Computer Simulations of Water and Methanol in Carbon Nanotube

Confinement and transport of fluids through nanopores is a subject of great interest. It has been observed that the confined fluids can have a greater degree of order compared to the bulk. Here we report molecular dynamics simulations of the filling and emptying of water and methanol inside carbon nanotubes of varying length and diameter. First we study the filling and emptying kinetics of water in a long (6,6) carbon nanotube open to water reservoir. A switching parameter is introduced to tune the nanotube-water attractive interactions. Filling of the nanotube is extremely sensitive to the strength of water-carbon potential. The nanotube can be made to switch continuously back and forth between filled and empty states as well as a completely filled or a completely empty state by fine tuning the nanotube-water interaction potential. The time dependence of filling and emptying follows a simple kinetic scheme. The transition time to empty, once initiated, is greater for the long nanotube. These results have implications on using polarity changes to drive filling and emptying transitions in nanotube devices. Second, we investigate the thermodynamics of filling of a short (6,6) open carbon nanotube solvated in water. The energy and entropy of transfer are determined from simulations at temperatures ranging between 280K and 320K. Results are compared with previous calculations for a periodically replicated nanotube in vacuum of same length and diameter containing different numbers of water molecules. The differences in behavior of the two types of nanotubes can be traced to the lower energy and entropy of transfer of water into the open nanotube which are finely balanced and to the differences in the end effects. Finally we extend our studies to simulations of liquid methanol in carbon nanotubes of three different pore sizes. Although methanol is a larger molecule, it easily enters the hydrophobic nanotube from the bulk. The H-bonded wire observed in water inside nanotube is not seen in case of methanol. Also, the methanol filled nanotubes did not show complete emptying for the same range of the switching parameter used to tune the nanotube-water interactions. This may be useful in separating water-methanol mixtures