GO-Assisted Supramolecular Framework Membrane for High-Performance Separation of Nanosized Oil-in-Water Emulsions

Abstract

Intercepting tiny droplets in nano-emulsions is crucial for the development of membrane materials with pore diameters smaller than the droplet size, as per the size screening mechanism. While this method achieves high separation efficiency, it results in a decrease in separation flux. On the one hand, the use of macro-porous materials can increase the flux, but it does not guarantee high efficiency on the other hand. Fabricating superwetting materials that exhibit both high efficiency and flux in separating nanosized emulsions provides opportunities for overcoming the bottleneck yet how to extend the applicable range with high efficiency remains a challenge. To address this issue, we propose a strategy to enhance the hydrophilicity of supramolecular framework nanosheets by modifying hydrophilic graphene oxide (GO). By incorporating GO into the supramolecular framework (SF) composite membrane through a sequential pumping process onto a commercial matrix, we create a GO-assisted SF composite membrane capable of separating oil-in-water (O/W) emulsions containing nanosized droplets. The framework intercepts the dispersed tiny droplets in the emulsions through uniform nanoscale pores while also facilitating the demulsification process through electrostatic interaction on its negatively charged surface. The hydrophilic GO modification on the composite membrane enhances its water affinity and promotes the formation of a hydrated layer on the membrane surface. The resulting composite membrane exhibits a nanoscale cut-off size, a negatively charged surface, and oleophobicity under water. Importantly, it achieves high water flux and resistance to oil contamination. By leveraging the size screening and demulsification effects, the composite membrane efficiently removes nanosized oil droplets dispersed in O/W emulsions stabilized by non-ionic, anionic, and cationic surfactants. Particularly for emulsions containing ionic surfactants, no residual droplets are detected through dynamic light scattering (DLS) after separation. The filtrate exhibits a total organic carbon (TOC) content of less than 10 ppm, corresponding to a separation efficiency greater than 99.9%, which surpasses the standards of many countries and organizations. Furthermore, compared to the original SF membrane, the composite membrane demonstrates approximately 3.5 times higher separation permeation during the separation process of various emulsions. Additionally, the composite membrane exhibits an anti-fouling effect and achieves a high flux recovery rate, ensuring stable separation performance for 5 cycles through simple water washing treatment. The composite membrane retains its components throughout repeated use, exhibits thermal stability up to 150 °C, and can withstand corrosive chemical environments, including 1 mol·L⁻¹ HCl, 0.01 mol·L⁻¹ NaOH, and 1 mol·L⁻¹ NaCl. In this study, we realize the combination of two components with distinct structural and surface characteristics to fabricate a composite membrane through a simple method and achieve high-performance separation of nanosized O/W emulsions through synergistic functionality.

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