Investigating the\nImpact of Water on a Menthol-Based\nDeep Eutectic Solvent: A Combined Experimental and Molecular Dynamics\nStudy

Deep eutectic solvents (DESs) have emerged as potential\nalternatives\nto traditional solvents, offering unique properties and potential\napplications. However, the excessively viscous nature of DESs limits\ntheir applications in extraction, separation, electrodeposition, battery\nand energy storage, and drug delivery. The presence of water as a\ncosolvent can substantially reduce the viscosity and density of DESs\nand open widespread applications. The aim of this research is to elucidate\nthe nanostructures of DESs in the presence of water, which is key\nto understanding and tuning their properties for various applications.\nMultitechnique approaches such as Raman spectroscopy, differential\nscanning calorimetry (DSC), <sup>1</sup>H nuclear magnetic resonance,\nand an all-atom molecular dynamics (MD) simulation were employed to\ndetermine how water affects the molecular-level arrangement, dynamics,\nand interactions of a L-menthol/acetic acid (AA)-based hydrophobic\nDES at various hydration levels. A pure DES exhibits significant H-bonding\ninteractions with menthol and AA. As water is added, these interactions\nweaken, new H-bonding interactions between the DES system and water\nemerge, and the DES turns into a binary mixture as evident from visual\ninspection and DSC and Raman experiments. As the contacts between\nmenthol and AA weaken and the number of contacts between water molecules\nincreases, about 50%–60% of water leads to a different system.\nAdditionally, principal component analysis assists us in identifying\nthe sequences in which the characteristics of the DES change with\nthe gradual addition of water. The changes detected in the nanostructural\ncharacteristics of the L-menthol/AA DES can aid to develop a less\nviscous and dense DES suited for special applications. The combined\nRaman spectroscopic and MD simulation analysis provided deep molecular-level\ninsights into the structural and dynamic changes occurring in the\nMen–AA system with varying water content. The findings obtained\nfrom this research not only enhance our comprehension of the influence\nof water on altering the characteristics of DES but also facilitate\nthe development of DES systems specifically designed for industrial\noperations that need reduced viscosity and density.