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

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 (Λcalcd), calculated considering only the ion pair formation (K1 = 1.11 × 103) of lithium perfluorobutyrate (LiCF3(CF2)2CO2) fit the observed ones (Λobsd), however, they deviated much positively from Λobsd values, e.g. the relative error of Λcalcd was + 9.6% to Λobsd at Cs = 3.0 × 10− 3 mol dm− 3. The re-calculation including the formation of both the ion pair (K1 = 1.06 × 103) and the quadrupole (the dimer of ion pairs, K41 = 1.2 × 102) 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 (LiCHF2CO2), the limiting molar conductivity (Λ0(direct) = 181.2), estimated directly by the Shedlovsky method was extremely larger than the intrinsic value (Λ0(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 Λ0(direct) and Λ0(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.