Demixing of Polymerized Ionic Liquid/Ionic Liquid Mixtures by Infiltration in Nanopores

Abstract

Ion transport through membrane nanopores is pertinent to several applications, including water desalination and energy harvesting. We synthesized a series of polymerized ionic liquids (PILs) based on the 1-butyl-3-vinylimidazolium cation ([BVIM]⁺ with three different anions ([X]⁻: [TFSI]⁻, [BF₄]⁻, [PF₆]⁻). We explored how mixtures of the PIL with the corresponding IL (poly[BVIM]⁺[X]⁻/[BMIM]⁺[X]⁻) penetrate the nanopores. For this purpose, we employ ex situ reflection optical microscopy of the evolution of the imbibition length and in situ conductivity measurements by nanodielectric spectroscopy. The latter provides details of ion motion during and following imbibition. In the bulk, symmetric poly[BVIM]⁺[X]⁻/[BMIM]⁺[X]⁻ mixtures are locally heterogeneous, composed of nearly pure IL domains and mixed PIL/IL domains. When the mixture is placed on top self-ordered nanoporous aluminum oxide templates (AAO), the ionic liquid is dragged by capillary action into the pores. During imbibition the two components partially demix. At the end of the filling process the pores contain an excess of the IL and a minority of PIL chains. Subsequently we explored the effect of polymer adsorption and surface functionality on the kinetics of ion transport. The results suggest the possibility to separate a mixture of ionic compounds (IL and PIL in this case) by the difference in the imbibition kinetics of its constituent components. Applications of AAOs as separation membranes for ionic systems are discussed.