Molecular Dynamics Simulations of CO₂ at an Ionic Liquid Interface: Adsorption, Ordering, and Interfacial Crossing

The ionic liquid−CO2 system is of interest because ionic liquids have potential to be used for CO2 capture. Using classical molecular dynamics simulations, the vacuum−liquid and CO2 gas−liquid interfaces of the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) have been studied at a range of temperatures and pressures. Interfacial ordering and orientational tendencies of the ionic liquid at the vacuum interface generally agree with previous experimental and simulation studies. The interfacial structure of the IL remains relatively unperturbed when the liquid is in contact with CO2. CO2 adsorbs rapidly onto the liquid interface, forming a dense layer. Diffusion into the bulk occurs on a much slower time scale. Interfacial fluxes and diffusivities were determined. The potential of mean force for interfacial crossing and corresponding residence time distributions of interfacial crossing events of CO2 were also calculated. CO2 desorption from the liquid was also simulated. A high density CO2 layer forms for desorption with CO2 present but not for desorption into a vacuum. The interfacial behavior and transport dynamics have been characterized by studying these properties.