Methane Partitioning and Transport in Hydrated Carbon Nanotubes

Abstract

The well defined shape and size of carbon nanotubes (CNTs) makes them attractive candidates for theoretical and experimental studies of various nanoscopic phenomena such as protection and confinement of molecular species as well as transport of molecules through their interior pores. Here we investigate solute partitioning and transport using molecular dynamics simulations of CNTs in mixtures of hydrophobic solutes and water. The hydrophobic pores of CNTs provide a favorable environment for partitioning of hydrophobic solutes. We find that the transfer of a methane molecule from aqueous solution into the CNT interior is favored by about 16 kJ/mol of free energy. In 50 molecular dynamics simulations, we observe that methane molecules replace water molecules initially inside the nanotubes, and completely fill their interior channels over a nanosecond time scale. Once filled with methane molecules, the nanotubes are able to transport methane from one end to the other through successive methane uptake and release events at the tube ends. We estimate a net rate of transport of about 11 methane molecules per nanotube and nanosecond for a 1 mol/L methane concentration gradient. This concentration-corrected rate of methane transport even exceeds that of water through nanotubes (∼1 per nanosecond at a 1 mol/L osmotic gradient). These results have implications for the design of molecule-selective CNT devices that may act through mechanisms similar to those of biological transmembrane channels.

Simulation optimization of spherical non-polar guest recognition by deep-cavity cavitands

“…28 Similar to pocket-like hosts, carbon nanotubes exhibit a strong affinity for the adsorption of hydrophobic species into their interiors. 29,30 In an effort to better understand factors that determine hydrophobic guest selectivity by OA, we have conducted molecular simulations of cavitand host interactions with adamantane and a series of Lennard-Jones (LJ) guests to form 1:1 complexes in water. Adamantane is a nearly spherical hydrocarbon that snugly fits within cavitand pockets and its derivatives comprise a class of strongly binding compounds.…”

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

“…These results also have implications on understanding the urea-induced denaturation of proteins by providing further evidence of the potential existence of a "dry globule"-like transient state [65] during early stage of protein unfolding and the "direct interaction mechanism" whereby urea attacks protein directly via favorable dispersion interaction, rather than disrupts water structure as a "water breaker". In addition, this study points out the crucial role of dispersion interaction in the selective absorption of molecules inside hydrophobic nanopores [54][55][56], which might be important for nanoscience and nanotechnology.…”

“…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].…”

“…Water wires have many interesting properties, such as wavelike density distributions [1,46], rapid and concerted motions [1,40,43], orientation-ordered structures and collective flips [1,21,41,48], and excellent on-off gating behaviors [46,47]. In addition, it has been observed that the methane [56], methanol [54], and gas molecules (O 2 , H 2 , and CO 2 ) [55] preferentially bind to the interiors of narrow SWNT over water and form 1D molecular wires. Despite the above progress, the properties of molecular wires have not been fully understood, particularly for the molecular wires formed by larger polar organic molecules.…”

Two coarse-graining studies of stochastic models in molecular biology

“…In figure 1.1 we show a snapshot of a molecular dynamics simulation [54,14], conducted by Shekhar Garde's research group, of a buckyball with 4200 "simple point charge" surrounding water molecules. The computational details for the simulation are similar to those used in [20] for hydrated carbon nanotube simulations. Our main interest will be to obtain a useful statistical parameterization of the dynamics of the water molecules in this detailed molecular dynamics simulation.…”