Rinse-Free Fabrication, Surface Modification, and Performance of Molecular-Layer-by-Layer (mLbL) Polyamide Reverse Osmosis Membranes for Scaling and Fouling Resistance
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Reverse osmosis (RO) membranes are vital for seawater desalination and industrial water treatment. Most commercial RO membranes are fabricated via interfacial polymerization (IP), a process in which a thin polyamide selective layer is formed at the interface between an aqueous solution of diamines and an organic phase containing acid chlorides. While IP offers appreciable water permeability and salt rejection, it produces membranes with rough, heterogeneous surfaces and poorly defined polymer architectures. This is primarily due to the rapid, diffusion-limited nature of the reaction
highly water-soluble monomers such as m-phenylenediamine (MPD) diffuse into the organic phase containing trimesoyl chloride (TMC), resulting in non-uniform polymerization rates and the formation of a ridge-and-valley morphology. The uncontrolled reaction front and local monomer depletion further lead to spatial variation in crosslinking and surface chemistry, making IP-derived membranes more prone to fouling and scaling.Molecular layer-by-layer (mLbL) deposition was developed as a solution to these limitations, offering nanoscale control over film thickness and crosslinking through alternating monomer deposition and intermediate rinsing steps. However, conventional mLbL methods are hindered by the need to rinse away unreacted monomers after every deposition cycle, which significantly increases solvent use and fabrication time up to 80% of the total process time is devoted to rinsing. This thesis introduces a modified, rinse-free mLbL method that eliminates intermediate rinsing by lowering monomer concentration and delivery volume to control polymer growth at the surface. Using MPD and TMC, we fabricated polyamide membranes with uniform thickness (~20 nm), high crosslink density, and minimal unreacted functionalities. Rinse-free mLbL membranes achieve 99.9% NaCl rejection within 4 hours, with a water permeance of 0.9 L·m⁻²·h⁻¹·bar⁻¹ surpassing conventional mLbL and commercial brackish water RO membranes in rejection and stabilization time. In addition to superior performance and process efficiency (reducing chemical usage by 93% and fabrication time by 90%), the rinse-free platform enables precise surface chemical modifications. By tuning membrane surface properties such as charge and hydrophilicity, we demonstrate the potential to design polyamide films with enhanced resistance to silica scaling and organic fouling—two of the most persistent challenges in membrane operation. This work positions rinse-free mLbL as a scalable and versatile approach for next-generation membrane fabrication, enabling both high performance and fundamental understanding of fouling and scaling resistance mechanisms.
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