Water in biodegradable glucose-water-urea deep eutectic solvent: modifications of structure and dynamics in a crowded environment.

Abstract

Molecular dynamics simulations have been performed on a highly viscous (η ∼ 255 cP) naturally abundant deep eutectic solvent (NADES) composed of glucose, urea and water in a weight ratio of 6 : 4 : 1 at 328 K. The simulated system contains 66 glucose, 111 water and 133 urea molecules. A neat system with 256 water molecules has also been simulated. In this study, the water structure and dynamics in a crowded environment have been investigated by computing inter-species radial distribution functions (RDFs), quantitative and qualitative analyses of intra-species water H-bonds, heterogeneity timescales from the anomalous mean square displacements, and two-point and four-point density-time correlation functions. The simulated structures indicate asymmetric interactions between water and glucose molecules, and considerable water-clustering. In addition, a dramatic distortion of the orientational order has been reflected. A severe decrease in the average number of water-water H-bonds and the corresponding participation of water molecules have been detected, although the water H-bond length distribution does not differ much from that for the neat system. The participation populations of water for H-bonding with itself and the other two species have been expressed by constructing a pi-chart. Only ∼16% of the total water molecules have been found to be simultaneously H-bonded with glucose and urea molecules. A qualitative picture of water clustering has been proposed through the interpretation of the observed drastic deviation of water angle distributions. Centre-of-mass translations and structural H-bond relaxations have been found to be significantly slowed down relative to those in neat water. Evidence of hop-trap movements for DES water has been found.