Quantitively unveiling the activity-structure relationship of polyamide membrane: A molecular dynamics simulation study

Abstract

The outstanding separation properties of traditional polyamide (PA) membranes in desalination attract numerous interests about its internal microstructures and separation mechanism. Herein, the water permeation and salt rejection of PA membranes with different thicknesses and pore sizes were simulated by non-equilibrium molecular dynamics (NEMD) simulations. Results find that small cross-linking pores (1.00–4.00 Å) of PA are mainly responsible for salt rejection, and large stacking pores (4.00–6.00 Å) allow fast water transport. But when the pores are large enough to form the continuously connected pore channel, the PA membrane loses salt rejection. Furthermore, the water permeance of PA is found to be inversely proportional to the membrane thickness. The minimum thickness of PA is estimated to be 20 Å in order to maintain a high NaCl rejection. The water permeance of PA (Pw) is plotted with the pore size (R) and thickness (T) of PA as Pw = −5.9496 + 0.0648 ∗ R4 / T. The maximum water permeance of PA with high NaCl rejection is estimated to be 14.25 LMH/bar based on the minimum thickness (2 nm) and the maximum pore size (5.00 Å) of PA. This simulation study provides microscopic insights into the quantitative structure-properties relationship of PA membrane.