CO2 solubility and diffusivity in 1-ethyl-3-methylimidazolium cation-based ionic liquids; isochoric pressure drop approach

Abstract

In this study, an experimental approach based on the isochoric pressure drop principle was implemented to measure (at low equilibrium pressure, 1.3–3.6 bar) the solubility and binary diffusivity of CO2 (constant and variable) in four imidazolium-based room-temperature ionic liquids (RTILs), namely, 1-ethyl-3-methylimidazolium acetate ([emim][Ac]), 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA]), 1-ethyl-3-methylimidazolium methyl sulfate ([emim][MeSO4]) and 1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO4]). The molar fraction of the dissolved CO2 and Henry's law constant were measured from experimental data. Constant binary diffusion coefficients were measured from the transient thin-film and semi-infinite volume models. In the original approach, the variable diffusivity coefficient model was used to measure the concentration-based variable diffusion coefficient. Anion interaction of the RTILs was found to have a strong influence on the CO2 solubility, and Henry's law constant and the studied anions were ranked as [Ac]− >> [EtSO4]− > [DCA]− > [MeSO4]−. Increasing temperature is not likely for CO2 solubility while it favors CO2 diffusivity. Thin-film model diffusivity measurements were found to be more suitable for [emim][MeSO4] and [emim][EtSO4], while semi-infinite volume model was suitable for [emim][Ac] and [emim][DCA]. Variable diffusion coefficients were found not to be a monotonous function of time, which increased in early stages and decreased after a certain time. Gibbs free energy, enthalpy, and entropy values indicate the strength of the favorable interaction between CO2 and RTILs, mildly exothermic in nature and falling in the range of physisorption.