Asymmetric dipole distribution of single-file water molecules across nanochannels under high pressure gradient

The dipole distributions of water molecules across the channel of single-walled carbon nanotubes (SWCNTs) under high pressure gradient P are studied using molecular dynamics (MD) simulations. It is found that the profiles of the dipole distributions are highly symmetrical with two peak values around 30° and 150° at zero or low pressure gradient (P ≤ 6MPa), and remarkably asymmetric with the main peak around 30° under high pressure gradient (P ≥ 12MPa). The reason is mainly that molecules can enter into the SWCNTs from two ends under low pressure water, whereas the water molecules can only enter into the tube from one end under high pressure. Further, it also implies that water molecules prefer to retain the hydrogens forward when entering into and passing through the nanotube. Our observations may be instructive for future applications in nanometer scale fluid devices and biological channels.