resin) increases as the dielectric constant of the organic solvent decreases. 

In mixed solvent systems the difference in the solvating abilities of solvent molecules S and S2 causes a selective solvation of cations and anions [119,120],... 

This structure type evidently requires the simultaneous existence of two differently sized halide types, and thus exists only in mixed hahde systems. Recent investigations revealed this cubic stracture also for cluster phases with other interstitial atoms, i.e., the cation-free Si-centered [(Zr6Si)Cli2-xl2+x] [28]. 

John D. Corbett once said There are many wonders still to be discovered [4]. This certainly holds generally for all the different areas and niches of early transition cluster chemistry and especially for the mixed-hahde systems. The results reported above so far cover a very Hmited selection of only chloride/iodide systems and basically boron as the interstitial. Because of the very sensitive dependence of the stable stracture built in the soHd-state reaction type on parameters like optimal bonding electron counts, number of cations present, size and type of cations (bonding requirements for the cations), metal/halide ratio, and type of halide, a much larger mixed-hahde cluster chemistry can be expected. Further developments, also in mixed-hahde systems, can be expected by using solution chemistry of molecular clusters, excised from solid-state precursors. 

Although Taube s pyrazine Ru"—Ru dimer was produced by the Ag oxidation of [(NHjljRu—NC4H4N—Ru(NH3)5] , attempts to prepare similar Ru"-Ru " complexes from [(NH3)5Ru(C5H4N)2Ru(NH3)5]" and [(NHjljRu—NC5H4C2H4C5H4N—Ru(NH3)5]" were unsuccessful. Cyclic voltammetric data indicated a two-electron oxidation to Ru" -Ru " dimers. In view of the identical ligands around each Ru atom, Mayoh and Day have questioned the localization of the Ru valencies in Taube s dimer into discrete Ru" and Ru " centres. However, a theoretical calculation of the conditions necessary for valence trapping in any mixed valence system, showed that the condition is indeed satisfied by the above Ru compound. Other workers have suggested that the available data on this complex could also be explained by a molecular orbital scheme in which the Ru ion and pyrazine-filled n (or k ) molecular orbitals are mixed, and the unpaired electron is mainly but un-symmetrically shared by the two cations. ... 

Anton RE, Gomez D, Graciaa A, Lachaise J, Salager JL (1993) Surfactant-oil-water systems near the affinity inversion. Part IX. Optimum Formulation and Phase behavior of mixed anionic-cationic systems. J Dispers Sci Technol 14 401-416... 

Yu WL, Pei J, Huang W, Zhao GX (1997) Formation of CdS nanoparticles in mixed cationic-anionic surfactant vesicle system. Mater Chem Phys 49 87-92... 

The general observations made regarding structural influences on melting points are transferable across cation type, and apply in each case. The primary focus is on l-alkyl-3-methylimidazolium cations, coupled with simple organic and inorganic anions. Complex anions, such as mixed /MX systems, are mentioned, as are other series of cations (including some examples of tetraalkylammonium salts). 

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