New insights into confined diffusion mechanisms of end-capping reagent regulated interfacial polymerization

The permeability and selectivity of polyamide (PA) nanofiltration membranes depend on the physicochemical structure of the active layer including the crosslinking degree, uniformity, microstructure, and thickness. The additive control strategy has been regarded as a successful method to regulate nanofiltration performance. Herein, the functionalized end-capping reagent, sulfanilic acid, as a co-reactive additive is introduced in the aqueous phase to adjust the homogeneity and thickness of polyamide layer by changing the diffusion behavior of the amine and cross-linking degree of nascent membrane. The end-capping reagent regulates the interfacial polymerization process, which can optimize the distribution of charge density and structural characteristics of polyamide layer, and improve the aperture property by a simple one-step method. The PA-10 with a combination structure of the round-like and ridge-and-valley possesses a favorable water permeance of 23.2 L m⁻² h⁻¹ bar⁻¹, which has at least 50% increment in flux while retaining the Na₂SO₄ rejection compared to the PA-0 (without the additives). Furthermore, the mechanism of interfacial polymerization regulated by end-capping reagents has been studied by the Doppler broadened energy spectrum, thermodynamic molecular reactivity, and molecular dynamics behavior. The loose nascent layer generated by end-capping reagents results in confined diffusion of monomer in the aqueous phase, thus favoring the formation of a thin compact separation layer and improving the membrane separation performance. This work elucidates the mechanism of end-capping reagent sulfanilic acid regulating the interfacial polymerization process and the connection between the composition and performance of nanofiltration membranes, which provides guidance for the controllable design of nanofiltration membranes.