Does interfacial vaporization of organic solvent affect the structure and separation properties of polyamide RO membranes?

Abstract

Nanovoids in polyamide rejection layers of thin film composite (TFC) reverse osmosis (RO) membranes are responsible for their characteristic “ridge-and-valley” surface roughness and have profound impact on their separation performance. However, mechanisms leading to these void-containing roughness features remain poorly understood. The current work presents compelling evidence that vaporization of the organic solvent contributes to the formation of nanovoids during the exothermic interfacial polymerization (IP) process. We used a series of alkane solvents with systematically varying chain length and vapor pressure to prepare TFC membranes. Our study revealed that an organic solvent with higher vapor pressure generated more vapor during the IP reaction, which in turn resulted in larger size of the voids in the polyamide thin film and higher membrane water permeability. We further designed a strategy to suppress the vapor effect by preparing polyamide thin films at a free interface. This led to the disappearance of nanovoids and nearly identical membrane permeability regardless of the organic solvent used for the IP process, in good agreement of the weakened confinement to the organic vapor generated by interfacial heating. The current study provides new mechanistic insights to interpret the formation of the voids-containing morphology of TFC polyamide membranes, which would facilitate improved understanding of membrane transport mechanisms and better control of membrane structural features.