Effect of water on the thermo-physical properties of Reline: An experimental and molecular simulation based approach

Abstract

Increasing applications of ionic liquids and their analogues, namely Deep Eutectic Solvents (DESs), requires further investigation into the effect of moisture content on the physico-chemical characteristics of these fluids. Although it is common practice to synthesize these fluids in a moisture-controlled environment, as moisture is generally considered to have an impact on their properties, there are no systematic studies on this. We herein examine the effects of water on Reline, a Type-III DES composed of urea and choline chloride. Experiments were performed to obtain the physical properties of aqueous Reline solution. We observed moderate changes in density, speed of sound, refractive index, and pH with increasing water fraction; however, the change in viscosity and conductivity was strong and exponential. In addition, molecular dynamics simulations were performed to analyze the intermolecular interactions of Reline and aqueous Reline solutions. The simulations primarily present the significance of urea-anion interaction to explain the low melting point of the DES. In the presence of water, the anion is preferentially hydrated as compared to urea or the cation. More interestingly, simulations help to classify the effects of water into different regimes. At low water fractions (<5%) the urea-urea interactions are enhanced, as is revealed through the hydrogen bond analysis. Beyond 25% water fractions, the components of Reline are individually hydrated and have high diffusivity, which is further reflected in the change in transport properties. The results presented herein provide valuable information on aqueous Reline solutions both in terms of experimental data and molecular insights, which in turn, we believe, might assist in developing further applications of Reline and other related DESs.

Exploring Orange Peel Treatment with Deep Eutectic Solvents and Diluted Organic Acids

“…Up to water contents of 25 wt.%, the intermolecular hydrogen bonding network between the DESs' components is still predominant [24,25]. Water solvation of the individual DES components occurs between water contents of 25 and 50 wt.%.…”

“…In literature, it is suggested that the physicochemical properties of DESs, e.g., viscosity, can be tuned upon water addition [24][25][26]. Up to water contents of 25 wt.%, the intermolecular hydrogen bonding network between the DESs' components is still predominant [24,25].…”

Characteristics and intermolecular interaction of eutectic binary mixtures: Reline and Glyceline

“…Excess thermodynamic properties have been successfully employed in evaluating the variation in properties of ionic liquid (IL) mixtures [12,13], aqueous mixtures of ILs and mixtures of molecular solvents with IL [14][15][16]. Studies of variations in physical properties using excess properties have also been carried out on aqueous mixture of DESs [17][18][19][20]. However, extensive studies on the variation of the intermolecular forces and thus physical properties of a DES upon mixing using excess thermodynamic properties have not been carried out.…”

The Effect of Water upon Deep Eutectic Solvent Nanostructure: An Unusual Transition from Ionic Mixture to Aqueous Solution

“…A relatively new approach leverages the favorable physicochemical properties of DES/water mixtures, such as lowered viscosity 8. Trends in these properties suggest that there is an upper limit to this hydration above which DESs are more like aqueous solutions 9, 10, 11. However, it is not known how far such mixtures can be hydrated before they cease to be DESs on a nanostructural level because only a limited compositional range has been probed experimentally in detail by NMR spectroscopy,12 which has also been used extensively for ionic liquid (IL)/water mixtures 13, 14.…”