Effect of Surface Conduction on Propagation of Ion-Concentration Shock Waves in Isotachophoresis

Isotachophoresis (ITP) is a widely used nonlinear electrophoretic technique for preconcentration and separation of ionic species. Typically, ITP is performed in microchannels where the effect of surface conduction due to electric double layer (EDL) at channel walls is negligible compared to bulk conduction. However, when electrophoretic techniques such as ITP are integrated in nanochannels or shallow microchannels, surface conduction can alter bulk electrophoretic transport. The existing mathematical models for multispecies electrophoretic transport do not account for the competing effects of surface and bulk conduction. We present a mathematical model for multispecies electrophoretic transport incorporating the effects of surface conduction on bulk ion-transport. Our one-dimensional model is capable of describing electrophoretic systems consisting of arbitrarily large number of co-ions, having same charge polarity as the wall charge, and a single counter-ion. Based on numerical solutions of the governing equations, we show that unlike in conventional ITP where surface conduction is negligible, the zone concentrations do not obey the Kohlrausch regulating function when surface conduction is prominent. Moreover, our simulations show that surface conduction alters the propagation speeds of ion-concentration shock waves in ITP. In addition, surface conduction results in additional shock and expansion waves in ITP which are otherwise not present in conventional ITP.