Probing the tricationic ionic liquid/vacuum interface: insights from molecular dynamics simulations

Abstract

The surface properties of three symmetric linear tricationic ionic liquids (LTILs) with the common anion, bis(trifluoromethylsulfonyl)imide ([NTf₂]⁻), were studied using atomistic molecular dynamics simulation and identification of the truly interfacial molecules (ITIM) analysis. A refined version of the CL&P force field with the partial charges based on quantum calculations for isolated ion quartets was used to calculate the number densities, orientation of the cations, charge densities and surface tensions. The densities obtained from the simulation of the interface are on average 3% smaller than the densities of bulk NPT simulations, which is due to applying long-range corrections in the simulations of bulk liquids. New observations were reported for this new class of ILs through density profiles and orientational analysis. The ITIM analysis shows that the surface of the LTILs is more populated with anions rather than cations and it becomes smoother with a decrease in the alkyl chain length of the cations. The microscopic structural analysis of the orientational ordering at the interface shows that although for LTIL-1 and LTIL-2, the surface is more populated with anions and therefore has a negative charge, for LTIL-3 the surface is more populated with linkage alkyl chains and therefore has a small positive charge. This difference in the interfacial structures arises from the difference in the alkyl chain lengths. The results show that the LTILs with shorter alkyl chains (i.e. n = 3 and 6) form an inverse-arc shape structure while LTILs with longer alkyl chains (i.e. n = 10) form a sinuous like structure at the interface. The surface tension values of these ILs were calculated at 298 K using the mechanical definition. The simulations resulted in acceptable values for surface tension compared to the experimental trends.