(184f) A Computational Study of the Behavior of Ionic Liquids Confined Inside Multi-Walled Carbon Nanotubes

Ionic liquids (ILs) have attracted extensive attention in recent years. When confined inside nanoporous materials, ILs can show completely different physical properties from those in bulk systems (e.g., drastic changes in melting point and crystalline structures). In this work we have used molecular simulation to investigate systems of ILs inside multi-walled carbon nanotubes (MWCNTs). Our goal is to understand how several properties of the ILs (e.g. mass and electron density, structure and orientation, and diffusivity) depend on the size of the pores and interactions between the ILs and the pore walls. Such a fundamental understanding will be crucial to optimize the synthesis of hard-templated 1D nanostructures (nanorods, nanotubes, nanowires) based on organic salts, which may have properties (e.g., magnetic, optical) that are desirable for different applications (magnetic hyperthermia cancer treatment, medical imaging, sensors, organic light emitting diodes, photovoltaics). The macroscopic properties (magnetic, optical) of these nanostructures are affected by the molecular-level properties of the organic salts inside the nanoporous templates, which can be readily determined from our molecular simulations.

We have studied the IL [BMIM+][PF6-] confined inside MWCNTs with diameters ranging between 1.5 and 3.7 nm, using molecular dynamics (MD) simulations in the NVT ensemble. The IL was modeled using the force field developed by Lopes et al [J. Phys. Chem. B 2006]. We found that the nanotube diameter influences the stacking, distribution and structure of the confined IL. The radial profiles of the mass and electron densities of the IL are oscillatory, with a maximum close to the surface. We also observed that the imidazolium ring of the cation lies approximately flat near the wall due to a very strong attraction between the ring and wall carbon atoms. Results for the dynamics of our systems indicate that the displacement of the atoms in the axial direction is less than the size of the ions, suggesting that there is limited diffusion in the axial direction. We also studied the effect of pore loading on the structural and dynamical properties of the ILs. MWCNTs partially filled with IL exhibit regions of low density in the center of the pore; in contrast, close to the nanotube surface the density is similar to that observed when the pore is completely filled. The dynamics of the ions are strongly influenced by the pore loading, with slower dynamics observed as the pore loading increases.