Modeling nanovoid-enhanced water permeance of thin film composite membranes

Nanovoids in polyamide layers shape the roughness structures of thin film composite (TFC) reverse osmosis (RO) and nanofiltration (NF) membranes and have a critical influence on their water permeance. Although there have been numerous experimental works reporting enhanced water permeance by incorporating nanovoids, a systematic modeling for assessing the underlying mechanisms is still lacking. In this work, we perform numerical simulations to investigate water transport behaviors in a polyamide layer with or without a nanovoid. An analytical model, which quantifies the competition among substrate-induced unfavorable funnel effect and nanovoid-induced favorable multi-effects (i.e., surface area effect and gutter effect), is also developed and agrees well with simulation results. The model results suggest that even a tiny nanovoid of a few nanometers in size could provide an excellent self-gutter effect to minimize water transport distance. The available water permeance can be further improved by the increased effective surface area. Our modeling results reveal that the nanovoid-containing membrane can potentially offer 1–2 orders of magnitude enhancement in membrane permeance. The enhancement is greater for larger nanovoids with higher aspect ratios and for less porous substrates. Furthermore, the presence of nanovoids greatly improves the flux distribution uniformity over the membrane surface, which could potentially alleviate membrane fouling. The theoretical framework established in this work provides essential guidance for the future development of TFC membranes.