Ion triple

The physical picture in concentrated electrolytes is more apdy described by the theory of ionic association (18,19). It was pointed out that as the solutions become more concentrated, the opportunity to form ion pairs held by electrostatic attraction increases (18). This tendency increases for ions with smaller ionic radius and in the lower dielectric constant solvents used for lithium batteries. A significant amount of ion-pairing and triple-ion formation exists in the high concentration electrolytes used in batteries. The ions are solvated, causing solvent molecules to be highly oriented and polarized. In concentrated solutions the ions are close together and the attraction between them increases ion-pairing of the electrolyte. Solvation can tie up a considerable amount of solvent and increase the viscosity of concentrated solutions. 

The experimentally noionic accessible limiting conductivities A — A — A of the triple ion must be estimated with consideration of ion sizes yielding Ai =A0/3 [101,102] or 2A0/3 [103], with preference for the latter value. 

Ion-pair association constants K A determined with the set of conductivity equations (7)—(15) agree with those obtained from Eq. (18) and (19) [100]. Salomon and Uchiyama have shown that it is also possible to extend the directly Fuoss-Hsia equation to include triple-ion formation [104],... 

Conductivity curves (A versus c ) of salts in solvents of low-permittivity commonly show a weakly temperature-dependent minimum around 0.02 molL-1 followed by a strongly temperature-dependent maximum at about 1 mol L 1. According to Fuoss and Kraus [101,102] the increase of conductivity behind the minimum is due to the formation of new charge carriers from the ion pairs. They assume that coulombic forces suffice to form bilateral cationic [C+A-C+] and anionic [A C+A ] triple ions in solvents of low-permittivity ( <15) if the ions have approximately equal radii. 

In contrast to bilateral triple-ion formation, unilateral triple-ion formation may also occur in solvents of high permittivity, when ion-pair association is increased by noncoulombic specific ion-ion interactions in solvents of low basicity such as PC or AN. Exclusive formation of anionic tripleions [A-C+A-] ", is observed in these solvents when large organic molecular anions A interact with small cations such as Li + or H+. For example, in contrast to lithium acetate in DMSO [97], where ion association is moderate, ion association as well as unilateral triple-ion formation is observed in the solvent PC [105] due to the much lower basicity of this solvent, (see Table 2)... 

Despite the results from various experiments such as transference number measurements, polarographic studies, spectroscopic measurements, and dielectric relaxation studies in addition to conductivity measurements, unilateral triple-ions remain a matter of debate. For experimental examples and other hypotheses for the interpretation of conductance minima the reader is referred to Ref. [15] and the literature cited there. 

Table 5 contains a selection of ion-pair association constants, triple ion formation constants, and limiting conductivities for various electrolytes which have been studied in connection with the optimization of battery electrolytes. It shows... 

Table 5. Association constants, triple ion formation constants and limiting conductivities of some lithium electrolytes.

Figure 5. (a) The ( A, SO,) anion symmetric streching mode of polypropylene glycol)- LiCF,SO, for 0 M ratios of 2000 1 and 6 1. Solid symbols represent experimental data after subtraction of the spectrum corre-ponding to the pure polymer. Solid curves represent a three-component fit. Broken curves represent the individual fitted components, (b) Relative Raman intensities of the fitted profiles for the ( Aj, SO,) anion mode for this system, plotted against square root of the salt concentration, solvated ions ion pairs , triple ions, (c) The molar conductivity of the same system at 293 K. Adapted from A. Ferry, P. Jacobsson, L. M. Torell, Electrnchim. Acta 1995, 40, 2369 and F. M. Gray, Solid State Ionics 1990, 40/41, 637. 

Fuoss, R. M. Kraus, C. A. (1933). Properties of electrolytic solutions. IV. The conductance minimum and the formation of triple ions due to the action of Coulomb forces. Journal of the American Chemical Society, 55, 2387-99. 

Results from infrared spectroscopy indicate that the only species present in 50 % phosphoric acid are H3PO4, HjPOj and their oligomers (Wilson Mesley, 1968). There is evidence that HgPjOg, the phosphoric acid dimer, and HsPjOg, the triple ion H2PO4.. HjPOj, are also present (Elmore,... 

Further, in the case of virtually non-existent ion-solvent interactions (low degree of solvation), so that solute-solute interactions become more important, Kraus and co-workers47 confirmed that in dilute solutions ion pairs and some simple ions occurred, in more concentrated solutions triple ions of type M+ X M+ orX M+X andinhighly concentrated solutions even quadrupoles the expression triple ions was reserved by Fuoss and Kraus48 for non-hydrogen-bonded ion aggregates formed by electrostatic attraction. 

In the original paper these ions were designated triple ions however, the author prefers to use this designation according to Fuoss and Kraus48 only for M + X M+orX M + X , formed merely by electrostatic attraction (see p. 255). 

Several workers have attempted to use the common ion technique to depress [Pn+] and thus to achieve a monoeidic Pn+A system, as was done so successfully for anionic systems. However, because generally the solvents used for cationic polymerisations are much more polar, the KD of the chain-carriers and of the common-ion salts are considerably greater than in the anionic systems. Therefore the electro-chemical situation is likely to be complicated by triple ion formation and the effects of ionic strength on the KD and on the rate-constants, so that any results obtained by extrapolations to infinite ionic strength need to be scrutinised most carefully. 

A variety of localized lithiated carbanions, such as aryllithiums and sulfur- and silicon-substituted alkyllithiums, have been found, by application of C, Li, and Li NMR techniques, to form triple ions in THF-HMPA solution. Thus, change to triple ion structures (18a-g) could be discerned as HMPA (2-5 equiv.) was added to solutions of monomeric structures (17a-g) in 4 1 THF-diethylether. The amount of triple ion is sensitive to ortho substitution monomeric (17a) and (17b) form 65-80% triple ion in presence of 1-3 equiv. HMPA whereas (17c) and (17e) form less than 20% at 5 equiv. HMPA. Pyridylthio-substituted carbanion (19) forms bis-chelated triple ion (20). 

A stable triple ion species of a different nature can be isolated as crystalline solid (227) on reacting pentafluorophenyl iodide with pentafluorophenyllithium (equation 40). XRD... 

Propagation of two-ended (bifunctional) propagating species often proceeds at a rate lower than that of the corresponding monoanion species as a result of triple-ion formation [Bhattacharyya et al., 1964 Smid, 2002]. For example, ionic dissociation of the counterion from one end of the cesium salt of a two-ended propagating species XXX yields XXXI in which the newly dissociated anionic center remains near the ion pair at the other end of... 

XXXI. The result is XXXII, referred to as a triple ion. The triple ion propagates faster than a simple ion pair hut slower than a free ion [Muller, 1989]. The major effect of triple ion formation is a lowering of the concentration of free ions and the overall result is a decrease... 

The higher reactivity of 2-vinylpyridine relative to styrene has been attributed to a combination of intramolecular solvation and triple-ion formation [Sigwalt, 1975 Soum et al., 1977]. 

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