Stability of supported ionic liquid membranes as studied by X-ray photoelectron spectroscopy

This work evaluates the use of room temperature ionic liquids (RTILs) in supported liquid membranes (SLMs). The physicochemical properties of RTILs, namely their reduced solubility in water and high interfacial tension, make their use very attractive in order to obtain stable supported liquid membranes. Preliminary results showed that, under relatively mild stirring conditions, there was no evidence of ionic liquid displacement from the membranes' pores. In this work, further studies were conducted in order to assess the SLMs' operational stability under dynamic conditions. Dynamic stability studies were performed to evaluate the stability of a SLM with [C8MIM][PF6] when submitted to more aggressive and well-defined hydrodynamic conditions. Additionally, X-ray photoelectron spectroscopy (XPS) was used to characterise the SLMs obtained by immobilizing the ionic liquids [CnMIM][PF6] (n = 4, 8) and [C10MIM][BF4] in a polyvinylidene fluoride (PVDF) supporting membrane. This technique allows for the determination of the surface chemical composition of a sample and was used for the characterization of both the supported liquid membranes and the ionic liquids. The SLMs were analysed immediately after preparation and after being immersed for I week in de-ionised water. This way, it was possible not only to assess the presence of the RTILs in the membrane after preparation, but also to identify whether there were any chemical modifications on the SLMs' surface after contact with an aqueous solution, thus obtaining valuable information about their integrity and stability. (c) 2005 Elsevier B.V. All rights reserved.