Thermophysical and Spectroscopic Studies of Pure 1-Butyl-3-methylimidazolium Tetrafluoroborate and Its Aqueous Mixtures

Abstract

Intermolecular interactions in the aqueous mixtures of the room temperature ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), have been studied. The thermophysical properties: density ρ, speed of sound u, specific conductivity κ and refractive index nD have been measured over the whole composition range at different temperatures (293.15–323.15 K) and are discussed. The results from thermophysical measurements are explained with the help of spectroscopy. In order to interpret the nature of molecular interactions occurring between [BMIM][BF4] and water molecules, as well as to identify the moieties in which interactions are taking place, 1H, 13C NMR and FT-IR spectra of the solutions have been studied. Excess molar volume VE, excess molar isentropic compressibility \( K_{S}^{\text{E}} \), partial molar excess volume \( V_{i}^{\text{E}} \), partial molar excess isentropic compressibility \( K_{S,i}^{\text{E}} \), deviation in specific conductivity ∆κ, and deviation in refractive index ∆R have also been determined and analyzed to have a better understanding of the interactions taking place between the different components. Additionally, the excess ultrasonic speed uE and excess isentropic compressibility \( k_{S}^{\text{E}} \), in terms of volume fractions, have been calculated and analyzed. It has been observed that temperature has a significant effect on the thermophysical properties of the studied system. Spectroscopic measurements confirm the disruption of ion-pair interactions of [BMIM][BF4] and hydrogen-bonded network of water in the aqueous mixture of [BMIM][BF4].