Diffusivities and solubilities of carbon dioxide in deep eutectic solvents

Abstract

Deep eutectic solvents (DESs) have been identified as promising solvents for CO2 capture because of their low volatility and tunable properties. Detailed information on solute solubility and diffusivity is crucial for the design and operation of processes involving mixtures composed of DESs and dissolved gases. The infinite-dilution diffusion coefficients of CO2 in 33 DESs (both hydrophilic and hydrophobic) were gravimetrically determined with a magnetic suspension balance at 298.15 and 313.15 K along with the Henry’s constants of CO2 in these DESs. Interestingly, the diffusion coefficients of CO2 at infinite dilution in hydrophobic DESs were found to be much higher than those in hydrophilic DESs. In order to better understand this phenomenon, CO2 diffusion coefficients under infinite dilution conditions in traditional solvents and ionic liquids (ILs) were summarized from literature, which revealed that the CO2 diffusivity is often approximately inversely proportional to the square root of the viscosity. However, the hydrophilic DESs and some ILs/hydrophobic DESs with large molar volumes displayed a significantly different behavior. The OH group seems to hinder CO2 diffusion, while the presence of multiple long flexible alkyl chains in the DES constituents can facilitate the diffusion. Modified Wilke–Chang correlations were successfully developed to describe the infinite-dilution diffusion coefficients in the measured DESs. Tetraoctylammonium bromide - decanoic acid (1:2) was found promising for further investigation for application in CO2 capture processes with supported liquid membranes or physical absorption.