Structure of Quaternary Ammonium Ionic Liquids at Interfaces: Effects of Cation Tail Modification with Isoelectronic Groups

Herein we use molecular dynamics simulations to investigate the effects of cation tail modification with isoelectronic groups on interfacial structure of room-temperature ionic liquids (RTILs) when confined between two oppositely charged or two neutral graphene sheets. The RTILs chosen in this regard are triethyloctylammonium bis(trifluoromethylsulfonyl)imide (N-2228(+)/NTf2-) and (2-ethoxyethoxy)ethyltriethylammonium bis-(trifluoromethylsulfonyl)imide (N-222(2O2O2)(+)/NTf2-). For all the systems studied, we determined number density, free energy, electric field and electrostatic potential profiles along the axis normal to graphene sheet plane. Our results predict that in both the systems, the positively charged graphene sheets are screened predominantly by anionic atoms and cationic tail groups that are embedded in the cavities formed by the anions in the interfacial region, and the extent of screening for the RTIL containing alkyl-substituted cations is slightly higher than that in the diether-substituted analog. Near the negatively charged sheet, while the probability of finding cation tails is relatively enhanced in the diether system, increased density augmentation of cation head groups in the N-2228(+) system is observed in comparison with that in the N-222(2O2O2)(+) system. We observe both perpendicular and parallel orientations of cation tail near the negatively charged wall, but it appears that the probability of parallel orientation for N-222(2O2O2)(+) is higher than that for N-2228(+). In the case of neutral sheets, the anionic distribution at the interface is similar in both the cases and cation tail enrichment in the first layer close to the neutral wall is increased in the system with a non-polar tail. The probability of finding cation head groups in the vicinity of neutral wall is slightly larger in the case of diether system. The simulated zone-resolved tangential radial distribution functions (TRDFs) in planar slabs parallel to the sheets show that the cation heads (or anions) in the interfacial region closest to the negatively (or positively) charged sheet are ordered more than that in the corresponding bulk liquid. For both the isoelectronic analogs, the overall charge density fluctuations and electric field oscillations near the positively charged walls are larger than that near the negatively charged walls.