Exploring carbon nanotubes and their assemblies as molecular channels: Theory and molecular simulations

Carbon nanotubes (NTs) hold great promise as components in molecular-scale devices that could be used for a variety of applications such as chemical sensing, separations, and biomedical uses. Previous experimental and computer simulation studies have reported that water can wet the interior of an open-ended NT. Rigid cylindrical pores of NTs could thus be used as molecular channels in the design of novel nanofluidic devices. The primary goal of my research is to obtain a fundamental understanding of the filling and transport of molecular species in the quasi one-dimensional geometry of NT pores. We employ molecular dynamics (MD) simulations of a variety of systems involving NTs and their assemblies and also develop a molecular theory for fluid flow at a microscopic level.; As a first step, we study the mechanism of filling of open-ended NT pores with simple solutes, such as methane molecules. Thermodynamic, structural and kinetic aspects of pore filling are investigated. We also study single-file flow of water molecules through a membrane made of hexagonally-packed NTs in an osmotic setup; in which a salt concentration difference between the two ends of the NTs drives water flow. Contrary to a prediction of negligible flow from macroscopic hydrodynamics, we observe high flow rates of water through the NT. We find that macroscopic osmosis quantitatively describes the thermodynamics of the pump and, combined with the random-walk model, the hydrodynamics of the flow.; The second part of my research focuses on the effect of cosolvents and salts in biophysical systems. We have investigated thermodynamic and structural aspects of hydrophobic hydration in a variety of salt solutions. A range of behaviors from salting-in (increase in solubility) to salting-out (decrease in solubility) of hydrophobic solutes is observed depending on the size of salt ions. Changes in density of water in the vicinity of hydrophobic solutes is related with the thermodynamics of solute solubility. A fundamental level understanding of the effect of NaCl on hydrophobic interactions is provided and explained in terms of changes in water structure in the vicinity of hydrophobic solutes.