Charge-Density-Specific Response of Grafted Polyelectrolytes to Electric Fields: Bending or Tilting?

Abstract

The response of polyelectrolytes (PEs) to applied electric fields drives applications in energetics, diagnostics, materials development, and many more. Here we employ allatom molecular dynamics (MD) simulations to probe the response of grafted PE brushes to axial electric fields. For PEs with large charge densities, the electric field triggers a left−right asymmetric distribution of counterions around the PE backbone: consequently, depending on the location (left or right), there is an unequal screening and an unequal force on the PE functional groups causing a bending-driven brush height reduction. However, for the weakly charged PEs, the electric field causes a uniform distribution of the counterions across the brush, enforcing a uniform (without left−right asymmetry) partially unscreened PE brush: therefore, all the brush segments experience similar force causing a brush tilting-driven brush height reduction. Such bending versus tilting responses is commensurate with the electric-field-driven increase (decrease) in the flexibility of strongly (weakly) charged PEs.

Machine learning enabled quantification of the hydrogen bonds inside the polyelectrolyte brush layer probed using all-atom molecular dynamics simulations

“…While there has been significant experimental, [19][20][21][22][23] theoretical, [24][25][26][27][28][29][30] and simulation [31][32][33][34] studies focused on probing such responsiveness and behavior of the PE molecules as functions of the water and ion properties, a pinpointed atomistic understanding of the behavior of the brush supported water molecules and ions has remained very limited. Only recently, there have been efforts to study the PE brushes and the brush supported ions and water molecules using all-atom molecular dynamics (MD) simulations: [35][36][37][38][39][40] the utility of such an approach is that they provide unprecedented fundamental understanding about the atomistic structure, properties, and behaviors of brushsupported ions and water molecules, which in turn can be leveraged for a variety of potential applications (such as coion-driven electroosmotic transport 41 and simultaneous energy harvesting and flow augmentation 42 in PE brush functionalized nanochannels).…”

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

“…The electric field reduces the overall brush height [see Figure b]. The mechanism of the electric-field-mediated brush height reduction for the case of PAA PE brushes has been discussed in our recent paper . The axial electric field introduces an asymmetry in the structure of the complex formed between the PAA PE monomer and the counterion bound (condensed) to the PAA PE monomer: this complex gets oriented in the direction of the applied electric field .…”

“…This particular nature of the ion distribution (OS of the PE brush layer) meant an excess of coions in the brush-free bulk (as the system needed to be electroneutral), enforcing a most remarkable coion-dominated EOS transport. However, an increase in the electric field strength led to a brush height reduction (we described the detailed mechanism of such reduction in a follow-up paper) and disappearance of the OS effect, eventually triggering a counterion-dictated EOS transport (therefore, we also encounter an electric-field-strength-mediated reversal of EOS flow direction). In another study, we showed that such OS effect and the consequent coion-dominated EOS transport can be leveraged to trigger a most interesting electroslippage effect (which refers to simultaneous electrokinetic energy generation and flow augmentation) in the presence of an applied pressure-driven flow in such brush-functionalized nanochannels.…”

“…39 The axial electric field introduces an asymmetry in the structure of the complex formed between the PAA PE monomer and the counterion bound (condensed) to the PAA PE monomer: this complex gets oriented in the direction of the applied electric field. 39 Such a response of the PE-monomer-counterion complex makes the PE chain sparser in the direction of the applied electric field and leads to a brush height reduction in the presence of the applied electric field. 39 The reduced brush height means that the volume available for the coions and counterions present within the brush layer is also reduced, thereby forcing them to get squeezed out from the brush layer.…”

“…39 Such a response of the PE-monomer-counterion complex makes the PE chain sparser in the direction of the applied electric field and leads to a brush height reduction in the presence of the applied electric field. 39 The reduced brush height means that the volume available for the coions and counterions present within the brush layer is also reduced, thereby forcing them to get squeezed out from the brush layer. For the cases of Na + and Ba 2+ , the application of the electric field enforces a larger number of counterions to get squeezed out from inside the brush layer [leading to a larger charge contribution of the counterions in brush-free bulk, see Figure 2a,d], eventually enforcing Δq = q counter − q coion > 0 in brush-free bulk, which is tantamount to the disappearance of the OS effect [see Figure 2f].…”