The effect of increasing water on the physicochemical features of thymol: DL-menthol based natural deep eutectic solvent (DES)

The expanding use of ionic liquids and their analogs, specifically Deep Eutectic Solvents (DES), necessitate an examination of the impact of moisture content on their physicochemical features. Although these liquids are typically synthesized in moisture-controlled environments to mitigate the influence of moisture, precise studies on this topic are lacking. In this study, we investigated the effects of water on a Type-V DES composed of dl-menthol and thymol in a 1:1 ratio. We conducted tests to determine the physical features, including the density of the DES solution. We observed linear modifications in density with increasing water content; however, the change in density was exponential. Additionally, molecular dynamics simulations were applied to examine the intermolecular interactions of DES and DES solutions. The simulations highlighted the importance of dl-menthol-thymol interactions in explaining the low melting points of DESs. In the presence of water, the DES components become hydrated. Furthermore, the simulations helped categorize the influences of water into various systems. At low water content, DLM-DLM and Thy-Thy interactions are increased, as exposed by hydrogen bond analysis. In DES6 (80 % water), DES constituents are separately hydrated and exhibit varied diffusivity, reflected in changes in transport properties. The outcomes presented in this study provide valuable insights into aqueous solutions of DES, both from experimental data and molecular perspectives. We hope these findings will aid in developing further applications for this DES and related DES. © 2025 Elsevier B.V., All rights reserved.