Ionic pair formation

If considerable endothermic effect of the ionic association for acid can be explained by desolvation contribution of ionic pair formation (i.e., solvated moleeule of acid), the endothermic effect of ionic pair formation by such voluminous cations contradicts the physical model of the process. Estimation of desolvation energy of ionic pair formation of salt ions according to the equation shows that this energy is two orders of magnitude lower than the energy of heat movement of solvent molecules. For this reason, the process of ionic association of salt ions is exothermal, as seen from AH-p values. The exothermal character increases with permittivity decreasing, i.e., with increase of ion-ion interaction energy. 

The smaller the magnitude of the characteristic temperature for salt (389K), in comparison to acid, the smaller the barrier of the ionic pairs formation process. That is true because the association process in the case of acid is accompanied by energy consumption for desolvation. 

Even though less numerous, Raman studies of nitrate solutions in nonaqueous solvents have covered a range of metal ions (Li, Na, Ag, and Cu " ) in both protic and aprotic solvents [260-264]. Wooldridge et al. [260] carried out a vibrational study of alkali metal (lithium and sodium) nitrates dissolved in dimethylsulfoxide (0.5-2 M) at different temperatures (298-400 K). They found spectral evidence of an increase in the ionic pairing process when the temperature increases. Interestingly, in LiNOg solutions, the contact ionic pair formation promoted by the thermal increase is accompanied by a change in the ion-pair structure from monodentate to bidentate and by the partial desolvation of the alkaline ion ... 

Other kind of host molecules which has rendered good results in the detection pf PAHs are the quat molecules (16). These molecules include in their structure a positively charged quatemaiy nitrogen (quat) and an aromatic moiety. The quat moiety is tightly attached to the surface via an ionic pair formation with addition of chloride ions 17,18) and can interact with PAHs. 

Gutfelt et al. (1997) have evaluated various ME formulations as reaction media for synthesis of decyl sulphonate from decylbromide and sodium sulphite. The reaction rate was fast both in water-in-oil and in bicontinuous ME based on non-ionic surfactants. A comparison was made with this reaction being conducted in a two-phase. system with quats as phase-transfer catalyst but was found to be much less efficient. However, when two other nucleophiles, NaCN and NaNOj, were used the PTC method was almost as efficient as the ME media. It seems that in the case of decyl sulphonate there is a strong ion pair formation between the product and the PTC. The rate in the ME media could be further increased by addition of a small amount of a cationic surfactant. 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

As already mentioned, the criterion of complete ionization is the fulfilment of the Kohlrausch and Onsager equations (2.4.15) and (2.4.26) stating that the molar conductivity of the solution has to decrease linearly with the square root of its concentration. However, these relationships are valid at moderate concentrations only. At high concentrations, distinct deviations are observed which can partly be ascribed to non-bonding electrostatic and other interaction of more complicated nature (cf. p. 38) and partly to ionic bond formation between ions of opposite charge, i.e. to ion association (ion-pair formation). The separation of these two effects is indeed rather difficult. 

Lipophilic ion exchangers traditionally used for polymeric membrane preparation are the anionic tetraphenylborate derivatives and the cationic tetraalkylammonium salts. The charges on both lipophilic ions are localized on a single (boron or nitrogen) atom, but the steric inaccessibility of the charged center, due to bulky substituents, may inhibit ion-pair formation in the membrane and provide, when necessary, non-specific interactions between ionic sites and sample ions. 

As mentioned above, the most commonly used method for the analysis of cationic surfactants has been HPLC coupled with conductometric, UV, or fluorescence detectors, the latter typically utilizing post-column ion-pair formation for enhanced sensitivity. Analysis by GC is only possible for cationic compounds after a derivatisation step [33] because of the ionic character of this compound. However, structural information might be lost. 

The present author has developed a novel method called ion-association method. This is also a simple and versatile method for the preparation of ion-based organic dye nanoparticles in pure aqueous solution by the ion association approach [23]. It utilizes the control of hydrophilicity/hydrophobicity of the ionic material itself via ion-pair formation for example, addition of a cationic target dye solution into aqueous solution containing a certain kind of hydrophobic anions forms an electrically neutral ion-pair because of the strong electrostatic attraction, followed by aggregation of ion-pair species originated from van der Waals attractive interactions between them to produce nuclei and the subsequent nanoparticles (Fig. 3). In this case, hydrophobic but water-soluble anions, such as tetraphenyl-borate (TPB) or its derivatives (tetrakis(4-fluorophenyl)borate (TFPB), tetrakis [3,5-... 

Fig. 3 Concept of the ion-association method for fabricating ion-based organic dye nanoparticles in pure aqueous media. The approach is based on ion-pair formation between the ionic dye (for example, cationic dye) and the hydrophobic counterion that is soluble in water [for example, tetraphenylborate (TPB) or its derivative anion], which gives rise to a hydrophobic phase in water. For preparation, organic cosolvent is unnecessary. The size of the dye nanoparticles can be controlled by adjusting the interionic interaction between the dye cation and the associative hydrophobic counteranion...

The effect of solution anionic concentration is probably related to effects on activity coefficients and ion pair formation of more highly charged buffer species. In more concentrated solutions, the activity of the highly charged species is reduced by both ionic strength and ion pair formation. The effect on less charged, acidic species is less. Therefore, as solutions become more concentrated, the activity of basic species is reduced relative to that of acidic species, and at a given fraction... 

The electrophoretic mobilities of cationic analytes in water, in non-aqueous methanol and acetonitrile were measured and the influence of ionic strength and ion-pair formation on the mobility was determined. It was assumed that the mobility n of the ion at infinite dilution (jxQ ) and the viscosity of the pure solvent are constant ... 

The application of an organic solvent in CE offers a new possibility to change the selectivity of CE systems. Moreover, the various ion-solvent interactions and ion-pair formations can be exploited for increased separation efficacy. Its application for CE analysis of ionic synthetic dyes may be expected in the future [121],... 

S.P Porras, M.-L. Riekkola and E. Kenndler, Electrophoretic mobilities of cationic analytes in non-aqueous methanol, acetonitrile and their mixtures. Influence of ionic strength and ion-pair formation. J. Chromatogr.A 924 (2001) 31 12. 

Fig. 7.11 Reaction profiles for L-catalyzed isomerization of cis-to-trans ML2X2. In (A) an ionic intermediate is favored by a polar solvent. In (B) ion-pair formation arises with a less polar solvent. In (C) a non-polar solvent promotes a 5-coordinated intermediate. In (C), pseudo-rotation occurs. Based on D. G. Cooper and J. Powell, J. Amer. Chem. Soc. 95, 1102 (1973) see also Ref. 90. Reproduced with permission from D. G. Cooper and J. Powell, J. Amer. Chem. Soc. 95, 1102 (1973). (1973) American Chemical Society.

It can be shown that the virial type of activity coefficient equations and the ionic pairing model are equivalent, provided that the ionic pairing is weak. In these cases, it is in general difficult to distinguish between complex formation and activity coefficient variations unless independent experimental evidence for complex formation is available, e.g., from spectroscopic data, as is the case for the weak uranium(VI) chloride complexes. It should be noted that the ion interaction coefficients evaluated and tabulated by Cia-vatta [10] were obtained from experimental mean activity coefficient data without taking into account complex formation. However, it is known that many of the metal ions listed by Ciavatta form weak complexes with chloride and nitrate ions. This fact is reflected by ion interaction coefficients that are smaller than those for the noncomplexing perchlorate ion (see Table 6.3). This review takes chloride and nitrate complex formation into account when these ions are part of the ionic medium and uses the value of the ion interaction coefficient (m +,cio4) for (M +,ci ) (m +,noj)- Io... 

As a rule, if the unpaired electron density in the anion-radical is redistributed, the rotation barrier decreases. Thus, the barrier of the phenyl rotation in the benzaldehyde anion-radical is equal to 92 kJ mol", whereas in the 4-nitrobenzaldehyde anion-radical, the barrier decreases to 35 kJ mor (Branca and Gamba 1983). Ion-pair formation enforces the reflux of the unpaired electron from the carbonyl center to the nitro group. Being enriched with spin density, the nitro group coordinates the alkali metal cation and fixes the unpaired electron to a greater degree. The electron moves away from the rotation center. The rotation barrier decreases. The effect was revealed for the anion-radical of 4-nitrobenzophenone and its ionic pairs with lithium, sodium, potassium, and cesium (Branca and Gamba 1983 Scheme 6.19). 

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