Elucidating Interactions Between Ionic Liquids and Polycyclic Aromatic Hydrocarbons by Quantum Chemical Calculations

Abstract

Using quantum mechanical calculations performed at the density functional level of theory, the present study explores the binding energetics, orbital energies, and charge transfer behavior accompanying sorption of 12 different ionic liquids (ILs) onto 6 archetypal polyaromatic hydrocarbons (PAHs). The ILs were based on combinations of three different onium cations (i.e., 1-butyl-3-methylimidazolium, 1-butylpyridinium, 1-butyl-1-methylpyrrolidinium) paired with four common anions, that is, bromide, tetrafluoroborate, hexafluorophosphate, and bis­(trifluoromethylsulfonyl)­imide. In general, the size of the anion as well as interaction of the butyl side chain present on the cation with the paired anion exerted significant influence over the cation ring orientation with respect to the PAH surface. A smaller highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy band gap was observed for pyridinium-based ILs upon adsorption on the PAH surface in comparison to imidazolium and pyrrolidinium analogs, hinting at stronger interactions between PAHs and pyridinium ILs. Of the 12 ILs investigated, 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide displays the least favorable free energy of adsorption with PAHs whereas PAH interactions with 1-butyl-1-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide are the most favored thermodynamically. Charges determined from a Mulliken population analysis were consistent with charge transfer (CT) from the IL to the PAH. On the contrary, charges determined via electrostatic potential using the more reliable grid based analysis method (i.e., CHELPG) suggested the reverse direction of CT from the PAH to the IL. The direction of the CT occurring from the HOMO of the PAH to the LUMO of the IL, as shown by CHELPG analysis, is consistent with the physical location of the orbitals and the negative shift in the Fermi energy level observed for the IL–PAH complex. A more favorable enthalpy of adsorption for ILs onto a PAH is observed with an increase in the size of the PAH considered. The free energy of adsorption, however, does not change significantly with an increase in the PAH surface area. The adsorption of an IL on the PAH surface leads to a small change in the entropy of the adsorbate/adsorbent system. The thermochemistry computed at variable temperature indicates a significant increase in the free energy of adsorption (i.e., a less favorable adsorption) as temperature rises. Additionally, decomposition of the entropic contribution suggests a greater contribution from translational and rotational entropies upon cooling, again consistent with stronger association at lower temperatures. Overall, the thermochemical analyses suggest an entropically driven process of desorption of an IL from the PAH surface, generally leading to fairly weak interactions between ILs and ordinary PAHs under normal laboratory conditions