How monomer concentrations influence structures and separation performances of polyamide nanofiltration membranes?

Understanding the microscopic architecture as well as the formation mechanisms of polyamide separation layer is essential for designing nanofiltration (NF) membranes with better performance. The influence of monomer concentrations on the structure and separation performance of polyamide NF membranes are investigated using molecular dynamics simulations. By simulating the interfacial polymerization reaction between piperazine (PIP) and trimesoyl chloride (TMC) in a dual-solvent environment, four NF membranes are constructed using four different monomer ratios. Nonequilibrium molecular dynamics simulations are then employed to evaluate water permeance and ion rejection properties of the resulting membranes. Results show that monomer concentrations control critical membrane structural features like thickness and degree of network cross-linking (DNC) by tuning diffusion-reaction kinetics in interfacial polymerization, consequently altering water and ion transport pathways. Lower PIP concentrations promote decreased DNC and membrane density, enhancing water flux via the permeation paths. Molecular transport pathways and the dehydration behavior of ions reveal the synergistic effects of pore morphology and hydration shell modulation on membrane selectivity. These findings provide theoretical guidance for optimizing fabrication conditions and enhancing the separation performance of polyamide NF membranes.