Solvation of Carbon Nanotubes in a Room-Temperature Ionic Liquid

Single- and double-walled carbon nanotubes in the armchair configuration solvated in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4-) are studied via molecular dynamics (MD) computer simulations. Cations and anions show smeared-out, cylindrical shell-like distributions outside of the nanotubes irrespective of the nanotube diameter. The ion distributions inside the nanotubes vary markedly with their diameter. For example, in the case of (n,n) single-walled nanotubes, EMI+ and BF4- ions separately form single-shell zigzag and chiral distributions for (8,8) and (10,10), respectively, while (12,12) develops a second internal solvation structure. The first internal solvation shell of (15,15) nanotubes consists of alternating layers of cations and anions along the nanotube axis. In the azimuthal direction, these cations and anions, respectively, form a pentagonal structure, whereas the corresponding ions for (20,20) show disordered octagonal structures. The smallest nanotube that allows solvent ions inside the tunnel is (7,7) with a diameter of 0.95 nm, which shows a single file distribution of internal ions, Imidazole rings of cations in the first internal and external solvation shells are mainly parallel to the nanotube surface, indicating pi-stacking between the nanotubes and EMI+ ions there.