Study of ion-pair and triple-ion origination of an ionic liquid ([bmmim][BF4]) predominant in solvent systems

Abstract

Electrolytic conductivities, densities, viscosities, and FT-IR spectra of 1-butyl-2,3 dimethylimidazolium tetrafluoroborate ([bmmim][BF₄]) have been studied in tetrahydrofuran, dimethylacetamide and methyl cellosolve at different temperatures. The limiting molar conductivities, association constants, and the distance of closest approach of the ions have been evaluated using the Fuoss conductance equation (1978). The molar conductivities observed were explained by the formation of ion pairs and triple ion formation. Ion–solvent interactions have been interpreted in terms of apparent molar volumes and viscosity B-coefficients which are obtained from the results supplemented with densities and viscosities, respectively. The limiting apparent molar volumes, experimental slopes derived from the Masson equation and viscosity A and B coefficients using the Jones–Dole equation have been interpreted in terms of ion–ion and ion–solvent interactions respectively. However, the deviation of the conductometric curves (Λ vs. √c) from linearity in tetrahydrofuran indicated triple-ion formation and therefore corresponding conductance data have been analyzed by the Fuoss–Kraus theory of triple ions. The limiting ionic conductances have been estimated from the appropriate division of the limiting molar conductivity value of tetrabutylammonium tetraphenylborate as the “reference electrolyte” method along with a numerical evaluation of ion-pair and triple-ion formation constants (K_P ≈ K_A and K_T). The FT-IR spectra for the solvents as well as the solute in solvents have also been studied. The results are discussed in terms of ion–dipole interactions, hydrogen bond formation, structural aspects, and configurational theory.