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

Abstract

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.

Loosely Sandwich-Structured Membranes Decorated with UiO-66-NH2 for Efficient Antibiotic Separation and Organic Solvent Resistance

“…As depicted in Figure a, an ultrathin PA selective layer, having many tiny and rugged nanobubbles, is built on the explanate hydrophilic TA–PEI interlayer. It may be that the IP process is a heat-release fluidization reaction, the quantity of heat could erode gases’ solubility, making an unstable reaction interface that will produce nanobubbles to influence the membrane surface morphology. , The as-fabricated TFN membranes, in contrast, present a kind of Turing structure with typical “ridge-valley” morphologies (Figure b–f). The valleys on the TFN membrane surface are getting clearer, showing a deeper polygonal wrinkle pattern as the UiO-66-NH 2 content increases.…”

“…Recently, a thin-film nanocomposite (TFN) membrane has been verified to raise filtration selectivity, via rational design and optimization of the structure and characteristics of the porous substrate and selective layer. , Numerous functional nanomaterials like zwitterionic, molybdenum disulfides, metal–organic frameworks (MOFs), and covalent-organic frameworks have been studied in the TFN membrane. − Concrete manifestation in the reinforcement of polymer crosslinking degree, membrane hydrophilicity, and extra nanochannels facilitate water transportation. , By comparison, the amino-functionalized zirconium-based MOF (UiO-66-NH 2 ) is one of the most promising nanomaterials for efficient molecular separation, because of its high surface areas, ordered permanent porosity, superior chemical stability, and unique practical versatility. − For example, the hydrophilic amino groups in UiO-66-NH 2 could be covalently bound in branched polyamide (PA) macromolecules, inhibiting the surface defect formation in the TFN membrane, and then exhibiting moderate rejection rates for Na 2 SO 4 (99.9%) and NaCl (38.1%) . Replacing partial Zr 4+ to Ti 3+ in UiO-66-NH 2 could neutralize some of the positive charges in the PA-TFN membrane, via a facile ion exchange method, which also achieved excellent mono-/divalent cation selectivity .…”

“…Recently, a thin-film nanocomposite (TFN) membrane has been verified to raise filtration selectivity, via rational design and optimization of the structure and characteristics of the porous substrate and selective layer. 15,16 Numerous functional nanomaterials like zwitterionic, molybdenum disulfides, metal−organic frameworks (MOFs), and covalent-organic frameworks have been studied in the TFN membrane. 17−20 Concrete manifestation in the reinforcement of polymer crosslinking degree, membrane hydrophilicity, and extra nanochannels facilitate water transportation.…”

Demystifying the Role of Surfactant in Tailoring Polyamide Morphology for Enhanced Reverse Osmosis Performance: Mechanistic Insights and Environmental Implications

“…Future studies need to further optimize the surfactant loading to enhance its stabilization effect for better separation property. In addition, RO membranes often show unsatisfactory removal of viruses and bacteria, possibly due to the defects in the polyamide film. , Surfactant-assisted IP may act as a potential strategy for mitigating this penetration thanks to the reduced defects, which calls for more investigations.…”

Forward Osmosis Membranes: The Significant Roles of Selective Layer

“…For example, the addition of 1-methylimidazole in the aqueous phase can react with TMC to reduce the thickness of the polyamide layer, make it denser, and enrich the carboxylic acid groups on the surface, achieving a water flux of 72 Lm −2 ∙h −1 and rejection of 99.06% using 2000 mg/L NaCl as feed and at 15.5 bar pressure [ 130 ]. Another method to improve the retention rate of the membranes is by post-treatments such as heat treatment, secondary crosslinking, coating, etc., but these methods are likely to sacrifice the water permeability [ 131 , 132 , 133 ]. It should be noted that the NaCl rejection rate of the SWRO membrane can reach 99.5%, which is much higher than that of almost all FO membranes.…”