Unilateral Triple-Ion Formation

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. 

The investigation of ion-aggregate formation based on conductivity studies can be extended to quadruple-ion formation [108-111], which is thought to be the reason for the conductivity maximum and the subsequent decrease of conductivity. 

However, in our opinion the interdependence of up to four aggregation constants in addition to uncertainties in the determination of the limiting conductivity and activity coefficients makes their determination with recent equations [110] increasingly unsure. 

The last point has been studied more quantitatively for the electrolyte LiOjCCH Fy (x+y=3) / DMSO [97,105], Semiempirical quantum-mechanical calculations with the help of MOP AC [143] show that the mean electron density at the oxygen atoms q(0) decreases for these acetates by about 0.1 unit with increasing fluorine content of the anion [97]. As a consequence  

According to Wooster [203, 204] conductivity measurements can be evaluated at the level of Hmiting laws with the conductivity equation 

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Considering these different limiting forms of the recombination term an Important tentative conclusion emerges the concentration dependence of the reciprocal relaxation time is a direct measure of the main ionic recombination process and yields therefore information on the ionic species present in solution. A linear dependence on total ion-pair concentration would therefore indicate unilateral triple ion formation or, if both kinds of triple ions are present as indicated by conductance, a sufficient difference in their stability. At this point it should be noted that the usual method of Fuoss and Draus... 

Fig. 14. Unilateral triple ion formation in, from the top, 50,45, and 40% dimethoxy-ethane-benzene mixtures at 25° C. Reproduced, with permission, from ISE, Hirohara, Makjno, Takaya, and Nakayama Presented at the 17th Discussion Meeting of High Polymers, October, 1968, Matsuyama, Preprint p, 261...

The conductivity equation for electrolytes undergoing unilateral triple-ion formation, Eq. (71a) or Eq. (71b), is given by the relationship... 

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