Confinement Effects on Water Clusters Inside Carbon Nanotubes

Abstract

The effects of confinement on water clusters inside nonmetallic carbon nanotubes with radii ranging between 4 and 7.5 Å have been computationally investigated by means of global optimization and finite temperature simulations. The water−water interaction is described by the TIP4P rigid body potential, and a Lennard-Jones potential is used for the water− carbon interaction. Water clusters containing up to 20 molecules are found to form 1D chainlike configurations for the narrow (7, 5) nanotube and 2D ladderlike structures in the (7, 6) tube. In wider tubes, 3D configurations are then formed showing helical motifs, ringlike or closed cage structures, before the most stable structure on flat graphene is eventually found. The same results are obtained by replacing the fully atomistic water−nanotube potential by its continuous approximation [Bretoń, J.; Gonzaĺez-Platas, J.; Giradet, C. J. Chem. Phys. 1994, 101, 3334], indicating a negligible effect of corrugation. The effects of additional nanotubes were also considered with the adsorption energies being found to converge rather quickly already for the triple-wall tube. Parallel tempering Monte Carlo simulations of the water octamer reveal a counterintuitive decrease in the melting point relative to the free-standing case. Molecular dynamics simulations show that melting is concomitant with some axial diffusion of the water molecules, and with radial diffusion perpendicular to the tube axis remaining limited. In accordance with previous studies concerned with bulk water, the weakening of the cluster thermal stability is interpreted as being caused by the hydrophobic character of the carbon−water interaction.

Adsorption of molecular hydrogen on coronene with a new potential energy surface

“…Global minima were obtained using the BH 62 or Monte Carlo (MC) minimization 63 method, successfully employed before for both neutral 62,64,65 and charged atomic and molecular clusters [66][67][68][69][70][71] , along with many other applications. 72 This technique is a stochastic method that involves perturbation followed by minimization at each step.…”

Examination of the Feynman–Hibbs Approach in the Study of NeN-Coronene Clusters at Low Temperatures

“…This technique has been used successfully for both neutral [32,39,40] and charged atomic and molecular clusters [41][42][43][44][45][46], along with many other applications. [47] In the size range considered here the global optimization problem is feasible at a reasonable computational cost.…”

Water Clusters on Graphitic Carbon Surfaces

“…Particular attention was paid to the signatures in these vibrational spectra and the important role of hydrogen bonds in determining the structural, electronic, and spectral properties of INTs. Figure 6 depicts the optimized pentagonal, hexagonal, and heptagonal INTs with confinement in the cylindrical hollow spaces of zigzag (15, 0), (16,0), and (17, 0) SWCNTs. Our calculations shown that water molecules can form ordered cylindrical crystalline structures, referred to as INTs, by formation of perfect hydrogen bonding under confinement within SWCNTs.…”

“…When water is adsorbed on the substrates, it will minimize its energy by adopting a specific structure to that surface, and that the geometry is dependent on the relative strength of the water-surface interaction and the water-water interaction. In the past two decades, the interactions of water molecules with various graphitic materials, including graphene, carbon nanotubes, and graphite, have attracted wide spread attention among experimental and computational chemists [4][5][6][7][8][9][10][11][12][13][14][15][16]. The understanding of the water interactions with graphenes or single walled carbon nanotubes (SWCNTs) is very important for potential applications of these materials.…”