Influence of dissolved argon or carbon dioxide on the viscosity and surface tension of the imidazolium-based ionic liquids [OMIM][PF6] or [m(PEG2)2IM

Abstract

The present study investigates the influence of dissolved gases on the viscosity and surface tension of ILs at saturation conditions. As model systems, the imidazolium-based ILs, namely, the hydrophobic 1-methyl-3-octylimidazolium hexafluorophosphate ([OMIM][PF6]) and the hydrophilic 1,3-bis(2-(2-methoxyethoxy)ethyl)imidazolium iodide ([(mPEG2)2IM]I), were studied in combination with argon (Ar) and carbon dioxide (CO2) representing gases of relatively low and high solubility. By a combination of surface light scattering (SLS) and the pendant-drop (PD) method within one setup and sample cell, the surface tension determined by the PD method was used to access the saturated viscosity of the liquid bulk phase via SLS at temperatures of (303 and 323) K and pressures from (0.02 up to 8) MPa. At the same states and within the same setup, the gas solubility in the ILs was estimated on the basis of the evolution of the pressure during transient gas absorption. The addition of either gas causes a decrease in the surface tension of both ILs, which is more pronounced for CO2 at a given pressure and for the lower temperature. This behavior seems to be related to the weakening of intermolecular interactions at the gas–liquid interface by adding gas, facilitated by possible surface enrichment effects of CO2. The viscosity of both ILs is distinctly lowered with increasing CO2 pressure, amounting to a factor of 14 for [OMIM][PF6] at the largest pressure of 5 MPa. In the presence of Ar, the viscosity of [OMIM][PF6] reduces with increasing pressure by 11% at 8 MPa, while that of [(mPEG2)2IM]I increases by 7%. This behavior indicates that the fluidization effect on the liquid phase due to the solvation of Ar is dominant for the hydrophobic IL with higher Ar solubility, whereas it is outweighed by the competing compression effect of the gas for the hydrophobic IL.