Condutometric studies on higher ion-aggregation from lithium fluoroalkanoates in propylene carbonate and N, N-dimethylformamide

Abstract

In propylene carbonate (PC), the higher ion-aggregation behavior of lithium fluoroalkanoates (the total number of carbons: 2 to 8) was investigated by means of conductometry at 25 °C. At lower salt concentrations, the molar conductivities ( Λ calcd), calculated considering only the ion pair formation ( K 1 = 1.11 × 10 3) of lithium perfluorobutyrate (LiCF 3(CF 2) 2CO 2) fit the observed ones ( Λ obsd), however, they deviated much positively from Λ obsd values, e.g. the relative error of Λ calcd was + 9.6% to Λ obsd at C s = 3.0 × 10 − 3 mol dm − 3 . The re-calculation including the formation of both the ion pair ( K 1 = 1.06 × 10 3) and the quadrupole (the dimer of ion pairs, K 41 = 1.2 × 10 2) gave excellent consistency to the observed ones (relative error of less than ± 0.5%) over the whole range of the salt concentrations examined. For lithium difluoroacetate (LiCHF 2CO 2), the limiting molar conductivity ( Λ 0(direct) = 181.2), estimated directly by the Shedlovsky method was extremely larger than the intrinsic value ( Λ 0(indirect) = 27.7) which was indirectly evaluated by Kohlrausch's additivity law with strong electrolytes in the solvent. Such phenomena were also observed for other lithium fluoroalkanoate salts. Inconsistency between Λ 0(direct) and Λ 0(indirect) was attributed to the formation of higher ion-aggregates in addition to the ion pair formation. It was found that the longer the carbon-chain of perfluoroalkanoates, the smaller the association constants were obtained, however, the association constants became almost constant for higher perfluoroalkanoates having more than four carbon-atoms. The association constants of lithium perfluoroalkanoates in PC were consistent with the Mulliken charge densities, obtained with semi-empirical molecular orbital calculations (AM1 and PM3), on the oxygen atoms in the perfluoroalkanoate ions. In N, N-dimethylformamide (DMF), a more solvating solvent toward metal ions, however, the conductivity data of lithium perfluoroalkanoates were accounted for only by the ion pair formation and no higher ion-aggregation.