Structural Evidence for the Ordered Crystallites of Ionic Liquid in Confined Carbon Nanotubes

Abstract

Ionic liquids (ILs) are a class of new green materials that have attracted extensive attention in recent decades. Many novel properties not evident under normal conditions may appear when ionic liquids are confined to a nanometer scale. As was observed in the experiment, an anomalous phase behavior from liquid to high melting point perfect crystal occurred when 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6 ]) ionic liquid was confined in a carbon nanotube. In this work, we performed molecular dynamics (MD) simulations for [bmim][PF 6 ] ionic liquid and provided direct structural evidence that the ionic crystallizes in a carbon nanotube. The ordered ionic arrangement in both the radial and the axial directions can be observed inside the channels of the CNTs to induce the form of crystallites. The ionic stacking and distributing can be determined by the sizes of the CNTs. Hydrogen bonds remain the dominant interactions between cations and anions when the ionic liquid enters into the CNT from the bulk phase. The free energies as the thermal driven forces were calculated, and it is found that it is very difficult for a single anion to enter into the channel of the CNT spontaneously. A more favorable way is through an ion-pair in which a cation "pulls" an anion to enter into the channel of the CNT together. It is predicted that other ionic liquids that possess similar structures, even including the pyridinium-based ionic liquids, can show higher melting points when confined in CNTs.

Thermal conductivity of liquid/carbon nanotube core-shell nanocomposites

“…We used this bulk value due to a lack of more detailed data in the literature for this particular IL, which should act as the foundation for our comparison. Additionally, no particular confinement effect is expected, since the tubular area in our CNT is considerably larger than that of a thin CNT, where the available space in the tube and the hydrogen bond length become comparable 38,39 . The thermal conductivity of the shell λshell was measured to be 28 W/mK and tot A is the total conducting surface of the composite expressed as:…”

Temperature-Induced Molecular Rearrangement of an Ionic Liquid Confined in Nanospaces: An in Situ X-ray Absorption Fine Structure Study

“…Wang 28 illustrated that the compression of the H-bonding between cation and anion in ILs increases the melting point of the nanoconfined IL. Dong et al 20 also performed MD simulation to investigate the ordered ion arrangement of [Bmim][PF 6 ] within the CNTs. The drastic increase in the melting point of C 6 mimBr revealed that the IL within the nanospace is more stable than IL in the bulk system.…”

Small Molecules and Peptides Inside Carbon Nanotubes: Impact of Nanoscale Confinement

“…Molecules confined inside nanoscale space such as narrow nanotubes or membrane proteins can form one-dimensional (1D) molecular wires, which have attracted intense interest recently because of their scientific importance and potential applications in nanotechnology [1,21,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56]. Among them, it is of particular interest in determining the structure and dynami-cal behavior of water wires [1,21,[40][41][42][43][44][45][46][47][48][49] which have been found to exist in narrow nanotubes [1,21,[40][41][42][46][47][48] and biological channels [43][44][45].…”

“…We have calculated the urea flow, defined as the total number of urea molecules per nanosecond that have entered from one end and leave the SWNT from the opposite side. Given that the biological urea channel dvUT [52] has a length of ~ 16 Å (the number of urea molecules accommodated in the selectivity filter is about 3), we chose the 144-carbon (6, 6) SWNT (13.5 Å in length) as the nanochannel, wherein the resulting urea wire also consists of ~ 3 urea molecules. To facilitate a direct comparison with water wire, we performed additional simulations for the SWNT immersed in pure water.…”