Solvation Shell Structures of Ammonia in Reline and Ethaline Deep Eutectic Solvents.

Because of increasing atmospheric anthropogenic ammonia (NH) emission, researchers are devising new techniques to capture NH. Deep eutectic solvents (DESs) are found as potential media for NH mitigation. In the present study, we have carried out molecular dynamics (AIMD) simulations to decipher the solvation shell structures of an ammonia solute in reline (1:2 mixture of choline chloride and urea) and ethaline (1:2 mixture of choline chloride and ethylene glycol) DESs. We aim to resolve the fundamental interactions which help stabilize NH in these DESs, focusing on the structural arrangement of the DES species in the nearest solvation shell around NH solute. In reline, the hydrogen atoms of NH are preferentially solvated by chloride anions and the carbonyl oxygen atoms of urea. The nitrogen atom of NH renders hydrogen bonding with hydroxyl hydrogen of the choline cation. The positively charged head groups of the choline cations prefer to stay away from NH solute. In ethaline, strong hydrogen bonding interaction exists between the nitrogen atom of NH and hydroxyl hydrogen atoms of ethylene glycol. The hydrogen atoms of NH are found to be solvated by hydroxyl oxygen atoms of ethylene glycol and choline cation. While ethylene glycol molecules play a crucial role in solvating NH, the chloride anions remain passive in deciding the first solvation shell. In both the DESs, choline cations approach NH from their hydroxyl group side. We observe slightly stronger solute-solvent charge transfer and hydrogen bonding interaction in ethaline than those in reline.