Overscreening, Co-Ion-Dominated Electroosmosis, and Electric Field Strength Mediated Flow Reversal in Polyelectrolyte Brush Functionalized Nanochannels

Abstract

Controlling the direction and strength of nanofluidic electrohydrodyanmic transport in the presence of an externally applied electric field is extremely important in a number of nanotechnological applications. Here, we employ allatom molecular dynamics simulations to discover the possibility of changing the direction of electroosmotic (EOS) liquid flows by merely changing the electric field strength in a nanochannel functionalized with polyelectrolyte (PE) brushes. In exploring this, we have uncovered three facets of nanoconfined PE brush behavior and resulting EOS transport. First, we identify the onset of an overscreening effect: such overscreening refers to the presence of more counterions (Na + ) within the brush layer than needed to neutralize the negative brush charges. Accordingly, as a consequence of the overscreening, in the bulk liquid outside the brush layer, there is a greater number of co-ions (Cl − ) than counterions in the presence of an added salt (NaCl). Second, this specific ion distribution ensures that the overall EOS flow is along the direction of motion of the co-ions. Such coion-dictated EOS transport directly contradicts the notion that EOS flow is always dictated by the motion of the counterions. Finally, for large-enough electric fields, the brush height reduces significantly, causing some of the excess overscreeninginducing counterions to squeeze out of the PE brush layer into the brush-free bulk. As a result, the overscreening effect disappears and the number of co-ions and counterions outside the PE brush layer become similar. Despite that there is an EOS transport, this EOS transport, unlike the standard EOS transport that occurs due to the imbalance of the co-ions and counterions, occurs since a larger residence time of the water molecules in the first solvation shell of the counterions (Na + ) ensures a water transport in the direction of motion of the counterions. The net effect is the reversal of the direction of the EOS transport by merely changing the strength of the electric field.

Simultaneous Energy Generation and Flow Enhancement (Electroslippage Effect) in Polyelectrolyte Brush Functionalized Nanochannels

“…Such OS results in a highly nontrivial distribution of the mobile ions inside the nanochannel with an excess of co-ions or Cl – ions (relative to counterions or Na + ions) inside the brush-free bulk. Such an excess of co-ions in the brush-free bulk, coupled with a very weak flow strength inside the brush layer, had led to a co-ion-dominated EOS flow in the presence of low to medium electric fields, as revealed by our previous study . However, for the present case, the streaming current, caused by the left-to-right pressure-driven advection of the net charge density of the EDL induced by the charged brushes is counterion-driven (i.e., driven by the migration of the Na + ions).…”

“…Unlike the traditionally used continuum models that always predict a counterion dominated streaming current, it is difficult to predict the direction of streaming current in our system due to the presence of the OS. Our previous study has established that the OS-dictated ion distribution leads to the onset of a co-ion-driven EOS transport . This originates from the fact that there is little flow (due to the large drag) inside the PE brush layer, while there is an excess of co-ions in the brush-free bulk: as a result, the net motion of the water is toward the direction of migration of the co-ions.…”

“…While there are some studies probing the liquid transport in brush-grafted nanochannels through coarse-grained MD simulations − (they employ coarse-grained approaches to model the PE brushes and/or water molecules), all-atom MD simulations can play a significant role in capturing several highly nonintuitive phenomena associated with liquid transport in brush-functionalized nanochannels. For example, in one of our recent publications, we found that PE chains grafted on the inner walls of a nanochannel can experience overscreening or OS [a situation where a negatively (positively) charged PE brush layer is net positive (negative) due to the presence, inside the brush layer, of a larger number of counterions than needed for screening the charge of the PE brush layer], thereby giving rise to an excess of co-ions in the brush-free bulk in the presence of an added salt, and a co-ion dictated electroosmotic (EOS) flow under the application of small to moderate electric fields. In this manuscript, we employ all-atom MD simulations to study the pressure-driven transport of water in nanochannels grafted with backbone-charged PE brushes: our results focus on providing atomistic scale quantification of the streaming current, streaming electric field, the resulting electrokinetic energy generation, and the associated flow phenomenon (which is a combination of the pressure-driven flow and the induced EOS flow).…”

Non-monotonic variation of flow strength in nanochannels grafted with end-charged polyelectrolyte layers

“…Cao et al 49 studied the EOF through nanochannels with polymer patterning surfaces by MD simulations and observed that the polymer patterning induced anisotropy of the EOF when the direction of the electric field was changed. In a recent work, Das and co-workers 50 studied the EOF through nanochannels functionalized with PE brushes by all-atom MD simulations and observed that the direction of the EOF changed by changing the electric field strength.…”

Visualization of Ion|Surface Binding and In Situ Evaluation of Surface Interaction Free Energies via Competitive Adsorption Isotherms

“…Consequently, the study of molecular interactions is a shared and fundamental interest in seemingly unrelated fields, such as biophysics 1 and adhesion, 2 corrosion science, 3 and stem cell research, 4 or electro-osmosis in ion channels. 5 In essence, competing molecular interactions, such as competitions of different specific and unspecific bonds, drive subtle molecular balances and equilibria in the complex machinery of life and in technology.…”