Specific Ion and Electric Field Controlled Diverse Ion Distribution and Electroosmotic Transport in a Polyelectrolyte Brush Grafted Nanochannel

Abstract

Controlling ion distribution inside a charged nanochannel is central to using such channels in diverse applications. Here, we show the possibility of using a charged polyelectrolyte (PE) brush-grafted nanochannel for triggering diverse nanoscopic ion distribution and nanofluidic electroosmotic transport by controlling the valence and size of the counterions (that screen the charges of the PE brushes) and the strength of an externally applied axial electric field. We atomistically simulate separate cases of fully charged polyacrylic acid (PAA) brush functionalized nanochannels with Na + , Cs + , Ca 2+ , Ba 2+ , and Y 3+ counterions screening the PE charges. Four key findings emerge from our simulations. First, we find that the counterions with a greater valence and a smaller size prefer to remain localized inside the brush layer. Second, for the case where there is an added chloride salt with the same cation (as the screening counterions), there are more coions (Cl − ions) in the brush-free bulk than counterions (for counterions Na + , Ca 2+ , Ba 2+ , Y 3+ ): this is a manifestation of the overscreening (OS) of the PE brush layer. Contrastingly, the number of Cs + ions remain higher than the Cl − ions inside the brushfree bulk, ensuring that there is no OS effect for this case. Third, large applied electric field enables a few Na + , Cs + , and Ba 2+ counterions to leave the brush layer and to go to the bulk: this makes the OS of the PE brush layer disappear for the cases of PE brushes being screened by the Na + and Ba 2+ ions. On the other hand, no such electric-field-mediated disappearance of OS is observed for the cases of Ca 2+ and Y 3+ screening counterions; we attribute this to the firm attachment of these counterions to the negatively charged monomers. Free energy associated with a counterion binding to a PE chain corroborates this diversity in the counterion-specific response to the applied electric field. Finally, we demonstrate that such diverse ion distributions, along with specific electric-field-strength-dependent ion properties, lead to (1) electroosmotic (EOS) transport in nanochannels grafted with PAA brushes screened with Cs + ions to be always counterion dominated, (2) EOS transport in nanochannels grafted with PAA brushes screened with Ca 2+ and Y 3+ ions to be always coion-dominated, and (3) EOS transport in nanochannels grafted with PAA brushes screened with Na + and Ba 2+ ions to be coion dominated for smaller electric fields and counterion dominated for larger electric fields.

Hydrophilic and Apolar Hydration in Densely Grafted Cationic Brushes and Counterions with Large Mobilities

“…Other notable studies that have employed all-atom MD simulations for studying the behavior of the uncharged polymer brushes and the brush-supported water molecules include the papers by Kubo and co-workers and Yagasaki and co-workers . Very recently, we started to explore in great detail, using all-atom MD simulations, the behavior of charged polyelectrolyte (PE) brushes and the brush-supported water molecules and the counterions. − Our simulations primarily focused on anionic PE brushes, such as poly­(acrylic acid) [PAA] or poly­(styrenesulfonate) [PSS] brushes, and yielded several interesting findings, such as identifying water-in-salt electrolyte (WISE) like behavior of the PAA-brush-supported counterions and water molecules, , the alteration of the hydrogen bonding energetics inside the PE brush layer, the role of multivalent counterions in regulating the counterion bridging effect inside the brush layer, the PE brush charge-density-dependent changes in the orientation between the PAA and PSS brushes, and co-ion-driven electrokinetic transport in nanochannels grafted with PAA brushes. − In this regard, there has been very little work in probing the behavior of cationic brushes and such brush-supported water molecules and counterions (anions) using all-atom MD simulations. Very recently, we studied PMETAC brushes using all-atom MD simulations and used machine learning methods to identify the alterations in the characteristics of the water–water hydrogen bonds inside such a brush layer .…”

All-atom molecular dynamics simulations of polymer and polyelectrolyte brushes

“…The findings of our previous study prompted us to investigate the EOF behavior inside the PAA-brush-grafted nanochannel for separate cases where the PAA brushes were screened with counterions of different valences (Na + , Cs + , Ca 2+ , Ba 2+ , and Y 3+ ). 159 Additionally, for each of these individual cases, we considered an added salt which is the chloride salt of the corresponding screening counterions. For example, for the cases where the screening counterions were Na + , Ba 2 , and Y 3+ , we considered the added salt as NaCl, BaCl 2 , and YCl 3 .…”

All-Atom Molecular Dynamics Simulations of Cationic Polyelectrolyte Brushes in the Presence of Halide Counterions

“…For example, uncharged polymer brushes and charged PE brushes have shown a variety of responses depending on the nature of the salt present in the medium as well as the nature of screening counterions (in the case of the PE brushes). For example, in the presence of multivalent screening counterions (e.g., Ca 2+ , Mg 2+ , Y 3+ ions), PE brush layers have demonstrated extensive shrinking behavior, − reduced lubricity, formation of inhomogeneous lateral structures, improved regulation of nanofluidic electroosmotic transport, etc. Similarly, cationic PE brushes have demonstrated distinct swelling behavior in the presence of halide ions (I – , Br – , Cl – , and F – ) and Hoffmeister anions .…”