Probing the ion-dipole interactions between the imidazolium-based ionic liquids and polyethylene glycol 200 using excess thermodynamic and spectroscopy studies

Abstract

The use of volatile organic compounds in industrial applications has generally been considered as detrimental to the environment and been a contributor to unsustainable activity. Ionic liquids (ILs) are generally considered as “green” and ecologically friendly solvents compared to volatile organic solvents. In the application of ILs in various processes, they are typically mixed with other solvents in order to modify their physicochemical properties. Thus, the focus in the present study on the interactions between ILs and polymer via thermophysical properties together with FT-IR analysis. In this study, density (ρ), viscosity (η), speeds of sound (u), and refractive index (nD) of the binary mixtures of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+PF6-), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM+BF4-) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4-) with polyethylene glycol 200 (PEG200) were measured at (293.15, 303.15, 313.15, 323.15 and 333.15) K and at p = 0.1 MPa. Excess thermodynamic properties: excess molar volume (VE), excess isentropic compressibility (KsE), excess viscosity (ηE), excess refractive index (nDE) and excess thermal expansion coefficient (αPE) were calculated from the thermophysical properties for the new systems studied. Excess thermodynamic properties were best correlated using the Redlich-Kister polynomial equation. Excess thermodynamic properties such as excess molar volume (VE), excess isentropic compressibility (KsE), and excess thermal expansion coefficient (αPE), displayed negative deviations, while excess viscosity (ηE) and excess refractive index (nDE) exhibited positive deviations throughout the entire composition range for the new systems. In addition, FT-IR spectroscopy was further utilized to confirm the presence of ion–dipole interactions/hydrogen bonding between the molecules of ILs and PEG200. The results from FT-IR spectra support the results obtained from excess thermodynamic properties. The results from both excess thermodynamic properties and FT-IR spectroscopy analysis confirmed the presence of ion–dipole interactions/hydrogen bonding. The strength of the ion–dipole interactions formed between molecules of PEG200 and ions of ILs follows the order: EMIM+BF4- >BMIM+PF6- > BMIM+BF4-. This study of the ILs + PEG200 binary mixtures contributes to the knowledge of physicochemical properties of these mixtures which can be utilized in specific applications such polymeric electrolytes in different electrochemical devices.