Molecular dynamics study of pore inner wall modification effect in structure of water molecules confined in single-walled carbon nanotubes

The behavior of water molecules under nanoscale confinement has received considerable attention, especially for the influence caused by the modified groups of pores. To better design bionic nanodevices for future research, we anchored carboxyl acid (- COOH) groups onto the inner wall of a single-walled armchair carbon nanotube's (CNT's) central region to model the pore shape of aquaporin-1 and investigated the effect of modified groups on the structure of water molecules. The orientations and density distributions of water molecules in the CNTs and near the tube mouths have been studied by molecular dynamics simulation. The results indicate that water molecules confined inside the two unmodified regions have opposite and steady preferential dipole orientations pointing toward the - COOH groups on the central region of the CNT. Meanwhile the orientations of water molecules near the tube mouths which are certain distances away from the - COOH groups are also affected. This phenomenon becomes stronger as the number of - COOH groups increases and the CNT diameter decreases. In addition, the results show that the - COOH groups on the inner wall of the central region have a slight effect on the axial density distribution of the water molecules near the tube mouths, but a strong impact on that of water molecules inside the CNTs. Different distances between the - COOH groups and tube mouths can create diverse axial density distributions of water molecules.