Tunable Surface Chemistry and Smooth Morphology of Molecular Layer-by-Layer Deposited Polyamide Membranes Enable Reversible Silica Scaling

Abstract

Silica scaling is one of the most problematic forms of inorganic fouling in reverse osmosis (RO) systems, often causing irreversible performance loss and demanding frequent, costly cleaning. We systematically investigated the mechanisms of silica scaling on thin (≈20 nm), ultrasmooth (<5 nm roughness), and highly cross-linked polyamide films fabricated via rinse-free molecular layer-by-layer (mLbL) deposition. This platform enabled precise tailoring of terminal surface chemistries, independent of surface roughness, to decouple the effects of charge and hydrophilicity on scaling behavior. Through a combination of RO performance tests and quartz crystal microbalance with dissipation (QCM-D) measurements, we demonstrate that silica deposition is governed not solely by electrostatic interactions but also by the interplay of membrane interfacial water molecule orientation, hydrogen bonding, and silanol chemistry. Besides surface chemistry, surface morphology plays an equally important role in silica scaling reversibility. The mLbL membranes enabled the detachment of silica deposits, with simple deionized water rinsing restoring nearly complete water flux, in sharp contrast to interfacial polymerized membranes, where ridge-and-valley roughness makes scaling nearly irreversible. A strong correlation was observed between water flux decline of the RO membranes and silica deposition on the polyamide films measured by QCM-D. Together, these findings establish mLbL as a promising membrane fabrication strategy that offers precise control of chemistry and morphology to advance the design of antiscaling RO membranes.