Molecular Dynamics Simulation Study of Polyamide Membrane Structures and RO/FO Water Permeation Properties

Area of Science:

• Materials Science
• Chemical Engineering
• Computational Chemistry

Background:

• Polyamide (PA) membranes are crucial for reverse osmosis (RO) desalination, enabling selective water permeation and salt rejection for producing drinking water.
• Designing high-performance RO membranes requires a deep understanding of microscopic PA structures, including molecular-level water transport and ion rejection mechanisms.

Purpose of the Study:

• To investigate water molecular transport properties through virtual PA membranes with varying structures and densities using molecular dynamics (MD) simulations.
• To examine water permeability and ion rejection mechanisms on a molecular scale for PA membranes under RO and forward osmosis (FO) conditions.

Main Methods:

• Construction of two virtual PA membrane models with different structures and densities on a computer.
• Direct reverse/forward osmosis (RO/FO) filtration molecular dynamics (MD) simulations, including a quasi-non-equilibrium MD technique with applied pressure differences.
• Verification of a simple NVT (Number, Volume, and Temperature constant ensemble)-RO MD simulation method.

Main Results:

• Simulations of RO and FO water permeability for a dense PA membrane model agreed with experimental RO mode values.
• The dense PA membrane model demonstrated complete rejection of Na⁺ and Cl⁻ ions within a simulation time of several nanoseconds.
• The study examined the influence of PA structure void size on water permeability.

Conclusions:

• Naturally dense PA structures exhibit excellent ion rejection capabilities, crucial for effective desalination.
• Molecular dynamics simulations provide valuable insights into the molecular-level mechanisms governing water transport and ion rejection in PA membranes.
• Understanding these molecular-level details is indispensable for the rational design of advanced, high-performance RO membranes.