The impact of Span80 single molecule and micelle forms on the structure and performance of polyamide nanofiltration membranes

Abstract

The utilization of surfactants in the fabrication of thin-film composite (TFC) membranes has proven to be an effective way for the regulation of interfacial polymerization (IP) process in order to obtain ideal polyamide (PA) layers. However, the uncontrollable migration of surfactants and the variety in the aggregated state of surfactants result in difficulty in regulation of the morphology of PA layer. Here, nonionic surfactant Span80 is introduced in organic-phase before IP process, and the impact of the transformation of Span80 from single molecules to micelles, as well as the morphology the PA layers is systematically investigated. The function of Span80 in IP process and its impact on improving the uniformity, compactness, and separation performance of the PA layer are demonstrated by molecular simulations. As has been proved that Span80 single molecules or micelles tend to form a surfactant layer composed by Span80 single molecules at the water-organic interface due to the lower interaction energy during their migration from the organic phase to the interface. The formation of this monolayer significantly reduces the water-organic interfacial tension, which promotes the diffusion of piperazine monomer, and accelerates the IP process. As a result, with the increasing Span80 concentration, the morphology of PA layer transforms from granular structure to denser micro-spherical structure. The more uniform PA layer helps to improve the performance of this nanofiltration membrane, with the Li⁺/Mg²⁺ separation ratio increasing from 2.41 to 14.72, representing an approximately 6-fold enhancement in separation efficiency.