Influence of an Oxygen Functionalization on the Physicochemical Properties of Ionic Liquids: Density, Viscosity, and Carbon Dioxide Solubility as a Function of Temperature

The physicochemical properties (density, viscosity, and carbon dioxide solubility) of ionic liquids based on pyridinium, pyrrolidinium, and ammonium cations were studied at atmospheric pressure and as a function of temperature between (293 and 343) K. The influence of the inclusion of oxygen functional groups (hydroxyl and ester) in the cations was assessed by comparing their behavior with that of similar nonfunctionalized ionic liquids. We observed that the presence of oxygen groups does not affect the density significantly. The inclusion of an ester group in the alkyl-side chain of pyridinium or ammonium cations greatly increases the viscosity of bis(trifluoromethylsulfonyl)imide ionic liquids (5 times for pyridinium, 2 times for ammonium-based ionic liquids at 293 K), while the presence of hydroxyl groups only slightly increases their viscosity (16 % increase for ammonium at 293 K). Carbon dioxide solubilities are not significantly influenced by the introduction of oxygen functional groups in the cations for the ammonium-based ionic liquids. In the case of the pyridinium-based ionic liquids, the solubility of carbon dioxide significantly decreases (up to a 48 % decrease in mole fraction) due to a defavorable entropic contribution to the Gibbs energy of solvation.