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​ZIWR Weekly Seminar

Wednesday, December 2nd 2020 at 13:15

Via zoom at https://us02web.zoom.us/j/82462191010

Counter-ion condensation in ion-exchange membranes: Impact on transport properties

Jovan Kamcev*

Department of Chemical Engineering and Macromolecular Science and Engineering, University of Michigan, Ann Arbor, USA

*jkamcev@umich.edu

Polymeric membranes with ionizable functional groups covalently bound to their backbone (ion exchange membranes, IEMs) are used in various technologies for water treatment (e.g., electrodialysis, membrane capacitive deionization, etc.) and energy generation/storage (e.g., fuel cells, flow batteries, etc.). The role of IEMs in these technologies is to mediate the transport of various solutes. For example, electrodialysis membranes must selectively permeate counter-ions (ions with opposite charge to that of the fixed ions) but prevent transport of co-ions (ions with similar charge to that of the fixed ions) and water. Rational design of high-performance membranes with transport properties specifically tailored for a given application requires fundamental understanding of the influence of polymer structure on solute transport across IEMs. However, despite a longstanding academic and industrial interest in IEMs, the current state of such fundamental understanding is incomplete. According to the solution-diffusion framework, solute transport in dense polymer membranes is governed by two phenomena – equilibrium solute partitioning within a membrane (thermodynamic phenomenon) and solute diffusion across a membrane (kinetic phenomenon). Significant progress on the fundamental understanding of equilibrium ion partitioning in IEMs has been recently made. For the first time, experimental ion partitioning results for several IEMs were predicted a priori from basic knowledge of polymer structure by incorporating ideas from Manning's counter-ion condensation theory into the classical Donnan approach for ion sorption modeling. Manning's theory, which was originally developed for polyelectrolyte solutions, states that counter-ions condense (or localize) onto the polymer backbone to neutralize a fraction of the fixed charge groups when the distance between fixed charge groups on a polyelectrolyte chain is less than the Bjerrum length. Counter-ion condensation has been observed in various types of charged polymers, including biological charged polymers such as DNA, but this phenomenon has not been discussed much in the membrane literature until recently. The impact of counter-ion condensation on ion transport in IEMs and implications for membrane design are the central themes of this presentation.

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Meeting ID: 824 6219 1010