Distinct Solvation Structures of CO₂ and SO₂ in Reline and Ethaline Deep Eutectic Solvents Revealed by AIMD Simulations

Abstract

Deep eutectic solvents (DESs) are emerging as an alternative media for the sequestration of greenhouse gases such as CO 2 and SO 2 . Herein, we performed ab initio molecular dynamics (AIMD) simulations to elucidate the solvation structure around CO 2 and SO 2 in choline chloride-based DESs, namely, reline and ethaline. We show that in all four systems the structures of the nearest neighbor shells around these molecules are distinct. We observe that because of the electrophilic character, the carbon atom of CO 2 and the sulfur atom of SO 2 are preferentially solvated by the chloride anions. The strength of the correlation between the chloride anions and the sulfur atom is much stronger because of charge transfer, which is more profound in ethaline DES. In both DESs, the choline cations are found to be closer to the oxygen atoms of CO 2 and SO 2 . We observe that upon changing the solute from CO 2 to SO 2 , the nearest neighbor solvation structure changes drastically; while the chloride anions prefer to stay in a circular shell around the carbon atom of CO 2 , they are found to be much more localized near the sulfur atom of SO 2 . The solvation shells formed by the urea molecules in reline and EG molecules in ethaline also overlap with that of the chloride anion around CO 2 . In ethaline, the hydroxyl group of the choline cation is found to be closer to the solute molecules as compared to its ammonium headgroup.

Computer Simulations of Deep Eutectic Solvents: Challenges, Solutions, and Perspectives

“…DFT approaches have not only been fruitful for studies of distribution patterns of DES components, but also in studies of interactions of DESs with various functional substances in gas separation. In particular, free energy changes and structural analyses have been used to develop new solutions towards desulfurization of liquid fuels [ 73 , 78 , 85 , 86 , 89 ], capturing greenhouse gases such as CO 2 or SO 2 [ 72 , 79 , 90 , 91 ], metronidazole extraction from plasma [ 92 ], developing efficient mercury removal strategies from different gases [ 93 ], extractive detoxification of feedstocks for the production of biofuels using new hydrophobic DESs [ 74 ], capturing NH 3 [ 94 ], and for separating phenolic compounds from oil mixtures [ 95 ].…”

Computer Simulations of Deep Eutectic Solvents. Challenges, Solutions, and Perspectives

“…DFT approaches have not only been fruitful for studies of distribution patterns of DES components, but also in studies of interactions of DESs with various functional substances in gas separation. In particular, free energy changes and structural analyses have been used to develop new solutions towards desulfurization of liquid fuels [56,61,68,69,72], capturing greenhouse gases such as CO2 or SO2 [55,62,73,74], metronidazole extraction from plasma [75], developing efficient mercury removal strategies from different gases [76], extractive detoxification of feedstocks for the production of biofuels using new hydrophobic DESs [57], capturing NH3 [77] and for separating phenolic compounds from oil mixtures [78].…”

“…It was shown that in hydrous reline, water competes for the anions, and the hydrogen atoms of urea have similar propensities to bond to the chloride (Cl − ) ions and the oxygen atoms of urea and water. The same level of theory was used by Malik et al [73] to elucidate the solvation structure around CO2 and SO2 in ChCl-based DESs, namely, reline and ethaline. Zahn et al [93] revealed significantly reduced ion charges in several choline-based DESs by using ab initio molecular dynamics in the GPW representation.…”

“…Among the subsequent AIMD simulations of DESs, a number of papers have focused on gas capture [73,85,142]. Since many-body effects and polarizability are directly included in AIMD, it is excellent for elucidating the local solvation structure around the gas molecules.…”

“…Since many-body effects and polarizability are directly included in AIMD, it is excellent for elucidating the local solvation structure around the gas molecules. For example, Malik et al [73] demonstrated for CO2 and SO2 in reline and ethaline that charge transfer between the solute and the chloride anion determines the shapes of the solvation shells, while the nature of the H-bond donor (HBD) is responsible for its organization around the solute. AIMD allows one to investigate the solvation of not only small gas molecules in DES, but also, for example, mercury solvation, as its capture represents a major challenge in natural gas processing [143].…”