Improved stability of a supported liquid membrane process via hydrophobic modification of PVDF support by plasma activation and chemical vapor deposition

Abstract

• PVDF membrane support was modified by plasma activation and CVD of a silane PFOCTS. • Water contact angle of support increased from 124.3° to ∼ 142.4° after surface fluorination. • Instability of an SLM process was analyzed by electrical impedance spectroscopy (EIS) • Stability of an SLM containing [BMIM]PF 6 was enhanced with a more hydrophobic support. • Results of in-situ EIS monitoring were verified by a macroscopic mass transfer analysis. In this research, hydrophobic modification of a poly(vinylidene fluoride) (PVDF) membrane through plasma activation and chemical vapor deposition with trichloro(1H,1H,2H,2H-perfluorooctyl)silane was investigated. The modified membrane was used as a support to improve the stability of a supported liquid membrane (SLM) process. Various plasma conditions such as power (6.8, 10.5, and 18 W) and treatment time (1, 2.5, 5, 7.5, and 10 min) were explored. Physicochemical properties of the pristine and modified membranes including the morphology, hydrophobicity, and chemical structure were first analyzed. The hydrophobicity of modified membranes increased, due to successful incorporation of a fluorine-containing silane (a reagent for reducing surface free energy) on the outer surface of the membrane and the inner surface of open pores nearby. The impact of hydrophobic modification on the stability of SLMs was real-time explored by a non-destructive technique, electrical impedance spectroscopy (EIS), where phenol and ionic liquid 1-butyl-3-methylimidazole hexafluorophosphate were selected as the model solute and membrane liquid (organic phase). The stability issues of SLMs deduced from EIS analysis were further verified by mass transfer analysis and feed pH monitoring. Results showed that the modified membranes provided more stable and prolonged SLM processes but relatively slower transport. The optimal plasma condition was found to be 18-W power and 5-min treatment time, at which the hydrophobicity of PVDF support was maximized (water contact angle 142.0°) and the most stable SLM was obtained (impedance reduction at 0.5-h time by ca. 60%). The mechanism causing the instability of the present SLM was finally identified. Plasma treatment and chemical vapor deposition with fluorine-containing silanes was an effective approach for enhancing the stability of an SLM and EIS was a powerful technique for real-time monitoring the instability behavior.