Molecular wire of urea in carbon nanotube: a molecular dynamics study

We perform molecular dynamics simulations of narrow single-walled carbon nanotubes (SWNTs) in aqueous urea to investigate the structure and dynamical behavior of urea molecules inside the SWNT. Even at low urea concentrations (e.g., 0.5 M), we have observed spontaneous and continuous filling of SWNT with a one-dimensional urea wire (leaving very few water molecules inside the SWNT). The urea wire is structurally ordered, both translationally and orientationally, with a contiguous hydrogen-bonded network and concerted urea's dipole orientations. Interestingly, despite the symmetric nature of the whole system, the potential energy profile of urea along the SWNT is asymmetric, arising from the ordering of asymmetric urea partial charge distribution (or dipole moment) in confined environment. Furthermore, we study the kinetics of confined urea and find that the permeation of urea molecules through the SWNT decreases significantly (by a factor of ∼20) compared to that of water molecules, due to the stronger dispersion interaction of urea with SWNT than water, and a maximum in urea permeation happens around a concentration of 5 M. These findings might shed some light on the better understanding of unique properties of molecular wires (particularly the wires formed by polar organic small molecules) confined within both artificial and biological nanochannels, and are expected to have practical applications such as the electronic devices for signal transduction and multiplication at the nanoscale.