Optical and antibacterial properties of 1-butyl-3-methylimidazolium ionic liquids with trifluoromethanesulfonate or tetrafluoroborate anion

Abstract

Two room temperature ionic liquids (RTILs) having the same cation (1-butyl-3-methylimidazolium [BMIM]+) and different anions (tetrafluoroborate [BF4]−or trifluoromethanesulfonate [TfO]−) have been investigated by ATR-FTIR, NMR, TGA/DTA, UV–vis spectroscopy, agar-disk diffusion, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) assay. Structural investigations by ATR-FTIR and NMR spectroscopy demonstrated that the [BMIM]+ cation exhibits hydrogen bonding with the [BF4]−and [TfO]− anions via the C (2)-H and C (4, 5)-H of the imidazolium cation ring in addition to the protons of the alkyl and methyl groups. UV–vis spectroscopy revealed an energy gaps of 3.79 ± 0.13 and 3.37 ± 0.04 eV for [BMIM][BF4] and [BMIM][TfO], respectively. Single term Sellmeier oscillator model and the Drude-Lorentz model were employed for the first time to investigate the optical and dielectric properties of ionic liquids. Applying the former model, enables us to extract the average inter-band oscillator wavelength (λ0), the average oscillator strength (S0), single oscillator energy (E0), dispersion energy (Ed) while the latter gives the plasmon frequency (ωp), reduced resonance frequency (ω0) and the electron lifetime at femtosecond level. Antibacterial assay by MIC and MBC demonstrated that both ionic liquids have antibacterial activity against gram-positive and gram-negative bacterial strains.