Investigation of the melting of ionic liquid [emim][PF6] confined inside carbon nanotubes using molecular dynamics simulations

Abstract

We have performed molecular dynamics (MD) simulations to study the thermodynamics, structure, and dynamical behavior of 1-ethyl-3-methylimidazolium hexafluorophosphate [emim][PF₆] during the melting process inside carbon nanotubes (CNTs) with different radii. Our structural results indicate the layering behavior for the IL inside the zigzag and armchair CNTs with the local maximums in the densities close to the CNT walls. The sharp and intense peaks are also observed at low temperature. However, as the temperature increases, the peaks related to the layer structure near the CNT wall become broader and less intense. It is also shown that the average number of hydrogen bonds decreases as the temperature increases. The internal energy also increases slightly with increasing temperature, followed by a dramatic jump. The temperature at this jump merely corresponds to the starting melting temperature, and another turning point in the energy curve corresponds to the entirely molten temperature, observed to be nearly 500 and 900 K, respectively. It is also shown that the diffusion coefficient increases as the temperature increases and a dramatic increase in the diffusion of the IL molecules occurs at the same temperature as the jump in total energy and thus marks the melting transition without ambiguity.