Cation-anion pairs liquids

Stabilization of Cation-Anion Pairs by Crown Ethers Liquid Clathrates... 

STABILIZATION OF CATION-ANION PAIRS BY CROWN ETHERS LIQUID CLATHRATES... 

Ionic liquids. Ionic liquids are formed by an eqnimolar mixture of a traditional solid matrix, such as CHCA and DHB, with an organic base (e.g., butylamine) [41-44], These cation-anion pairs allow homogeneous sample preparation, which improves reprodncibility and increases signal intensity. 

Effects of anion on surface tension. Surface tension measurements by the du Nouy ring method kept the type of cation constant while the anions were varied to include [BMIM][Bp4], [BMlM][PFg], BMlM][Tf2 N], and [BMIMJpFjSO ] and four additional cation-anion pairs. The dry ionic liquids showed the expected decreasing trends in surface tension with increasing temperature for each ionic liquid cation-anion series [5]. Increasing anion size appeared to correlate with a decrease in surface tension of the ionic liquid - as noted in reference [34]. 

Figure 12.4 Typical ionic liquid GC stationary phase structure Note the long nonpolar hydrocarbon spacer with charge-balanced organic cation-anion pairs at either end. (Reprinted with permission of Supelco, Inc., Bellefonte, PA, www.sigmaaldrich.com/supelco.)...

Ionic Liquids, Structures, Fig. 3 Structure of the isolated cation-anion pair of l-butyl-3-methylimidazolium tetrafluoroborate... 

The other three cases are less common. In the case of chemically anchored anionic species, mostly carboxylate [78,79] or sulfonate groups have been investigated. Examples for chemically fixed cation/anion pairs are scarce here, ammonium carboxylate [80,81] or, more related to water-immiscible ionic liquids, guanidinium sulfonimide ion pairs [82] have been reported. The chemical grafting of zwitterionic species is nearly always related to immobilized ammonium or imidazolium sulfonate zwitterions that are easily formed upon reaction of nucleophiles (amines or imidazoles) with sultones [83-85]. 

Fig. 1 Examples of common cations and anion pairs used in the formation of ionic liquids, and general progression of changes in IL properties with anion type.

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]). 

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