  Electrokinetic transport behavior in nanochannels is different to that in
larger sized channels. Specifically, molecular dynamics (MD) simulations in
nanochannels have demonstrated two little understood phenomena which are not
observed in microchannels, being: (i) the decrease of average electroosmotic
mobility at high surface charge density, and (ii) the decrease of channel
conductance at high salt concentrations, as the surface charge is increased.
However, current electric double layer models do not capture these results. In
this study we provide evidence that this inconsistency primarily arises from
the neglect of the viscoelectric effect (being the increase of local viscosity
near charged surfaces due to water molecule orientation) in conventional
continuum models. It is shown that predictions of electroosmotic mobility in a
slit nanochannel, derived from a viscoelectric-modified continuum model, are in
quantitative agreement with previous MD simulation results. Furthermore,
viscoelectric effects are found to dominate over ion steric and dielectric
saturation effects in both electroosmotic and ion transport processes. Finally,
we indicate that mechanisms of the previous MD-observed phenomena can be
well-explained by the viscoelectric theory.
