Melting Transition of Ionic Liquid [bmim][PF6] Crystal Confined in Nanopores: A Molecular Dynamics Simulation

In this paper, the melting process of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) crystals confined in carbon nanotubes were investigated using molecular dynamics simulation. The confined ions formed a "shell-chain" structure within the nanopore. Our results revealed that this "shell-chain" structure possessed long-range crystalline order at low ternperature and initiated melting at approximately 500 K, which well fit with experimental observations of ionic liquids in carbon nanotubes. This melting process was also confirmed by the potential energy profile of the confined ions. It was found that below 500 K, the ions within the nanopores were almost frozen around their positions. As the temperature increased above 500 K, the average number of hydrogen bonds for each confined anion began to decline linearly. This decline led to a dramatic change in the packing arrangement of the confined ions, followed by a steep rise in the ionic diffusivity.