Massively non-linear increase in electroosmotic transport in nanochannels grafted with cationic polyelectrolyte brushes*

In this study, we employ all-atom molecular dynamic (MD) simulations for probing the electroosmotic (EOS) transport in nanochannels grafted with cationic [poly(2-

(methacryloyloxy)ethyl) trimethylammonium chloride] (PMETAC) polyelectrolyte (PE) brushes. Our central finding is that there is a massively non-linear increase in the EOS flow strength with the electric field: with an increase in the electric field strength by two-fold, the maximum of the EOS velocity increases much more than two-fold. To explain this phenomenon, we first study the behavior of the brushes and the brush-supported counterions in presence of the applied electric field. At larger electric fields, the chloride counterions inside the brush layer moves very fast: this is possible due to the significantly large mobility of the chloride ions inside the PMETAC PE brush layer. The brush chains that interact strongly with these ions, as a result, experience significant bending and bucking in the direction of the counterion motion. Such changes in the brush configuration ensure an accumulation of a large number of counterions at the brush-layer-bulk interface. The water molecules hydrating these interface-localized counterions experience much smaller drag force from the brush layer, thereby demonstrating very high velocity (i.e., EOS velocity becomes very large) at larger electric fields. This leads to the highly intriguing significantly large non-linear increase in the EOS velocity in PEMTAC-brush-grafted nanochannels.