Extraordinarily fast transport of water in carbon nanotubes (CNTs) in recent experiments has been generally attributed to the smoothness of the CNT surface. Using molecular dynamics simulations we investigate water flow in (16,16) CNTs and show that the enhanced flow rates over Hagen-Poiseuille flow arise from a velocity "jump" in a depletion region at the water nanotube interface and that the water orientations and hydrogen bonding at the interface significantly affect the flow rates. For nanotube with the same smooth wall structure but with more hydrophilic Lennard-Jones (LJ) parameters of silicon, the enhancement is greatly reduced because it does not have "free" OH bonds pointing to the wall as in CNTs that would reduce the number of hydrogen bonds in the depletion layer. Roughness in the tube walls causes strong hydrogen-bonding network and no significant flow enhancement is attained in rough tubes.

Confinement can induce unusual behavior in the properties of matter. Using molecular dynamics simulations, we show here that water confined to carbon nanotubes of a critical size under ambient conditions (1 bar, 300 K) can undergo a transition into a state having icelike mobility with an amount of hydrogen bonding similar to that in liquid water. The onset of this behavior occurs rapidly, raising the possibility that confinement inside nanotubes, and perhaps even buckyballs, can provide an environment in which the dynamics of phase changes may be studied directly by simulation. Moreover, because of a variety of evidence suggesting that water ordering may modulate proton conductance via a "proton wire" hydrogen bonding network, the ability to modulate water ordering with geometry suggests a possible mechanism for a switchable nanoscale semiconductor.

We report the fabrication of devices in which one single-walled carbon nanotube (SWCNT) spans a barrier between two fluid reservoirs, enabling direct electrical measurement of ion transport through the tube.

Understanding the diffusion of small molecules in hydrogel system is of major importance in a variety of applications including drug delivery systems, tissue engineering and contact lens. Cross-linking density of hydrogels has been commonly used to tune key parameters like mesh size and molecular weight between cross-linkers, in order to change macroscopic properties of hydrogels. In this thesis, molecular dynamics investigations of chemically-cross-linked poly(ethylene glycol) (PEG) hydrogels are reported with the aim of exploring the diffusion properties of water, ions, and rhodamine within the polymer at the molecular level.The water structure and diffusion properties were studied at various cross-linking densities with molecular weights of the chains ranging from 572 to 3400. As the cross-linking density is increased, the water diffusion decreases and the slowdown in diffusion is more severe at the polymer-water interface. The water diffusion at various cross-linking densities is correlated with the water hydrogen bonding dynamics. The diffusion of ions and rhodamine also decreased as the cross-linking density is increased. The variation of diffusion coefficient with cross-linking density is related to the variation of water content at different cross-linking densities.Comparison of simulation results and obstruction scaling theory for hydrogels showed similar trends.ii To Father and Mother.iii