Structure–property relationships in transport and thermodynamic properties of imidazolium bistriflamide ionic liquids for CO2 capture

Abstract

The solubility and diffusivity of CO2 in 12 different 1,3-difunctionalized imidazolium bis(trifluoromethylsulonyl)amide (bistriflamide) ionic liquids (ILs) were determined using the transient thin film absorption method, and values were correlated to the physicochemical properties of the ILs including density, viscosity, self diffusivity and ionicity. The ionicity of the IL was quantified in terms of the Haven ratio, calculated using the self-diffusion coefficient and conductivity. The Henry’s Law constant for CO2 in the ILs showed a strong correlation to the square of the difference in the solubility parameters of the ILs and CO2, consistent with regular solution theory. A stronger correlation was observed when both the solubility parameter and the Haven ratio were included in the correlation. CO2 diffusivities in the ILs were found to be linearly related to the self-diffusion coefficient of the IL cations. Interestingly, structural isomers of ILs were found to have the same CO2 solubility, but distinct CO2 diffusivities. The product of solubility and diffusivity (i.e., permeability) was found to correlate well with IL viscosity alone, despite the fact that solubility and diffusivity were each found to depend upon both IL viscosity and molar volume.