Spatial variations of pH and electrokinetic transport in micro–nanofluidic systems under ion concentration polarization

Through the integration of ion-selective interfaces into microchannels, micro–nanofluidic systems can realize diverse macroscale functions such as seawater desalination, lithium-magnesium separation, and trace particle enrichment based on ion concentration polarization (ICP). Although ion transport and electrokinetic flow in membrane-embedded microchannels have been extensively studied, the effects of dissociation reactions on spatial variations of pH and electroconvection remain unclear. This study investigates the electrokinetic transport characteristics coupled with dissociation equilibria, including electroosmotic flow, desalination, ion enrichment, and overall pH distribution within the microchannel. The results show that the buffer reactions maintain the ionic balance of the system when high-mobility ions accumulate near the membrane and, by generating co-ions for charge compensation, limit the enrichment of a single ion species so that its concentration remains below the electroneutrality limit. The acid dissociation reaction is significantly accelerated in the Ohmic and limiting current (LC) regimes due to ion enrichment, resulting in a maximum increase of approximately 7.8% in flow velocity. The coupling between the dissociation reactions and the electroosmotic pumping effect extends the pH disturbance from the local region to the entire channel. In strong electrolytes, the upstream pH rises to approximately 8.3, whereas in weak electrolytes, an overall acidification occurs, with the pH decreasing to approximately 6.1. These findings provide theoretical guidance for optimizing desalination efficiency, manipulating electroosmotic pumping, and enriching pH-sensitive particles in microfluidic systems.