Effect of Cation on Physical Properties and CO2 Solubility for Phosphonium-Based Ionic Liquids with 2-Cyanopyrrolide Anions

A series of tetraalkylphosphonium 2-cyanopyrrolide ([P-nnnn][2-CNPyr]) ionic liquids (ILs) were prepared to investigate the effect of cation size on physical properties and CO2 solubility. Each IL was synthesized in our laboratory and characterized by NMR spectroscopy. Their physical properties, including density, viscosity, and ionic conductivity, were determined as a function of temperature and fit to empirical equations. The density gradually increased with decreasing cation size, while the viscosity decreased noticeably. In addition, the [P-nnnn] [2-CNPyr] ILs with large cations exhibited relatively low degrees of ionicity based on analysis of the Walden plots. This implies the presence of extensive ion pairing or formation of aggregates resulting from van der Waals interactions between the long hydrocarbon substituents. The CO2 solubility in each IL was measured at 22 degrees C using a volumetric method. While the anion is typically known to be predominantly responsible for the CO2 capture reaction, the [P-nnnn][2-CNPyr] ILs with shorter alkyl chains on the cations exhibited slightly stronger CO2 binding ability than the ILs with longer alkyl chains. We attribute this to the difference in entropy of reaction, as well as the variation in the relative degree of ionicity.