Water molecules in CNT-SiN membrane: Properties and the separation effect for water-alcohol solution.

Water confined in carbon nanotubes (CNTs) has been intensively studied because of its unique properties and potential for various applications and is often embedded in silicon nitride (SiN) membranes. However, the understanding of the influence of SiN on the properties of water in CNTs lacks clarity. In this study, we performed molecular dynamics simulations to investigate the effect of the SiN membrane on water molecules inside CNTs. The internal electric field generated in the CNTs by the point charges of the SiN membrane changes the structure and dynamical properties of water in the nanotubes, causing it to attain a disordered structure. The SiN membrane decreases the diffusivity of water in the CNTs; this is because the Coulomb potential energy (i.e., electrostatic interaction) of water decreases owing to the presence of SiN, whereas the Lennard-Jones potential energy (i.e., van der Waals interaction) does not change significantly. Furthermore, electrostatic interactions make the water structure more stable in the CNTs. As a result, the SiN membrane enhances the separation effect of the water-methanol mixture with CNTs in the presence of an external electric field. Furthermore, the threshold of the external electric field strength to induce water-methanol separation with CNTs is reduced owing to the presence of a silicon nitride membrane.