Intermolecular interactions and solvation effects of dimethylsulfoxide on type III deep eutectic solvents

Reline, a typical type III Deep Eutectic Solvent (DES), composed of urea and choline chloride, is widely used and highly investigated for its thermo-physical properties and intermolecular interactions. The formation of hydrogen bonds between urea and chloride ions and a strong suppression in the hydrogen bonds between the urea molecules lead to a significantly low melting point of reline. Considering commercial applications, aqueous solutions of reline have been extensively analyzed; however, water, being a protic solvent, strongly affects the hydrogen bond network in reline and disrupts its structure. A mixture of reline with aprotic solvent has not been systematically investigated in the literature. To this end, we herein explore the intermolecular interactions between reline and dimethylsulfoxide (DMSO), a commonly used industrial solvent, which unlike water, does not have labile hydrogen. Our experiments revealed that a decrease in the density of the mixture with increasing DMSO amount is less significant as compared with water. Furthermore, molecular dynamics simulation results, as analyzed in terms of the hydrogen bonds between the components and interaction energies, suggest a very limited effect on urea-urea, urea-chloride, and urea-choline interactions in the presence of DMSO. Moreover, DMSO preferentially interacts with urea in the mixture; whereas in the case with water, the chloride ion was preferentially hydrated. Radial distribution analysis indicates that reline, while getting diluted by DMSO, maintains its structure up to 0.7-mole fraction of DMSO. Taken together the results reveal that DMSO reserves intermolecular interactions of reline better than water.