Refining Molecular Dynamics of Choline Chloride:Lactic Acid Deep Eutectic Solvent: Impact of Lactic Acid Ratio

Deep eutectic solvents (DES) are of significant interest due to their eco-friendly and efficient solvent properties. Choline chloride:lactic acid (ChCl:LA) in particular stands out for its ability to extract lignin from biomass, a process whose efficiency is influenced by the ratio of LA molecules and whose underlying mechanism remains unknown. This study investigates the structure of the ChCl:LA DES, employing ab initio molecular dynamics simulations to assess the accuracy of the transferable and polarizable Cl\&Pol force field in describing this system. When both compared, we observe that the Cl\&Pol force field qualitatively captures the primary interactions occurring within the system, albeit with some numerical discrepancies that are anticipated due to its transferable nature. Aiming to refine the original force field, we conduct calculations that incorporate two improvements to the initial model: tuning the $\sigma$ parameter of the strongest hydrogen-bond interactions and incorporating the Tang-Toennis dumping function to correct the overpolarization of the chloride ions. The first adjustment not only enhances the description of the specifically targeted interactions but also significantly improves the short-range structure of the entire hydrogen-bond network. The second refinement, although minimally impacting the structure at low LA ratios, proves critical at higher ratios by correcting the oversegregation of ionic molecules arising from the original setup of the force field. Consequently, it becomes indispensable for reliably depicting the medium and long-range structure of the system, highlighting that the specific parameter of the force field to be refined depends on the scale of the structure under investigation. Concomitantly, the structural evolution observed as the LA ratio increases sheds light into the origin of improved processing of lignin as the LA ratio increases, suggesting an enhanced availability of the interacting moieties at high LA contents.