Unveiling the Growth of Polyamide Nanofilms at Water/Organic Free Interfaces: Toward Enhanced Water/Salt Selectivity

The permeance and selectivity of a reverse osmosis (RO) membrane are governed by its ultrathin polyamide film, yet the growth of this critical film during interfacial polymerization (IP) has not been fully understood. This study investigates the evolution of a polyamide nanofilm at the aqueous/organic interface over time. Despite its thickness remaining largely constant (∼15 nm) for the IP reaction time ranging from 0.5 to 60 min, the density of the polyamide nanofilm increased from 1.25 to 1.36 g cm–3 due to the continued reaction between diffused m-phenylenediamine and dangling acyl chloride groups within the formed polyamide film. This continued growth of the polyamide nanofilm led to a simultaneous increase in its crosslinking degree (from 50.1 to 94.3%) and the healing of nanosized defects, resulting in a greatly enhanced rejection of 99.2% for NaCl without sacrificing water permeance. Using humic acid as a molecular probe for sealing membrane defects, the relative contributions of the increased crosslinking and reduced defects toward better membrane selectivity were resolved, which supports our conceptual model involving both enhanced size exclusion and healed defects. The fundamental insights into the growth mechanisms and the structure–property relationship of the polyamide nanofilm provide crucial guidance for the further development and optimization of high-performance RO membranes.