Bulk andLiquid–Vapor Interface of Pyrrolidinium-BasedIonic Liquids: A Molecular Simulation Study.

Using molecular dynamics simulations,we have studied the structureof three 1-butyl-1-methylpyrrolidinium ionic liquids whose anionsare triflate, bis(trifluoromethanesulfonyl)imide, and tris(pentafluoroethyl)trifluorophosphate.The structure of the bulk phase of the three ionic liquids has beeninterpreted using radial and spatial distribution functions and structurefactors that allows us to characterize the morphology of the polarand nonpolar domains present in this family of liquids. The size ofthe polar regions depends on the anion size, whereas the morphologyof the nonpolar domains is anion-independent. Furthermore, the surfaceordering properties of the ionic liquids and charge and density profileswere also studied using molecular simulations. The surface tensionof the liquid–vapor interfaces of these ionic liquids was alsopredicted from our molecular simulations. In addition, microscopicstructural analysis of orientational ordering at the interface anddensity profiles along the direction normal to the interface suggestthat the alkyl chains of the cation tend to protrude toward the vacuum,and the presence of the interface leads to a strong organization ofthe liquid phase in the region close to the interface. In the interfacialarea, the polar regions of the ionic liquids are more structured thanthose in the bulk phase, whereas the opposite behavior is observedfor the nonpolar regions. [ABSTRACT FROM AUTHOR]