Anion and cation pair

At ionic strengths between about 2 and 3.5 mol/kg the high salt concentrations lead to further interaction between anion and cation pairs, but also to important interaction between pairs of ions of the same charge and simultaneously between three ions where two have the same charge. The Pitzer model can accurately account for such effects with a complex series of interaction terms added to a... 

Electrostatic Adhesion Between Anion and Cation Pairs... 

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

Much more simply, the same result can be attained with bipolar membranes, membranes consisting of an anion- and cation-permeable (an anion- and cation-exchange) membrane laminated together. At such a membrane, when mounted between electrodes so that the cation-exchange layer faces the anode, water is split into and OH ions so that the acidic and alkaline solutions required for regeneration as above are produced at the respective surfaces of the bipolar membrane. When such membranes are suitably integrated into the sequence of membranes in the electrodialysis unit above, gas evolution at the electrodes is not needed the acid-base pair is produced with about half the power. 

The salting-in effect may be used to increase the solubility of a drug substance through the formation of associated ion pairs, most commonly making use of anionic countering (hydrochloride being the most popular). Detailed reviews of pharmaceutical salts have been published, which contain extensive tables of anions and cations acceptable for pharmaceutical use [44,47]. These articles also describe useful processes for the selection of the most desirable salt... 

The ease of formation of hydrophobic ion pairs, and hence the rate acceleration, will be determined by the hydrophobic and electrostatic interactions between the anionic and cationic species. Lapinte and Viout (1974) found that the nucleophilic order OH- > CN > C6H50- in water was completely reversed in CTAB micelles hydrophobic phenoxide ion is activated better by the micelle. The micellar binding of phenols and phenoxides was determined by Bunton and Sepulveda (1979). Similarly, hydrophobic hydroxamates are activated much better than their hydrophilic counterparts. In the same vein, the extent of activation correlates approximately with the hydrophobic nature of aqueous aggregates as estimated by Amax of methyl orange (Table 7) and of picrate ion (Bougoin et al., 1975 Shinkai et al., 1978f Table 5). 

Ion-pair formation needs its due recognition because it very often gives rise to unexpected extractions. In true sense, ion-pair may be regarded as a close association of an anion and cation, and therefore, it usually takes place either in a polar or a non-polar solvent. In reality, the ion-pairs are invariably formed by virtue of the union between comparatively large organic anions and (much smaller) cations. Interestingly, the resulting ion-pairs have been found to show their appreciable solubility in polar solvents and hence, these species may be extracted conveniently under such experimental parameters where neither individual component ion could. 

C, showing that the anions and cations exist as contact ion pairs.103 The possibility of pn-dn conjugation in arylphosphines has been discussed in terms of overlap integrals and the energies of planar and pyramidal forms.139 Free-electron MO calculations have been applied to the absorption frequencies of unsymmetrical phosphocyanines derived from cyclopentadienylidenetriphenylphosphorane (115).140... 

The third intra-pair reaction to be discussed involves bond formation between radical anion and cation without intervening transfer both singlet and triplet radical ion pairs can couple. For example, the bifunctional radical cation 24 generates two chloranil adducts, most likely via zwitterions (e.g., 74 and 75 ), initiated by forming a C O bond. The CIDNP results indicate that 74 and 75 are formed from a singlet radical ion pair. Adduct 75 is a minor product, as the major spin density of 24 + is located in the allyl function which, therefore, is expected to be the principal site of coupling. 

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