Systematic experiments and multiscale simulation calculations reveal chemical stability differences in dry/wet nanofiltration membranes

Abstract

This study investigated the chemical stability differences between dry and wet nanofiltration membranes when immersed in strong acid and base environments. Systematic experimental studies were conducted on both types of nanofiltration (NF) membranes, supplemented by Molecular Dynamics (MD) simulations and Density Functional Theory (DFT) calculations. The results indicate that wet NF membranes, soaked in deionized water for a week, exhibit higher chemical stability compared to dry membranes treated only with air. Traditional air heating causes membrane pore contraction and excessive crosslinking. However, after a week of immersion in water, the polyamide (PA) layer gradually becomes a loose and well-extendable PA layer. MD simulations revealed that H+ and OH− ions possess lower diffusion coefficients and extended residence times in dry NF membranes, making them more prone to damage. Additionally, DFT calculations disclosed that the Gibbs free energy (ΔG) for amide bond hydrolysis in wet NF membranes, in the presence of water molecules, is elevated, significantly delaying their hydrolysis rate under extreme conditions. This study highlights the pivotal role of membrane wetness in the chemical durability of NF membranes and offers a novel perspective for improving membrane lifespan and efficiency.