Antielectrophoretic Response-Driven Bending–Tilting Deformation of Cationic Polyelectrolyte Brushes Drives Nonlinear Electroosmotic Transport in Brush-Grafted Nanochannels

Abstract

In this paper, we use all-atom molecular dynamics (MD) simulations to describe a nonlinearly enhanced electroosmotic (EOS) flow, where, in a nanochannel grafted with cationic PMETAC ([poly(2-(methacryloyloxy)ethyl) trimethylammonium chloride]) brushes, a 2-fold increase in the electric field strength leads to a several-fold (more than 2-fold) increase in the EOS flow strength and volume flow rate. The electric field enforces the PMETAC brushes to undergo a bending–tilting-driven deformation, with a significant portion of the brush layer becoming parallel to the grafting surface. In response, a substantial fraction of the counterions leave the brush layer (hence become more mobile) but instead of going into the bulk, accumulate at the brush–bulk interface, i.e., stay in proximity to the brush segments aligned parallel to the grafting surface. This creates an interesting situation where the counterions are not completely within the brush layer, yet they fully screen the brush charges. Such “freer” conditions enable the counterions to achieve very high velocity, thereby ensuring that the water solvating the counterions themselves moves very fast, triggering the significantly augmented EOS transport. Probing deeper, we can identify that the bending–tilting-driven brush deformation, enforcing the brushes to align parallel to the substrate, results from the antielectrophoretic behavior of the brushes, where, despite being positively charged, the brushes move against the electric field direction. Such an antielectrophoretic behavior of the PE brushes, which has not been reported before, can be associated with the very fast velocities of the negatively charged counterions and the electrostatic and hydrodynamic coupling of the counterions with the positive functional groups of the brushes. We anticipate that the findings of this paper will shed light on strategies for nanochannel flow, the antielectrophoretic response of charged polymer chains, and the significance of capturing the detailed chemical architecture of polyelectrolytes in nanoscale science and engineering.