Carbon nanotubes in water: Structural characteristics and energetics

We study the structural properties of water surrounding a carbon nanotube using molecular dynamics simulations. The interaction potentials involve a description of the carbon nanotube using Morse, harmonic bending, torsion, and Lennard-Jones potentials. The water is described by the flexible Simple Point Charge (SPC) model by Teleman et al.,(1) and the carbon-water interactions include a carbon-oxygen Lennard-Jones potential, and an electrostatic quadrupole moment acting between the carbon atoms and the charge sites of the water. Vibration of the breathing mode of the carbon nanotube in water is inferred from the oscillations in carbon-carbon van der Waals energy, and the inverse proportionality between the radius of the carbon nanotube and the breathing frequency is in good agreement with experimental values. The results indicate, that under the present conditions, the presence of the water has a negligible influence on the breathing frequency. The water at the carbon-water interface is found to have a HOH plane nearly tangential to the interface, and the water radial density profile exhibits the characteristic layering also found in the graphite-water system. The average number of hydrogen bonds decreases from a value of 3.73 in the bulk phase to a value of 2.89 at the carbon-water interface. The inclusion of the carbon quadrupole moment is found to have a negligible influence on the structural properties of the water. Energy changes that occur by the process of introducing a carbon nanotube in water are calculated. The creation of a cavity in the bulk water to accommodate the nanotube constitutes the largest energy contribution. Results include an analysis of surface structure and energy values for planar and for concave cylindrical surfaces of water.