Interface engineering of ion exchange membranes
Aachen (2017, 2018) [Dissertation / PhD Thesis]
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Surface properties of membranes play decisive roles in the overall membrane separation processes. Membrane surfaces are elaborately altered during membrane formation or in a separate step afterward with the objective of enhancing some desired performances or mitigating drawbacks that are inherent to membrane separation processes. The work at hand investigates the surface/interface modification of both monopolar- and bipolar ion exchange membranes for electrodialysis applications. The ion transport phenomena investigated are particularly influenced by the state of the membrane surface/interface. The phenomena studied are overlimiting mass transfer through electrodialysis membranes, monovalent ion permselectivity of cation exchange membranes, and water dissociation catalysis at the interface of bipolar membranes. For the non-covalent surface modification of the membranes, two classes of polymers are applied: polyelectrolyte multilayers and microgels.The polyelectrolyte multilayers and microgels are successfully implemented – as homogeneous- and micropatterned layers – to tailor the interface of the ion exchange membranes without any prior treatment of the membranes. Their depositions are subsequently characterized with several surface analytical techniques. Results of the interface modification of bipolar membranes indicate enhanced performance when the anion exchange layer of the membranes are coated with certain polyelectrolytes, and more so when polyelectrolytes of higher charge density are applied. Out of the investigated Layer-by-Layer (LbL) assembly parameters, ionic strength and number of layers show the largest influence on the catalytic activity and surprisingly on the membrane ionic permselectivity as well. The membrane permselectivity was previously assumed to be only influenced by the bulk thickness of the anion- and cation exchange layers. Furthermore, the possibility of applying polyelectrolytes to induce monovalent ion permselectivity on standard cation exchange membranes is studied. Permselectivity comparable to that of a commercial monovalent-ion-permselective membrane is obtained with only six bilayers of polyelectrolytes, yet with significantly lower energy consumption per mole of Na+ ions transported through the membranes. At overlimiting current densities, the study further shows that polyelectrolyte multilayers allow switching on and turning off water splitting at the surface of ion exchange membranes. These membranes could be beneficial for applications in which ion permselectivity and pH regulation are needed at the same time. This thesis, in addition, attempts to answer the question whether polyelectrolytes can also be utilized to modulate overlimiting current phenomena in electrodialysis membranes. The study proves that controlled adsorption of polyelectrolyte multilayers at the interface between the ionic solution and membrane surface triggers a significantly earlier onset of electroconvection (as the main driver for the overlimiting current) at lower polarization potential. It further proves that their uniform regio-selective adsorption imposed by microcontact printing is even more effective, and enhances macroscopic electro-osmotic chaotic fluid instabilities. The experimental techniques, guided by theoretical studies, may pave the way for potentially ‘smart’ design of electrodialysis membranes for intense-current desalination.Apart from electrodialysis, the findings of this thesis may provide equally valuable insights to other areas of applications of ion exchange membranes, which require membranes with tailored surface properties.
Lammertink, Rob G. H.