The nanoconfined water is nowadays an important subject in chemical engineering processes. With the advance of nanotechnology, more and more nanoporous material is applied in the relevant chemical processes, such as the clearance of brackish water, the nanoparticle production and the desalination of seawater. The adsorption and distribution of water inside these materials is crucial for the function of these processes. It is noted that the water inside the nanoscale confinement would behave different from their bulk counterpart, and the wall-water interactions are considered to play a critical role in such anomalous behavior of water. The chemical properties of the nanoprous material, including their chemical patterns and charge distribution, determine the interactions of wall-water, which can be divided into two main types: the Van der Waals interactions and the electrostatic interactions. The latter is mainly determined by the polarization of the material and the former is mainly derived from the non-polar chemical properties of the material. Therefore, it is paramount to have an insight on their respective role in the adsorption and behavior of water inside nanotube, especially in the narrow pore. This can be set as step stone for the design and assessment of new functional material and their application in the chemical processes.

In this work, we performed a series of classical molecular dynamics simulations to investigate the filling and structure of water in the cylindrical nanotube with diameter of 0.81 nm under ambient conditions. The non-electrostatic wall-water interactions were tuned to “hydrophobic” and “normal” Carbon-Oxygen potentials. Four types of partial charges for the wall atoms were investigated, including -1.0e, -0.5e, zero, 0.5e and 1.0e. The simulation results show that the pore with “normal” potentials quickly filled with water, and a single-file water chain is formed in the pore. The partial charge of the tube has minute effect to the filling and structure of water in this “normal” nanotube. No filling of water is observed in the neutral nanotube with “hydrophobic” potentials. The enhancement of the electrostatic interactions, with addition of the partial charge of the wall, could only make water fill the area of tube mouth, whereas the central part of the tube is still not filled. This suggests that, in the narrow nanotube, the non-electrostatic wall-water interactions determine the filling and structure of water inside pore other than the wall polarization. The electrostatic interaction may benefit the adsorption of water inside those hydrophobic narrow pores. However, such effect may be short-rang. As to the pore with larger length, the electrostatic-induced filling only happened near the pore-mouth.

Keywords: nanotube, simulation, confinement

Acknowledge: This work is supported by Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT 0732)