Molecular dynamics simulations of EMI-BF4 in nanoporous carbon actuators

An artificial muscle composite material consisting of carbide derived carbon (CDC) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4) ionic liquid was modeled using molecular dynamics (MD) simulations, in order to determine the molecular structural rearrangements causing actuation. CDC was represented as separate curved graphene-like flakes with charges of +2, 0 or -2 on each flake, with 24-27 aromatic rings each. The charge distribution in the flakes was determined by PM6 semi-empirical optimization. The pore size distribution of CDC and the density of the material were comparable to experimental data. Molecular structure analysis revealed a preferential parallel orientation for the cations over the negatively charged CDC surfaces, while cationic rotations and reorientations could be observed for positively charged CDC. Changes in the pore occupancy for each ionic type were observed for pore sizes between 4 and 7 angstrom, which, together with the replacement of large cations with smaller anions, could explain the volume decrease in the anodes (and, vice versa, the volume increase in the cathodes) in this type of actuator.