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The transition from single-file diffusion to Fickian diffusion in narrow cylindrical pores is investigated for systems of rigid single-walled armchair carbon nanotubes, solvated with binary mixtures of Lennard-Jones fluids (Ar/Ne, Ar/Kr, and Ar/Xe). A range of effects is examined including the mixture concentration, the size ratio of the two components, and the nanotube diameter. The transition from single-file to Fickian diffusion in varying carbon nanotube diameters is analyzed in terms of the Fickian self-diffusivity and the single-file mobility of the mixture components. It is found that the single-file to Fickian carbon nanotube transition diameter is a unique property of the individual molecule’s diameter and remains unchanged regardless of the mixture composition. In applications of binary mixtures, each component may crossover from single-file to Fickian diffusion in a different carbon nanotube diameter, giving rise to bimodal diffusion in some nanotubes. This transition allows for one species to diffuse in single-file while the other diffuses by a Fickian mechanism, yielding orders of magnitude difference between the self-diffusional rates of the two molecules. This phenomenon might be further extended to alter the diffusional motion of molecules in nanoporous materials. [ABSTRACT FROM AUTHOR]

The self-diffusion coefficients were calculated by molecular dynamics simulations and the effects of pore width, temperature, and fluid density on diffusion behavior of simple fluid argon and polar fluid water confined in micropores were analyzed and studied. A mathematical model describing diffusion behavior of fluids confined in micropores was proposed from the theories of molecular dynamics and molecular kinematics, and validated on the basis of the simulation results at various conditions. The model indicates that the diffusion coefficient is proportional to the square root of the pore width and to the temperature divided by the density squared. It is applicable to either liquid or gas states and only two parameters are required.© 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The density profiles and the diffusion behavior of fluid argon confined in micropores were studied by molecular-dynamics simulations. The effects of pore size (width0, temperature and number density on the density profiles and the self-diffusion coefficients in micropores were simulated with pore widths from 0.6 to 4.0 nm. The density profiles are greatly affected by the pore size. Strong inhomogeneities in the channel direction and vapor­liquid phase separation in the micropores were observed when initial conditions were chosen in the coexistence region of the fluid. The self-diffusion coefficient in the channel direction in the pores was found to be much lower than in the bulk, and decreasing with decreasing pore size, decreasing temperature, and increasing density. [ABSTRACT FROM AUTHOR]

Nanopores can serve as a molecule channel for transport of fluid, where water diffusion differs remarkably from that of simple particles. Hydrogen bonds play an essential role in the diffusion anomaly. Detailed investigations are carried out on the systems of rigid (6, 6), (7, 7), (8, 8), (9, 9), and (10, 10) armchair carbon nanotubes, solvated with Lennard-Jones water fluids. The role of hydrogen bonds is examined by diffusivity statistics and animation snapshots. It is found that in small (6,6) CNT, hydrogen bonds tend to aggregate water into a wire and lead to rapid collective drift. Confinement can stabilize the hydrogen bond of water molecules and enhance its lifetime. In relatively smaller CNTs, the diffusion mechanism could be altered by the temperature. Moreover, in larger nanotubes hydrogen bonding network allows the water to form regional concentrated clusters. This allows water fluid in extremely low density exhibit rather slow self-diffusion motion. This fundamental study attempts to provide insights in understanding nanoscale delivery system in aqueous solution.

This work focuses on the dynamic properties and energetics of the protein/peptide drug during its transport through carbon nanotubes (CNTs). A systematic study was performed on the interaction between the peptide and the CNTs. In the molecular dynamics (MD) simulations, the protein/peptide molecule Zadaxin is observed to be encapsulated inside the nanotube after its spontaneous insertion and oscillates around the center of the tube, where the van der Waals interaction energy is observed to be a minimum. Furthermore, it is found by performing steered MD simulations that the pulling force applied to the peptide reaches a maximum value, which demonstrates the ability of the CNTs to trap protein/peptide drugs. Such effects, attributed to van der Waals interactions, can be influenced by varying the lengths and diameters of the CNTs. Longer nanotubes provide a broader area to trap the peptide, while smaller nanotubes are able to encapsulate the peptide with a deeper interaction energy well. This investigation provides insights into nanoscale pharmaceutical drug delivery devices.

The dynamic behaviors of drug zadaxin getting into carbon nanotubes (CNTs) in different water surroundings were investigated by molecular dynamics simulation. It was found that the diameter (1.9 nm) of (14, 14) CNT is the critical size for inserting zadaxin into CNT at the present conditions. In addition, the length of CNTs is another factor for inserting. A certain length is needed. It implies that interactions of zadaxin with both the CNT and the water molecules are competitive in the insertion process. The CNT-zadaxin attractive interaction is found to be the main driving force with the lower density of water molecules in the surroundings, while the zadaxin-water interaction becomes dominant with the higher density. The study of the authors suggests that biomolecules-CNT systems can be further exploited for the potential applications to drugs, vaccines, and gene delivery.

The structural organization, the number of hydrogen bonds (H bond), and the self- and mutual diffusion coefficients of ethanol-water mixtures were studied by molecular dynamics simulation. It was found that both the numbers of H bonds per water and per ethanol decrease as the mole fraction of ethanol increases. The composition dependences and the relationships between the self- and the mutual diffusion coefficients were further discussed. The self-diffusion coefficient of water has a large drop as the concentration of ethanol increases from 0 to 0.3 and then it nearly keeps constant, while that of ethanol has a minimum around ethanol mole fraction of 0.5. The mutual diffusion coefficient could be divided into two parts, the kinematic factor and the thermodynamic factor. Both the kinematic and thermodynamic factors for ethanol-water mixtures were calculated. It was found that the change trend of mutual diffusion coefficients with the composition is mainly dependent on the thermodynamic factors.

The self- and mutual diffusion coefficients for binary mixtures of Ar-Kr both in the bulk and in the nanopores were studied by molecular dynamics simulations. The composition dependences and the relationships between the self- and the mutual diffusion coefficients both in the bulk and in the nanopores were further discussed. It was found that the simulation results (D(c.m.)) are close to the calculated ones (D(s)) for the Ar-Kr system. Both self- and mutual diffusion coefficients in nanopores are much lower than that of the bulk, and they ever decrease as the pore width decreases. Nevertheless, the self- and mutual diffusion coefficients increase as the mole fraction of Ar increases, and as expected, increase as the temperature increases. The self-diffusion coefficients of mixtures both in the bulk and in the nanopores are predicted by the Carman model and by the molecular cluster model.