Electrokinetics in polyelectrolyte grafted nanofluidic channels modulated by the ion partitioning effect

Abstract

The effects of ion partitioning on the electrokinetics in a polyelectrolyte grafted nanochannel, which is the representative of a soft nanochannel, are analyzed. Earlier studies in this regard have considered low polyelectrolyte layer (PEL) grafting density at the rigid nanochannel wall and, hence, an equal permittivity inside and outside the grafted layer. In order to overcome this shortcoming, the concept of Born energy is revisited. The coupled system of the modified Poisson-Boltzmann and Navier-Stokes equation is solved numerically, going beyond the widely employed Debye-Hückel linearization and low PEL densities. The complex interplay between the hydrodynamics and charge distribution, modulated by the ion partitioning effect, along with their consequent effects on the streaming potential and electrokinetic energy conversion efficiency (EKEC) have been systemically investigated. It has been observed that the ion partitioning effect reduces the EKEC in comparison to the case with equal permittivity up to a certain electrical double layer thickness after which it increases the EKEC. For a high concentration of mobile charges within the PEL, the net gain in the maximum EKEC due to the ion partitioning effect is about 10 fold that of the case when the ion partitioning effect is not considered. We delve into the various scaling regimes in the streaming potential and intriguingly point out the exact location of peaks in efficiency. The present study also reveals the possibility of improvement in streaming potential mediated energy conversion by the use of polyelectrolyte materials, which possess substantially lower dielectric permittivity than the bulk electrolyte.

Ionic current in nanochannels grafted with pH‐responsive polyelectrolyte brushes modeled using augmented strong stretching theory

“…Accordingly, we do not account for the ion partitioning effect that considers the Born energy difference between PE and electrolyte phase. Chakraborty and coworkers have first considered this effect , although for most of the other studies by the present group and other groups, this effect has been neglected.…”

“…The major weakness of most of the relevant theories stems from the fact that the electric double layer (EDL) electrostatics of the PE brushes is not obtained from the free energy minimization of the entire system that self‐consistently produces the correct brush configuration (i.e., the brush height and the monomer number density); as a consequence, the EDL electrostatics of the PE brush does not reflect the effect of the brush configuration and is based on an erroneous assumption of constant (salt and pH‐independent) brush height and uniform monomer (or equivalently, uniform distribution of the PE chargeable sites or PECS) distribution along the brush . A large number of other transport studies (e.g., liquid transport under different mechanisms) in such brush‐grafted nanochannels have also suffered from such brush‐configuration‐independent description of the PE brush EDL electrostatics .…”

Softness Induced Enhancement in Net Throughput of Non-Linear Bio-Fluids in Nanofluidic Channel under EDL Phenomenon

“…At interface of the electrolyte and PEL, the potential ( ψ ) and the gradient of the potential (∇ ψ ) satisfies the continuity in potential distribution. Since the relative permittivity of electrolyte and PEL region does not change, we here consider the permittivity of both the PEL and electrolyte to be same 26 . Below we write the boundary conditions as discussed above.…”

“…Considering all above pertinent issues involved with different bio-medical/biochemical applications, a thorough understanding of the underlying transport of non-Newtonian fluids through a soft narrow fluidic channel appears to be an important aspect for the modifications in the micro-electro-mechanical circuitry, primarily attributed to the complex interplay among different forcings associated with the underlying transport process. Accounting this aspect, a few studies as reported in the literature have delineated the physico-chemical interaction of electromechanics at small scales and fluid dynamics such as charge regulated surface, ion-partitioning effect, higher surface charge (relaxation effect) and surface potential, pH dependent charge density, Field Effect Transistor (FET) regulated surface potential, diffusion effect of ions, specific ion interaction, grafting of end charged polyelectrolyte brushes and their influential impacts on streaming current, streaming potential as well as electro-viscous effect 10 , 13 , 20 – 26 . But the non-intuitive molecular interactions of the PEL with the apparent viscosity of the non-Newtonian fluid fetches a new framework for fluid flow analysis owing to fit in the shear dependent nature of the fluid viscosity in different applications mentioned above 27 .…”

Electrokinetic flow and electric conduction of salt‐free solutions in a capillary

“…A charged solid wall bounding an ionic solution forms an electric double layer at the interface, in which the diffuse ions carry a total charge equal in magnitude but opposite in sign to that of the wall. When an ionic solution filled in a charged narrow channel is subject to an external electric field or pressure gradient that interacts with the double layer, electrokinetic phenomena such as electroosmosis and streaming potential will occur [1][2][3][4][5][6][7][8][9]. Electrokinetic flows are of many fundamental and practical interests in a variety of scientific and technological areas.…”