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

Abstract

Surfactant-assisted interfacial polymerization (IP) has shown strong potential to improve the separation performance of thin film composite polyamide membranes. A common belief is that the enhanced performance is attributed to accelerated amine diffusion induced by the surfactant, which can promote the IP reaction. However, we show enhanced membrane performance for Tween 80 (a common surfactant), even though it decreased the amine diffusion. Indeed, the membrane performance is closely related to its polyamide roughness features with numerous nanovoids. Inspired by the nanofoaming theory that relates the roughness features to nanobubbles degassed during the IP reaction, we hypothesize that the surfactant can stabilize the generated nanobubbles to tailor the formation of nanovoids. Accordingly, we obtained enlarged nanovoids when the surfactant was added below its critical micelle concentration (CMC). In addition, both the membrane permeance and selectivity were enhanced, thanks to the enlarged nanovoids and reduced defects in the polyamide layer. Increasing the concentration above CMC resulted in shrunken nanovoids and deteriorated performance, which can be ascribed to the decreased stabilization effect caused by micelle formation. Interestingly, better antifouling performance was also observed for the surfactant-assisted membranes. Our current study provides mechanistic insights into the critical role of surfactant during the IP reaction, which may have important implications for more efficient membrane-based desalination and water reuse.

Does Surface Roughness Necessarily Increase the Fouling Propensity of Polyamide Reverse Osmosis Membranes by Humic Acid?

“…At present, membrane surface roughness is typically characterized by AFM in terms of common roughness parameters such as average roughness R A , root-mean-square roughness R RMS , and maximum roughness R MAX . However, these roughness parameters are insufficient to completely describe a roughness structure. , For example, roughness structures that have identical roughness values (e.g., R MAX is given by half of the peak-to-valley height) can have different geometrical properties (e.g., aspect ratio), which may lead to very different mass transfer and fouling behaviors. Therefore, researchers need to be explicitly aware of such intrinsic limitations of roughness parameters obtained from AFM.…”

When self-assembly meets interfacial polymerization

“…The exact mechanism of the micelle-assisted IP process remains to be elucidated. Besides, multiple mechanisms may affect the IP reaction in surfactant-assisted IP ( 27 , 28 ), and the origin of membrane surface roughness as well as its impact on desalination performance are still poorly understood. As the PA layer exhibited subnanoscale internal free volumes and nanoscale thickness, and the IP process involves multidisciplinary knowledge, the heterogeneous nature of desalination membranes ( 29 ) calls for multiscale investigation on the fundamental aspect to gain a thorough understanding of the IP process.…”

Autonomous Interfacial Assembly of Polymer Nanofilms via Surfactant-Regulated Marangoni Instability

“…Interfacial degassing may partially contribute to the interfacial instability, but its effect may also not be strong in our case. This is because generated nanobubbles can be effectively eliminated when PA nanofilms are formed at the free interface . Excessive interfacial fluctuations produced at high TX-100 concentrations (>10 mM) could generate a Marangoni stress high enough to rupture and fragment the nascent PA layer, yielding a more irregular and defective PA structure (Figure E) .…”

“…Because IP is a complex process involving the simultaneous diffusion and reaction of various molecules, the nanofilm formed via IP is susceptible to interfacial instabilities via various mechanisms, possibly producing an irregular film structure. 1,10,23,28 Specifically, the spontaneous dissolution of highly surface-active TX-100 into n-hexane can create its concentration inhomogeneity and thus an interfacial tension gradient along the interface. This non-uniform interfacial tension drives liquid motion along the interface toward regions with higher interfacial tension, which sets the underlying liquid in motion, inducing a convective interfacial flow, called solutal Marangoni convection.…”

“…This is because generated nanobubbles can be effectively eliminated when PA nanofilms are formed at the free interface. 23 Excessive interfacial fluctuations produced at high TX-100 concentrations (>10 mM) could generate a Marangoni stress high enough to rupture and fragment the nascent PA layer, yielding a more irregular and defective PA structure (Figure 1E). 1 Meanwhile, significantly accelerated MPD diffusion could further contribute to the formation of the defective PA nanofilm by unbalancing reactant stoichiometry.…”