Computational Investigation on Water and Ion Transport in MoS₂ Nanoporous Membranes: Implications for Water Desalination

Abstract

A significant portion of current desalination techniques rely on porous membranes whose performance is highly dependent on the delicate trade-off between water permeability and ion rejection. At the nanoscale, water and salt transport are governed by the nanopore's geometry and charge distribution. In this Article, we mimicked the reverse osmosis process with MoS 2 nanoporous membranes using molecular dynamics simulations to shed light on how water and ion transport phenomena influence each other and how they are affected by the nanopore's size and charge distribution. We evaluated the system's water flow rate and salt rejection under real and artificially induced pore charge polarizations and different diameters. By manipulating the pore's partial charges while maintaining a fixed geometry, we were able to separate electrostatic contributions from those dependent on pore size. As expected, we found that an increase in the charge polarization of MoS 2 leads to a higher presence of ions inside the nanopores and a lowering of water permeability. We went further by quantifying this behavior and observed a high correlation between the fraction of pore volume occupied by ions and the decrease in water flow, indicating that the mechanism behind its performance is predominantly linked to geometric exclusion of water molecules rather than more complex changes in water structure. We also found intricate results indicating that inside pristine MoS 2 nanopores with a diameter of 1.33 nm, pore size and electrostatic interactions play comparatively important roles in regulating salt rejection, while in smaller pores (0.97 nm diameter), pore diameter dominates over charge distribution. Our results offer insights into the physics governing transport phenomena inside nanopores made of naturally occurring MoS 2 as well as in similar pores made of different materials with differing charge polarizations. We hope that such understanding can help in the design of more efficient desalination membranes.