Aromatic Polyamide Reverse-Osmosis Membrane: An Atomistic Molecular Dynamics Simulation

Polyamide
(PA) membrane-based reverse-osmosis (RO) serves as one
of the most important techniques for water desalination and purification.
Fundamental understanding of PA RO membranes at the atomistic level
is critical to enhance their separation capabilities, leading to significant
societal and commercial benefits. In this paper, a fully atomistic
molecular dynamics simulation was performed to investigate PA membrane.
Our simulated cross-linked membrane exhibits structural properties
similar to those reported in experiments. Our results also reveal
the presence of small local two-layer slip structures in PA membrane
with 70% cross-linking, primarily due to short-range anisotropic interactions
among aromatic benzene rings. Inside the inhomogeneous polymeric structure
of the membrane, water molecules show heterogeneous diffusivities
and converge adjacent to polar groups. Increased diffusion of water
molecules is observed through the less cross-linked pathways. The
existence of the fast pathways for water permeation has no effect
on membrane’s salt rejections.