Viscoelectric Effects in Nanochannel Electrokinetics

Abstract

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 viscoelectricmodified 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.

Accurate modeling of a biological nanopore with an extended continuum framework

“…67,68 The motility of water molecules is expressed by the NS equations as the fluid's viscosity. Hence, as also observed in MD simulations for water molecules near proteins 80 and confined in hydrophilic nanopores, [82][83][84] the water-solid interaction leads to a viscosity several times higher compared to the bulk values. Note that this is valid for hydrophilic interfaces only, as the lack of interaction with hydrophobic interfaces, such as carbon nanotubes, leads to a lower viscosity.…”

“…Hence, we propose an extended set of PNP-NS (ePNP-NS) equations, which improves the predictive power of the PNP-NS equations at the nanoscale and beyond infinite dilution. Our ePNP-NS framework takes into account the finite size of the ions using a size-modified PNP theory, 78 and implements spatial-dependencies for the solvent viscosity, 80,84 the ion diffusion coefficients and their mobilities. 54,79 It also includes self-consistent concentration-dependent properties (based on empirical fits to experimental data) for both all ions in terms of diffusion coefficients and mobilities, 60,87 and the solvent in terms of density, viscosity 92 and relative permittivity 93 .…”

Electroosmotic flow: From microfluidics to nanofluidics

“…On this basis, Hsu et al. investigated viscoelectric effects in nanochannels using a continuum VEDL model and indicated that viscoelectric effects are dominant over other effects such as ionic steric effects and dielectric saturation effects [56]. Due to the presence of the viscoelectric immobile layer, whose length scale becomes comparable to the channel dimension, the electroosmotic mobility can even decrease as surface charge increases, consistent with Qiao et al.…”

Low salinity effect on the recovery of oil trapped by nanopores: A molecular dynamics study

“…These slow dynamics of ions are responsible for the negligible ion depletion following the salinity reduction of the brine reservoir. The slowdown of ion diffusion is likely caused by the viscoelectric effect [34]. This effect is closely related to the modification of water molecules' orientation by highly charged solid surfaces and can slow down the diffusion of ions adjacent to these surfaces [34].…”

“…The slowdown of ion diffusion is likely caused by the viscoelectric effect [34]. This effect is closely related to the modification of water molecules' orientation by highly charged solid surfaces and can slow down the diffusion of ions adjacent to these surfaces [34]. In our simulations, we divide the Na + ions in the brine film into an inner layer (z < −2.55 nm in Fig.…”