Atypical dependence of electroosmotic transport on surface charge in a single-wall carbon nanotube

Electroosmotic phenomena is widely used to transport and mix fluids in micro- and nanoscale lab-on-a-chip applications. A single-wall carbon nanotube is a promising candidate for nanofluidic applications because of its extraordinary mechanical, electronic, and chemical properties. In this paper, we report on molecular dynamics simulation of electroosmotic transport of NaCl solution through a 5.42 nm diameter carbon nanotube with different surface charge densities. Simulation results indicate that if the carbon nanotube surface is negatively charged, a significant amount of Na+ ion is contact adsorbed on the nanotube wall and immobilized, resulting in negligible electroosmotic transport. However, if the carbon nanotube surface is positively charged, the contact adsorption of the Cl- ion is moderate and the adsorbed Cl- ions are not immobilized, thus generating a significant electroosmotic transport and a velocity slip on the carbon nanotube surface. The observed dependence of electroosmotic transport on the surface charge is significantly different from the results obtained using the conventional continuum theories, which predict that the electroosmotic transport will simply reverse its direction if the surface charge density is flipped. This anomalous dependence of electroosmotic transport on the surface charge is mainly caused by the different adsorption behavior of the counterions, which depends on the size of the counterion, the local electrostatic interactions between ion-water and ion-charged surface atoms, and on the external electric field.