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

Deep eutectic solvents (DESs) are emerging as an alternative media for the sequestration of greenhouse gases such as CO₂ and SO₂. Herein, we performed ab initio molecular dynamics (AIMD) simulations to elucidate the solvation structure around CO₂ and SO₂ 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 CO₂ and the sulfur atom of SO₂ 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 CO₂ and SO₂. We observe that upon changing the solute from CO₂ to SO₂, the nearest neighbor solvation structure changes drastically; while the chloride anions prefer to stay in a circular shell around the carbon atom of CO₂, they are found to be much more localized near the sulfur atom of SO₂. The solvation shells formed by the urea molecules in reline and EG molecules in ethaline also overlap with that of the chloride anion around CO₂. In ethaline, the hydroxyl group of the choline cation is found to be closer to the solute molecules as compared to its ammonium headgroup.