Unified Mesoscale Framework Bridging Equilibrium Thermodynamics and Transport Properties of Ionic Liquids and Their Mixtures

Abstract

Ionic liquids (ILs) have emerged as sustainable alternatives to molecular solvents because of their unique physicochemical properties. Although quantum chemical simulations provide fundamental insights into their intermolecular interactions, constructing coarse-grained models that preserve equilibrium thermodynamics while quantitatively reproducing the prescribed transport properties remains challenging. This simulation challenge comes primarily from the high viscosity and low diffusivity of ILs due to strong ion pairing, resulting in Schmidt numbers (Sc) that span multiple orders of magnitude. In this work, we present a unified mesoscale simulation framework that preserves thermodynamic consistency while quantitatively reproducing the transport properties of pure ILs and their mixtures. This framework integrates a bottom-up approach and an extended dissipative particle dynamics (DPD) formulation: equilibrium thermodynamics are derived from COSMO-RS analysis and fragment molecular orbital (FMO) calculations, while DPD resolves coarse-grained dissipative forces into parallel and perpendicular components. A key contribution of this study is a micromechanically grounded algorithm that systematically determines the dynamic parameters from constitutive correlations. Using pair correlation functions from bottom-up-parametrized equilibrium structures, two preliminary simulations establish the reference constitutive correlations, which are then used within the composition-dependent calibration procedure to match experimental transport properties across varying solvent compositions. We validate this framework using an imidazolium-based IL system, which demonstrates quantitative agreement with experimental transport properties and captures negative viscosity deviations from ideal mixing. Our framework bridges molecular chemistry and macroscopic rheology and will facilitate the screening, design, and implementation of IL-based processes for energy- and separation-related systems.