Structural and Interfacial Behavior of Choline Chloride-Based DESs (CholCl:EG and CholCl:Urea) at Various Weight Percentages in Water Mixture

Abstract

This study explores the impact of water on the molecular arrangement of reline and ethaline deep eutectic solvents (DESs) across varying concentrations ranging from 26 to 78 wt%. Given that water serves as the second hydrogen bond donor in reline and ethaline, understanding its influence on these systems is crucial. The controlled addition of water to DESs induces significant changes in their microscopic structure and thermodynamic properties. To investigate these effects comprehensively, we conducted large-scale atomistic molecular dynamics simulations and analyzed various parameters, including radial distribution functions, coordination numbers, the number of hydrogen bonds, mean square displacements, and diffusion coefficients. Our findings show how water weight percentage alters intermolecular interactions within ethaline−water and reline−water mixtures. Additionally, we provide detailed insights into the segregation patterns of DES components upon the introduction of water. The findings suggest distinct structural characteristics between aqueous solutions of the two investigated DESs. Specifically, as the weight percent of ethaline increases, the hydrogen bond donor (HBD) component, ethylene glycol (EG), exhibits segregation tendencies from water and choline chloride. In contrast, in reline aqueous solutions, the HBD component, urea, demonstrates a more uniform distribution with increasing mole fraction of reline. Water alters the composition of DES at the surface compared to the bulk. In pure reline, both urea and choline are present at the surface. Adding water depletes these molecules from the surface, allowing water molecules to accumulate there instead. In the ethaline system, EG molecules remain oriented towards the surface in both pure and aqueous forms. The orientational ordering of DESs at the liquid-vapor interface is investigated using bivariate orientational analysis, providing a deeper understanding of molecular orientation at aqueous DES interfaces.