Structural insights into carboxylic-acid based DES across H-bond donor ratios: impact of CL&Pol refinement

Abstract

Deep eutectic solvents (DES) are of significant interest due to their eco-friendly nature and vast applications. Carboxylic-acid-based choline chloride (ChCl) DES are notable for their roles in electrochemical, drug delivery, and biomass processing applications, with efficiency influenced by the ChCl : carboxylic acid ratio. Understanding these mechanisms requires detailed knowledge of their structure. This study investigates the choline chloride–lactic acid (ChCl:LA) DES structure using ab initio molecular dynamics simulations to assess the accuracy of the transferable and polarizable CL&Pol force field. We observe that the CL&Pol force field qualitatively captures primary interactions within the system, despite numerical discrepancies due to its transferable nature. To refine the original force field, we incorporate two improvements: tuning the σ parameter of the strongest hydrogen-bond interactions and incorporating the Tang–Toennies damping function to correct chloride ion overpolarization. The first adjustment enhances the targeted interactions and 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 in the original force field. Consequently, it becomes essential 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 structural scale under investigation. Notably, the long-range structure results from the competition between choline and carboxylic acid for chloride, rebalanced by the suggested modifications, especially the overpolarization correction.