Multicomponent Space-Charge Ion Uptake and Ion/Solvent Transport Models for Ion-Exchange Membranes

Case Western Reserve University, Cleveland OH

Investigators

Abstract

ABSTRACT CTS-0085679 Multicomponent Space-Charge Ion Uptake and Ion/Solvent Transport Models for Ion-Exchange Membranes Peter N. Pintauro Department of Chemical Engineering Tulane University New Orleans, LA 70118 Ion-exchange membranes are used in a variety of industrial processes and electrochemical devices, including electrodialysis separations, electrochemical reactors, sensors, and proton-exchange-membrane fuel cells. In order to understand better the mechanism of selective ion transport by these membranes, new space-charge equilibrium ion uptake and ion/solvent transport models are being developed and tested. The models consider: (i) multivalent counterion/fixed-charge-site ion-pair formation, (ii) concentration-dependent ion diffusivities and solvent viscosity within the pores of a membrane, (iii) variable Gibbs energy of ion solvation within the pore-fluid double layer, (iv) electrostatic interactions between ions and the membrane's fixed charges, and (iv) the orientation of solvent dipoles inside a membrane pore due to the strong electric field generated by the membrane's fixed-charge groups. The models are applied to a variety of systems: (i) commercially available cation-exchange and anion-exchange membranes, (2) solutions composed of monovalent/monovalent cation and anion salts, monovalent/divalent cation salts, alkali metal/quaternary ammonium salt mixtures, and NaCl/amino acid (glycine) mixtures, and (3) water, methanol, methanol/water, and acetonitrile/water solvents. Experimental measurements include ion and solvent fluxes and bulk solution concentration changes during multicomponent Donnan dialysis and electrodialysis as well as membrane-phase counterion concentration levels during multicomponent ion uptake. The data are used to test the space-charge membrane models. The development of sufficiently detailed structure/function models that describe accurately absorption and transport of multiple ionic species in ion-exchange membranes would be of great value to polymer engineers and membrane scientists who are developing new membrane applications and formulating new polymeric membrane materials. Such models could be used (i) to simulate the performance of a given ion-exchange membrane in a particular electrochemical device or separation application, and (ii) to indicate new membrane structures with desired ion-transport rates and selectivities. This research project seeks such models by expanding the fundamental knowledge-base regarding the molecular-level processes and interactions that control the separation of multicomponent salt solutions from aqueous and non-aqueous solvents in anion and cation-exchange membranes.