(565d) The Effect of the Structure of Ion-Exchange Membranes on Ion Concentration Polarization and Associated Electrokinetics

The phenomena resulting from ion concentration polarization occurring in ion selective systems under DC voltage are increasingly used in the field of microfluidics to develop novel diagnostic and screening platforms with the main applications in point-of-care testing. Ion exchange membranes that are traditionally employed in electromembrane separation processes as electrodialysis and electrodeionization are one example of a system exhibiting ion concentration polarization. They have recently been used for pretreatment of biological samples, preconcentration of biomolecules and their specific detection [1, 2]. The ion exchange membranes are commercially available and their chemical and physical composition is tailored for the main application in electromembrane separation. The composition of the membranes, however, affects the exhibited behavior when in DC electrical field and it can play an important role in designing point-of-care microfluidic platforms. Our work attempts to find important links between ion exchange membrane structure and the exhibited behavior under various experimental conditions. We use Micro X-ray computed tomography to analyze small pieces of commercially available ion exchange membranes in a state when they are ready for electrochemical characterization (fully swollen membranes in a given electrolyte). The 3D analysis of the membrane focuses on the composition of the membrane surface that is in contact with an electrolyte since the interaction of the membrane surface and the adjacent electrolytes especially on the depletion side of the membrane controls the behavior of the whole system. We further characterize these small pieces of the membrane by performing standard electrochemical measurements and by running fluorescent imaging of the membrane and adjacent layers of electrolyte to capture the ion concentration polarization and resulting electrokinetic phenomena. The electrokinetic phenomena that are responsible for the occurrence of an overlimiting current are still a subject of scientific discussions and depend on the type of the membrane and the experimental set up. Our goal is to describe the effects of the membrane surface on electrokinetics of ion exchange membranes and use the knowledge to design optimal structure of ion exchange membrane for a given function in point-of-care systems.

[1] Senapati, S., Slouka, Z., Shah, S. et al. An ion-exchange nanomembrane sensor for detection of nucleic acids using a surface charge inversion phenomenon, Biosensor&Bioelectronics, 2014, 60:92-100

[2] Slouka, Z., Senapati, S., Chang, H.C., Microfluidic systems with ion-selective membranes, Annual Review Analytical Chemistry, 2014; 7:317-335