A Molecular Simulation Study of Carbon Dioxide Uptake by a Deep Eutectic Solvent Confined in Slit Nanopores

Molecular dynamics simulations were performed to study the behavior of CO2 in varying amounts of a common deep eutectic solvent (DES), choline chloride and ethylene glycol (termed ethaline), confined in slit-like pores of width H = 5.2 nm with graphite or rutile walls at T = 318 K. In the absence of DES, CO2 adsorbs to the pore walls, but increasing, amounts of ethaline inside the pores quickly displace carbon dioxide into the gas/liquid interfaces, and into dissolution within the confined DES. This process is driven by strong interactions of the ethaline components with the pore walls, especially in the case of rutile systems, which also cause the local ratio of choline chloride:ethylene glycol inside the pores to depart from the bulk value of 1:2. As the amount of DES inside the pores increases, the diffusivity of CO2 reaches a maximum in partially filled pores and decays to the values observed in pores filled with DES, which are similar to the diffusion coefficients of CO2 in the bulk DES. The average number densities of CO2 near confined ethaline (i.e., dissolved in the DES, and adsorbed at the pore walls and at the gas/liquid interface) are significantly larger than the corresponding value observed in bulk ethaline; for pores partially filled with DES, the overall average number density of CO2 can be similar to 3.0-7.3 times the value observed in bulk ethaline. Even though larger number densities of CO2 are observed in pores with no ethaline adsorbed, our results suggest that systems of nanoporous materials partially filled with DESs could be further explored and optimized for separation of carbon dioxide and other gases, in analogy to the development of supported ionic liquid phase materials for similar purposes.