(590f) Molecular Dynamics Simulations of CO2 at An Ionic Liquid Interface: Adsorption, Ordering and Interfacial Crossing

Using classical molecular dynamics simulation techniques, the molecular scale characteristics of the gas-liquid interface of the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluromethylsulfonly)amide ([bmim][Tf2N]) have been studied. The vacuum-liquid surface of the ionic liquid, and the gas liquid interface of the ionic liquid with carbon dioxide, were simulated at a range of temperatures and pressures. The process of carbon dioxide adsorption on the liquid surface and absorption into the bulk liquid was quantified in terms of flux and concentration difference across the interface. Adsorption onto the interface occurs rapidly, in approximately 100 ps, but interfacial crossing and diffusion into the bulk is much slower. At the lower pressures the fraction of CO2 molecules adsorbed is very high, but as the pressure increases a monolayer is formed and a greater fraction of the CO2 molecules stay in the gas phase.

Density profiles with respect to the interfacial normal were computed for each atom in the ionic liquid and CO2. The dependence of the density profiles on the carbon dioxide pressure was evident from comparison of the simulations at different pressures. Interfacial ordering and orientational tendencies were indicated by the relative densities of different atoms as a function of position on the interface normal.

The distribution of angular orientation of selected bond directions, at regular points along the interface normal, was also calculated. This analysis was performed both for the ionic liquid molecules and the CO2 molecules. For the CO2 molecule orientations, a dependence on CO2 pressure is noted.

Residence time distributions of CO2 molecules in the interfacial region were created. It was observed that the rate at which molecules penetrate the surface, relative to the rate at which molecules desorb, is affected by the temperature. The analysis of the residence time distribution seems to indicate that adsorption and desorption occurs on a rapid timescale relative to diffusion through the interface. The time evolution of the density profile of CO2 was studied as it diffused across the interface over the 7 ns time period. Potentials of mean force were also computed for CO2 penetration into the liquid.