Ionic liquids (ILs) are organic salts that are in liquid state near room temperature. ILs have been proposed as alternative electrolytes for energy storage in electrochemical double-layer capacitors (EDLCs) . These devices have a structure similar to a battery, consisting of two carbon-based nanoporous electrodes, an electrolyte and an ion-permeable separator. Furthermore, inserting organic salts inside nm-sized cylindrical pores is one step in the synthesis of optically-active and magnetic 1D-nanostructures (nanorods, nanowires, nanotubes) based on organic salts [2-4]. These nanomaterials hold promise in biomedical applications ranging from magnetic hyperthermia cancer treatment to medical imaging.
A fundamental understanding of the behavior of organic salts inside nm-sized pores is crucial to optimize the previously mentioned applications. Here we report classical molecular dynamics (MD) simulation results of typical ILs (e.g., [bmim+][PF6-] and [dmim+][Cl-]) confined inside multi-walled carbon nanotubes of different diameters. Our results suggest that variables such as pore diameter and pore filling affect the structural properties of the confined IL (namely local density, layering behavior and orientation of the ions). We also discuss how the pore diameter and the density of the confined IL affect its dynamical properties (e.g., mean square displacements and single-particle time correlation functions of the ions). These molecular-level properties of the ILs determine relevant macroscopic properties of these systems (e.g., electrical capacitance and resistance, as well as magnetic and optical properties).
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