On the molecular mechanisms of H2/N2 uptake in confined ionic liquids: A computational study

Classical molecular dynamics and hybrid grand canonical Monte Carlo/molecular dynamics simulations were combined to analyze the gas uptake mechanism of hydrogen and nitrogen molecules inside carbon nanotubes filled with an ionic liquid. Several nanotube diameters (from 6 & Aring; to 12.24 & Aring;) and two different ionic liquids (ethylammonium nitrate and 1-ethyl-3-methylimidazolium tetrafluoroborate) were considered to study their effect on the gas capture capacity and on the location of gas molecules within the nanotubes. The simulations showed that nitrogen absorption ability is, in general, greater than that of hydrogen, with the aprotic ionic liquid being more efficient for gas confinement. In addition, gas capture was observed to increase from a scarce 0.4% in bulk ionic liquids up to 8%-25% inside small nanotubes, and the maximum gas uptake was observed for those nanotubes that allow for a greater degree of conformational freedom of the ionic liquid. However, our calculations show that, whereas hydrogen storage is mainly governed by the amount of accessible free volume, for understanding that of nitrogen solvation energetics must be also considered. In all cases, gas molecules are absorbed in the ionic liquid-rich-region, but the interactions with the other components of the system favour their accommodation closer to the carbon wall than to the nanotube centre. Finally, single-particle dynamics of gas molecules was analyzed by means of the velocity autocorrelation functions and the vibrational density of states, which show a blue-shifting when increasing the radius of the nanotube.