Charged nanoporous phosphorene as a water desalination membrane: insights from molecular dynamics

Abstract

Phosphorene, due to its remarkable properties such as self-passivation, stability, and anti-fouling, makes it a promising material for desalination membranes. Practically, these membranes acquire charges and affect the salt rejection and water flux. In this article, water desalination performance through positively charged (PC), negatively charged (NC), and charged but overall neutral (CN) single-layer nanoporous phosphorene (NPP) membrane of nanopore size ∼41 Å2 is investigated using pressure-driven molecular dynamics simulations. It is observed that the electrostatic interactions due to the distribution of charge around the nanopore edges strongly affect the desalination performance rather than steric hindrance. Overall, with an equivalent magnitude of total applied charge, the water flux through the CN membrane is more than PC and NC membranes. A membrane best suited for desalination performance among the charged NPP membranes is a CN membrane due to its high flux and adequate salt rejection, though it allows the passage of both ions. Comparatively, a PC or NC membrane has lower flux and allows the course of their counter ions respectively. To construe this observation salt ion density maps and molar concentration profiles are further examined. The degree of localization of counter ions around the nanopore edge increases with the increased total applied charge. While no such localization is observed for the CN membranes. PC and NC membranes provide more energetic barriers to co-ions due to strong coulombic repulsions and molecular layering of the adsorbed water, which hinder their transport. This study suggests the design of charged phosphorene membranes to maximize water transport while still maintaining the salt rejection potential.