(178l) Ionic Liquids Confined in Nanoporous Materials: Effects of Pore Size, Morphology and Material

Ionic liquids (ILs) have attracted extensive attention due to their unique properties and possible applications ranging from green solvents for chemical synthesis and separations to electrolytes for electrochemistry, among others. When confined inside nanoporous materials, ILs can show different physical properties from those observed in bulk systems (e.g., drastic changes in melting point and crystalline structures). Understanding the behavior of ILs inside nanopores is relevant to potential applications of these systems in electrochemical double layer capacitors (EDLCs) and dye sensitized solar cells (DSSCs). Such a fundamental understanding is also crucial to optimize the synthesis of hard-templated 1D nanostructures (nanorods, nanotubes, nanowires) based on organic salts, which may be imparted properties (e.g., magnetic, optical) that are desirable for different applications (magnetic hyperthermia cancer treatment, medical imaging, sensors).

In this work we have used molecular dynamics (MD) simulations to investigate systems of  representative ILs, [BMIM⁺][PF₆^-] and [EMIM⁺][TFMSI^-], inside several model materials (e.g., slit-shaped graphitic and titania pores, carbon nanotubes). We aimed at understanding how the structural and dynamical properties of the ILs depend on variables such as pore size and shape, porous material, and amount of IL inside the pore. Formation of different layers of ions was observed for the confined ILs irrespective of variations in pore size, shape and material. In all cases, change in pore loading leads to lower densities of ions in the center of the pore. The cations close to the pore walls tend to align with their imidazolium rings parallel to the pore surface in the case of carbon materials, and tilted to the surface in the case of rutile (110) pores. For all porous materials studied, the dynamics of the ions depend strongly on their location with respect to the surface; bulk-like dynamics are generally observed for the ions in the center regions of the pore, with the dynamics becoming slower as the ions get closer to the pore surfaces.