(247b) Interfacial Structure of Ionic Liquids Studied with Coarse-Grained Molecular Dynamics

The increased attention and utilization that ionic liquids have gained in the last couple of decades has been fueled not only by realization of their useful and unique physical and chemical properties, but also by a relatively new perspective in their analysis of recent years. All-atom molecular dynamics (MD) simulations, which expose these fluids’ customizability for use in vast technical applications, have adapted to produce coarse-grained (CG) simulation models for more computationally efficient computation of various systems. By reducing the mechanical degrees of freedom, these CG models make possible the simulation of larger time and length scales, enabling observation of complex structural behavior present in systems such as those of ionic liquids. However, even well-constructed CG models will not necessarily produce correct dynamic behavior, because reduction in degrees of freedom reduces “effective friction” between CG beads. To address this issue, the more recently developed probability distribution function coarse-graining (PDF-CG) method has been implemented. In this method, effective friction is reintroduced into the CG system through the stochastic Langevin equation. With the proposed approach, accurate prediction of structure and dynamics for CG systems like ionic liquids has been observed. This model can provide a key component in understanding ionic liquid behavior at various interfaces and chemical compositions, bridging the gap between the fluids’ impressive properties and development of next generation technology. The present work details application of the PDF-CG method to simulate two sets of ionic liquids: (a) N-methyl-N-butylpyrrolidinium bis(trifluoromethylsulfonyl)-imide ([pyr14][TFSI]), and (b) 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF₄]). First outlined is the development and validation of CG models, in which CG force fields were successfully obtained to produce correct structure and dynamics of the CG system in reference to all-atom MD simulations for both liquids. The system size was then increased by one order of magnitude. The developed CG models for both sets of ionic liquids were used to perform CG-MD simulations at liquid-vacuum interfaces to study inhomogeneous interfacial structure and density profiles that have been experimentally observed.