Surface functional groups related organic fouling on polyamide membranes: A molecular dynamics study

In many applications, organic fouling poses a significant limitation to polyamide reverse osmosis (RO) membranes. Extensive efforts in the past have mostly provided macroscopic-scale experimental validations; however, a deep understanding of the existing co-ion fouling mechanism is still in its infancy. Herein, using molecular dynamics (MD) simulations, we report how specific surface functional groups (–COOH, –NH₂, and –CONH–) on the RO membrane surface work and contribute to organic fouling. Our results indicate that functional group-initiated organic fouling is mainly ascribed to electrostatic interactions, supplemented by ionic bonding, hydrogen bonding, and hydrophobic interactions. Density functional theory (DFT) calculations revealed the RO system containing carboxylic acid groups was subject to a “bridge-effect” to typical foulants (e.g., alginate) on the membrane surface, whereas the amino groups had an even increased probability of bonding to alginate in solution, both of which were markedly influenced by the electrostatic interaction, exacerbating the organic fouling. In contrast, a hypothetical RO system with neat amide linkages was proven to have the least number of interactions between the membrane surface and contaminants, resulting in notably enhanced fouling resistance. Overall, our simulations offer a dynamic framework for elucidating molecular-level fouling mechanisms, which may navigate the future development of antifouling reverse osmosis membranes.