Polyvinylidene Fluoride Mixed Matrix Membranes with Ionic Liquid-Functionalized Carbon Nanotubes: Enhanced Desalination via Vacuum Membrane Distillation

Vacuum membrane distillation (VMD) is a promising technique for water treatment, but its application is often limited by a low permeation flux. In this study, the performance of VMD membranes was enhanced by incorporating carbon nanotubes (CNTs) and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) into a polyvinylidene fluoride (PVDF) matrix. CNTs improved the mechanical strength and transport properties of the membranes, while BMIMPF6 enhanced nanoparticle dispersion and compatibility. Response surface method (RSM) was employed to optimize the concentrations of PVDF, CNT-BMIMPF6, and poly(ethylene glycol) (PEG) as the pore-forming agent. The optimized membrane, containing 16.50 wt % PVDF, 0.77 wt % CNT-BMIMPF6, and 2.91 wt % PEG 400, exhibited a high water contact angle (WCA) of 135.5°, enhanced permeation flux from 42.66 to 63.01 L/h/m2, and improved salt rejection rate from 84.50 to 99.98%. Membranes’ structures were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Structural analysis results revealed enhanced porosity, mechanical stability, and crystallinity, along with the formation of finger-like cavities. Increased crystallinity contributed to greater thermal resistance at a feed temperature of 70 °C. Overall, this study demonstrated that CNT-BMIMPF6 nanofillers significantly improve performance and structural properties of PVDF-based membranes, providing a valuable strategy for advancing VMD technologies.