Hydration Harmony in Choline Chloride/Pyruvic Acid Deep Eutectic Solvent: Insights from Molecular Dynamics

Deep eutectic solvents (DESs) are innovative solvents that are revolutionizing green chemistry with their versatile properties, enabling sustainable transformations and cutting-edge technological advancements. Many choline chloride (ChCl)/carboxylic acid eutectic mixtures are hygroscopic and exhibit elevated viscosity due to strong hydrogen-bonding interactions. Even small amounts of water can significantly reduce their viscosity by disrupting the hydrogen bond network and promoting the formation of distinct DES-H₂O nanoclusters. This study examines the impact of water on HBA-HBD cluster formation within a 1:1 ChCl and pyruvic acid (PA) eutectic mixture across hydration levels ranging from 0 to 80 wt % through molecular dynamics (MD) simulations. The study initially investigated the role of hydrogen bonding among all components in both neat and hydrated DES systems through radial distribution functions (RDFs), hydrogen bond (HB) analysis, and free energy landscape (FEL) evaluations. In particular, the findings revealed that the hydrated eutectic mixture mainly forms two competing molecular structures, H₂O-in-DES and DES-in-H₂O structures, whose relative distribution varies within the hydration range of 0-25 wt %, with full solvation ultimately achieved at a hydration level of 50 wt %. Subsequent analysis of the self-diffusion coefficients (D) revealed two distinct trends below and above 25 wt % hydration, highlighting the structural stability of the hydrated eutectic system up to this critical point. Below 25 wt %, Ch⁺ shows the highest D values compared to Cl^- and PA. However, beyond this hydration level, the diffusion coefficients of Ch⁺ and Cl^- begin to converge, while H₂O consistently exhibits significantly higher mobility. This observation is further validated by experimental data on the density and ionic conductivity as functions of the DES mole fraction. The analysis was further advanced to determine shear viscosity using both the Green-Kubo formalism and Einstein relation, with the viscosity values derived from the Einstein relation showing close agreement with the experimental data.