Impact of Confinement and Zwitterionic Ligand Chemistry on Ion-Ion Selectivity of Functionalized Nanopores.

Membranes incorporating zwitterionic chemistries have recently emerged as promising candidates for facilitating challenging ion-ion separations. Transport of ions in such membranes predominantly occurs in hydrated nanopores lined with zwitterionic monomers. To shed light on the physics of ion-ion selectivity underlying such materials, we conducted molecular dynamics simulations of sodium halide transport in model nanopores grafted with sulfobetaine methacrylate molecules. Our results reveal that in both functionalized and unfunctionalized nanopores smaller ions prefer to reside near the pore center, while the larger ions tend to reside near the pore walls. An enhancement in the selective transport of larger anions is observed within the unfunctionalized nanopores relative to that in salt-in-water solutions. Upon functionalization of the nanopores with zwitterions (ZIs), the disparities in the anionic distribution profiles within the pores coupled with differences in the anion-ZI interactions result in a slowdown of larger anions relative to smaller anions. Increasing the ZI grafting density exacerbates these effects, further promoting the selective transport of smaller anions. Our results suggest that selectivity toward large anions can be realized by using nanoporous membranes with ZI content that is high enough to facilitate ion/water partitioning into the pores while preserving the characteristic tendency of the unfunctionalized pores to facilitate faster transport of the larger anions. On the other hand, selectivity toward smaller anions can be achieved by targeting ZI content within the pores that is high enough to significantly slow down the transport of large anions but not high enough to hinder the partitioning of ions/water molecules into the pore due to steric effects.

Role of Dielectric Drag in Circumventing the Solubility-Diffusivity Trade-off in Zwitterionic Copolymer Membranes.

Recent experiments have revealed that random zwitterionic amphiphilic copolymer (r-ZAC) membranes exhibit excellent Cl-/F- permselectivity circumventing the solubility-diffusivity trade-off. We conducted molecular dynamics simulations to investigate the origin of the experimental results on the transport of sodium halides in r-ZAC membranes. Our results indicate that the enhancement of Cl-/F- diffusivity selectivity in r-ZAC membranes (relative to that in bulk water) stems from the increase in dielectric drag dominating over the increase in Stokes drag, zwitterionic group-induced steric hindrance, and ion-polymer interactions. The importance of dielectric drag is further demonstrated by showing that reduction in ionic charges leads to a complete reversal of the diffusivity selectivity trends. We conclude that leveraging the impact of hydrophilic nanoconfinement on the dynamics of water can be utilized as a strategy to simultaneously augment solubility selectivity and diffusivity selectivity for separations, wherein the flux of the larger ionic species is desired over that of the smaller.

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

Energy generation through nanofluidics is a topic of great nanotechnological relevance. Here, we conduct all-atom molecular dynamics (MD) simulations of the transport of water and ions in a pressure-driven flow in nanochannels grafted with charged polyelectrolyte (PE) brushes and discover the possibility of simultaneous electrokinetic energy generation and flow enhancement (henceforth denoted as the electroslippage effect). Such PE-brush-functionalized nanochannels have been recently shown to demonstrate an overscreening (OS) effect (characterized by the presence of a greater number of screening counterions within the PE brush layer than needed to screen the PE brush charges), a consequent presence of excess co-ions within the PE brush-free bulk, and a co-ion-driven electroosmotic (EOS) transport in the presence of small to moderate applied axial electric fields. In this study, however, we find that the streaming current, which represents the current generated by the flow-driven downstream advection of the charge imbalance present within the electric double layer (EDL) that screens the PE brush charges, is governed by the migration of the counterions. This stems from the fact that the highest contribution to the overall streaming current arises from the region near the PE brush-water interface (where there is an excess of counterions), while the brush-free bulk yields a hitherto unreported, but small, co-ion-dictated streaming current. This downstream advection of the charge imbalance (and the resultant counterion-driven streaming current) eventually leads to the development of an electric field (streaming electric field) in the direction that is opposite the direction of the counterion-driven streaming current. The streaming current and the streaming electric field interact to generate the electrokinetic energy. Equally important, this streaming electric field induces an EOS transport, which becomes co-ion-driven, due to the presence of excess co-ions in the brush-free bulk. For the case of nanochannels grafted with negatively charged PE brushes, the streaming electric field will be in a direction that is opposite that of the pressure-driven transport, and hence the co-ion (or anion) driven EOS flow will be in the same direction as the pressure-driven transport. On the other hand, for the case of nanochannels grafted with positively charged PE brushes, the streaming electric field will be in the same direction as the pressure-driven flow, and hence the co-ion (or cation) driven EOS flow, will again be in the same direction as the pressure-driven flow. Therefore, whenever there occurs a presence of the OS and the resulting co-ion-driven EOS transport in PE brush grafted nanochannels, regardless of the sign of the charges of the PE brushes, this EOS transport will always aid the pressure-driven transport and will cause the most fascinating increase in the net volume flow rate across the nanochannel cross section, which is the electroslippage effect.

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

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 all-atom 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 co-ion-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 overscreening-inducing 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.

All-atom molecular dynamics simulations of weak polyionic brushes: influence of charge density on the properties of polyelectrolyte chains, brush-supported counterions, and water molecules.

All atom molecular dynamics (MD) simulations of planar Na+-counterion-neutralized polyacrylic acid (PAA) brushes are performed for varying degrees of ionization (and thereby varying charge density) and varying grafting density. Variation in the PE charge density (or degree of ionization) and grafting density leads to massive changes of the properties of the PE molecules (quantified by the changes in the height and the mobility of the PE brushes) as well as the local arrangement and distribution of the brush-supported counterions and water molecules within the brushes. The effect on the counterions is manifested by the corresponding variation of the counterion mobility, counterion concentration, extent of counterion binding to the charged site of the PE brushes, water-in-salt-like structure formation, and counterion-water-oxygen radial distribution function within the PE brushes. On the other hand, the effect on water molecules is manifested by the corresponding variation of water-oxygen-water-oxygen RDF, local water density, water-water and water-PE functional group hydrogen bond networks, static dielectric constant of water molecules, orientational tetrahedral order parameter, and water mobility. Enforcing such varying degree of ionization of weak polyelectrolytes is possible by changing the pH of the surrounding medium. Thus, our results provide insights into the changes in microstructure (at the atomistic level) of weak polyionic brushes at varying pH. We anticipate that this knowledge will prove to be vital for the efficient design of several nano-scale systems employing PE brushes such as nanomechanical gates, current rectifiers, etc.

Theoretical study on the massively augmented electro-osmotic water transport in polyelectrolyte brush functionalized nanoslits.

We demonstrate that functionalizing nanoslits with pH-responsive polyelectrolyte brushes can lead to extremely fast electro-osmotic (EOS) water transport, where the maximum centreline velocity and the volume flow rate can be an order of magnitude larger than these quantities in identically charged brush-free nanochannels for a wide range of system parameters. Such an enhancement is most remarkable given that the brushes have been known to retard the transport by imparting additional drag on the fluid flow. We argue that this enhancement stems from the localization of the charge density of the brush-induced electric double layer (and, hence, the EOS body force) away from the nanochannel wall (or the location of the wall-induced drag force). This ensures a much larger impact of the EOS body force triggering such fast water transport. Finally, the calculated flux values for the present brush-grafted nanochannels are found to be significantly larger than those for a wide range of nanofluidic membranes and channels, suggesting that the brush functionalization can be considered as a mechanism for enabling such superfast nanofluidic transport.

Strong stretching theory for pH-responsive polyelectrolyte brushes in large salt concentrations.

In this paper, we develop a theory for describing the thermodynamics, configuration, and electrostatics of strongly-stretched, pH-responsive polyelectrolyte (PE) brushes in the presence of large salt concentrations. The aim of the paper, therefore, is to study the properties of a PE brush in a salt concentration regime (namely, large concentrations of several molars) that has been hitherto unexplored theoretically in the context of PE brushes but can be routinely encountered in molecular scale simulations of the problem. The brushes are modelled using our recently developed augmented Strong Stretching Theory (SST), while the effect of the presence of the large salt concentration is accounted for by including the contributions of three different types of non-Poisson-Boltzmann (non-PB) effects to the free energy description of the PE brush induced electric double layer (EDL). These non-PB effects are ionic non-mean-field ion-ion correlations, solvent polarization, and the finite size effect of the ions and water dipoles. We study the individual influences of these different effects and show that the ion-ion correlations and solvent polarization effect reduce the brush height which consequently enhances the monomer density and leads to an electrostatic potential distribution of the brush induced EDL that has a larger magnitude at near-wall locations and becomes zero at shorter distances from the wall. The finite size effect, on the other hand, increases the brush height and therefore, weakens the monomer density and leads to a smaller near-wall magnitude of the EDL potential that becomes zero at larger distances from the wall. Eventually, we consider the impact of all the three non-PB effects simultaneously and show that the ion-ion correlations and solvent polarization effect dominate the size effects and dictate the overall brush configuration and the EDL electrostatics. We also point out that the influence of all the three non-PB effects becomes the largest for a larger salt concentration and a smaller bulk pH. Finally, we compare our theoretical predictions with those obtained from our recently developed all-atom MD simulation model and obtain an excellent match.

Coating for preventing nonspecific adhesion mediated biofouling in salty systems: Effect of the electrostatic and van der waals interactions.

Development of anti-biofouling coating has attracted immense attention for reducing the massively detrimental effects of biofouling in systems ranging from ship hulls and surgical instruments to catheters, implants, and stents. In this paper, we propose a model to quantify the role of electrostatic and van der Waals (vdW) forces in dictating the efficacy of dielectric coating for preventing the nonspecific adhesion mediated biofouling in salty systems. The model considers a generic charged lipid-bilayer encapsulated vesicle-like structure representing the bio-organism. Also, we consider the fouling caused by the nonspecific adhesion of the bio-organism on the substrate, without accounting for the explicit structures (e.g., pili, appendages) or conditions (e.g., surface adhesins secreted by the organisms) involved in the adhesion of specific microorganism. The model is tested by considering the properties of actual coating materials and biofouling causing microorganisms (bacteria, fungi, algae). Results show that while the electrostatic-vdW effect can be significant in anti-biofouling action for cases where the salt concentration is relatively low (e.g., saline solution for surgical instruments), it might not be effective for marine environment where the salt concentration is much higher. The findings, therefore, point to a hitherto unexplored driving mechanism of anti-biofouling action of the coating. Such an identification will also enable the appropriate choices of the coating materials (e.g., possible dielectric material with volume charge) and other system parameters (e.g., salinity of the solution for storing the surgical instruments) that will significantly improve the efficiency of the coatings in preventing the nonspecific adhesion mediated biofouling.

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

In this paper, we provide a theory to quantify the ionic current ( iion ) in nanochannels grafted with pH-responsive polyelectrolyte (PE) brushes. We consider the PE brushes to be modeled by our recently proposed augmented strong stretching theory (SST) model that improves the existing SST models by incorporating the effects of excluded volume interactions and an extended mass action law. Use of such augmented SST for this problem implies that this is the first study on computing iion in PE brush-grafted nanochannels accounting for the appropriate coupled configuration-electrostatic description of the PE brushes. iion is obtained as functions of PE brush grafting density, medium pH and salt concentration ( c∞ ), and the density of polyelectrolyte chargeable sites (PECS). For large c∞ , iion increases linearly with c∞ (as for such c∞ , iion becomes independent of the PE charge and is dominated by the bulk mobility and number density of the electrolyte ions), whereas iion is independent of c∞ at small c∞ (where the electric double layer electrostatics and the total number of ions in the system is dominated by the hydrogen ions). We further witness an enhancement of iion for smaller pH and larger grafting density at low and moderate c∞ , while there is little to no effect of the PECS density on the ionic current except for weakly grafted brushes at low c∞ . We anticipate that this study will serve as a theoretical foundation for a large number of applications that are based on the brush-induced modification of the ionic current in a nanochannel.

Electrokinetic energy conversion in nanochannels grafted with pH-responsive polyelectrolyte brushes modelled using augmented strong stretching theory.

In this paper, we develop a theory to quantify the electrokinetic energy conversion in electrolyte-filled nanochannels grafted with pH-responsive polyelectrolyte (PE) brushes. A pressure-driven flow drives the mobile electrolyte ions of the electric double layer (EDL) supported by the charged PE brushes leading to the generation of a streaming current, a streaming electric field and eventually an electrical energy. The salient feature of this study is that the brushes are described using our recently developed augmented Strong Stretching Theory (SST) model. In all the previous theoretical studies on liquid transport in PE-brush-grafted nanochannels, the brushes have either been assumed to be of constant height (independent of salt concentration or pH) or modelled using the Alexander-de-Gennes model that considers uniform monomer distribution along the brush height. Such simplifications have meant that the salt and the pH dependence of the brush height, the monomer distribution, and the resulting electrostatics have not been appropriately accounted for in the transport calculations. This paper addresses these limitations and provides a much more detailed description of the brushes while capturing the corresponding electrokinetic energy conversion. The results establish that the presence of the PE brushes ensures a localization of the average EDL charge density away from the grafting surface, thereby enabling the migration of the EDL ions with a larger background flow velocity; as a consequence, there is an enhancement of the streaming current, streaming electric field, and the resulting electrical energy generation under certain grafting densities of the PE brushes.

Interactions of gold and silica nanoparticles with plasma membranes get distinguished by the van der Waals forces: Implications for drug delivery, imaging, and theranostics.

Making a nanoparticle (NP) approach and interact with a plasma membrane (PM) through the receptor-ligand interaction is key for applications like targeted drug delivery, cellular imaging, and theranostics. In this paper, we show that the van der Waals (vdW) interactions dominate the electrostatics ensuring that a gold NP approached the PM more spontaneously as compared to a silica NP. The negative σ (charge density) of a PM induces a negative electrostatic potential at the surface of the approaching gold NP and the silica NP; however, there is very little difference between these induced values due to a small electric double layer at the physiological salt concentration (c∞). Hence there is very little difference in the electrostatic repulsion between the two cases, while the PM-NP vdW attraction is much more for the gold NP as a result of a larger Hamaker constant. Therefore, in comparison to the gold NP, the silica NP would (a) undergo a promotion of the specific adhesion and a prevention of the non-specific adhesion simultaneously for a larger σ - c∞ phase space including the physiological conditions, (b) necessitate a larger length of the ligands to trigger spontaneous receptor-ligand interactions, and (c) require a larger driving force for force-driven receptor-ligand interactions.

Electrostatics and Interactions of an Ionizable Silica Nanoparticle Approaching a Plasma Membrane.

The surface charge of the plasma membrane (PM) and the large salt content of the extracellular space ensure a significant role of the electrostatic effect dictating the interaction between the PM and an approaching nanoparticle (NP). In this article, we theoretically study the case of an ionizable silica NP approaching the PM. We witness that the surface charge of the silica NP, dictated by the surface ionization of the silica in the electrostatic environment created by the PM surface charge and the extracellular ion concentration, decreases as it approaches the PM. In other words, a silica NP is more negative away from the PM than in close proximity to the PM. Accordingly, we witness a significantly lower repulsion between the PM and NP favoring the approach and the interactions of the silica NP with the PM. Additionally, the presence of the silica NP in the vicinity of the PM induces a large nonisopotentiality, even across a fully permeable PM. We anticipate that these findings will be critically important in the better design of the widely used silica NPs for targeted drug and gene deliveries.

Revisiting the strong stretching theory for pH-responsive polyelectrolyte brushes: effects of consideration of excluded volume interactions and an expanded form of the mass action law.

In this paper, we develop a theory to account for the effect of excluded volume (EV) interactions in the strong stretching theory (SST) based description of pH-responsive polyelectrolyte (PE) brushes. The existing studies have considered the PE brushes to be present in a θ-solvent and hence have neglected the EV interactions; however, such a consideration cannot describe the situations where the pH-responsive brushes are in a "good" solvent. Secondly, we consider a more expanded form of the mass action law, governing the pH-dependent ionization of the PE molecules, in the SST description of the PE brushes. This expanded form of the mass action law considers different values of γa3 (γ is the density of chargeable sites on the PE molecule and a is the PE Kuhn length) and therefore is an improvement over the existing SST models of PE brushes as well as other theories involving pH-responsive PE molecules that always consider γa3 = 1. Our results demonstrate that the EV effects enhance the brush height by inducing additional PE inter-segmental repulsion. Similarly, the consideration of the expanded form of the mass action law would lead to a reduced (enhanced) brush height for γa3 < 1 (γa3 > 1). We also quantify variables such as the monomer density distribution, the distribution of the ends of the PE brush, and the EDL electrostatic potential and explain their differences with respect to those obtained with no EV interactions or γa3 = 1.

Highly enhanced liquid flows via thermoosmotic effects in soft and charged nanochannels.

Enhancing nanoscale liquid flows remains an existing challenge in nanofluidics. Here we propose the generation of highly augmented thermoosmotic (TOS) liquid flows in soft nanochannels (or nanochannels functionalized by grafting with end-charged polyelectrolyte or PE brushes) by employing an axial temperature gradient. The TOS transport is a combination of the induced-electric-field electroosmotic (EOS) transport and a thermo-chemioosmotic (TCOS) transport with the latter resulting from an induced pressure gradient on account of the changes associated with the imposition of the axial temperature gradient. The end-charged brushes develop an electric double layer (EDL) localized at the charged, non-grafted brush end. Depending on the system parameters, this EDL localization massively augments the influence of the EOS body force and the induced pressure-gradient resulting in a TOS transport in soft nanochannels that can be more than one order of magnitude larger than that in brush-free nanochannels. Given the existing notion that the presence of the brushes invariably reduces the flow strength, this result of massive flow augmentation is extremely significant and non-trivial serving as a paradigm shift in the study of liquid transport in brush-grafted nanochannels.

Ionic Diffusoosmosis in Nanochannels Grafted with End-Charged Polyelectrolyte Brushes.

In this paper, we develop a theory to study the imposed axial salt-concentration-gradient-driven ionic diffusioosmosis (IDO) in soft nanochannels or nanochannels grafted with end-charged polyelectrolyte (PE) brushes. Our analysis first quantifies the diffusioosmotically induced electric field, which is primarily dictated by the imposed concentration gradient (CG) with little contribution of the induced osmosis. This induced electric field triggers an electroosmotic (EOS) transport, while the net diffusioosmotic (DOS) transport results from a combination of this EOS transport and a chemiosmotic (COS) transport arising from the pressure gradient induced by the applied CG. Our results demonstrate that the DOS transport is massively enhanced in nanochannels grafted with PE brushes with weak grafting density stemming from the significantly enhanced EOS transport caused by the localization of the EOS body force away from the nanochannel walls. This augmentation is even stronger for cases where the COS transport aids the EOS transport. On the other hand, the DOS transport gets severely reduced in nanochannels grafted with dense PE brushes owing to the severity of the brush-induced additional drag force. We anticipate that these findings will help to unravel an entirely new understanding of induced electrokinetic transport in soft nanochannels.

Efficient electrochemomechanical energy conversion in nanochannels grafted with end-charged polyelectrolyte brushes at medium and high salt concentration.

We develop a theory to study the generation of the streaming potential and the resulting electrochemomechanical energy conversion (ECMEC) in the presence of pressure-driven transport in nanochannels grafted with end-charged polyelectrolyte (PE) brushes. Our theory gives a thermodynamically self-consistent coupled description of the PE-brush and the electrostatics of the electric double layer (EDL) induced by the PE charges. The end-charged brushes localize the maximum EDL charge density away from the wall, thereby enabling a larger magnitude of pressure-driven transport to stream the ions downstream. This effect is retarded by the drag force imparted by the brushes as well as by the enhanced electroosmotic transport in a direction opposite to the pressure-driven transport. An interplay of these three issues leads to highly non-trivial electrohydrodynamic transport that eventually allows us to converge on appropriate properties of the brushes (e.g., grafting density and the number of monomers) that lead to the generation of a significantly larger streaming potential and a much improved efficiency of the ECMEC as compared to the brush-free nanochannels particularly at medium and high salt concentrations.

Effect of Plasma Membrane Semipermeability in Making the Membrane Electric Double Layer Capacitances Significant.

Electric double layers (or EDLs) formed at the membrane-electrolyte interface (MEI) and membrane-cytosol interface (MCI) of a charged lipid bilayer plasma membrane develop finitely large capacitances. However, these EDL capacitances are often much larger than the intrinsic capacitance of the membrane, and all of these capacitances are in series. Consequently, the effect of these EDL capacitances in dictating the overall membrane-EDL effective capacitance Ceff becomes negligible. In this paper, we challenge this conventional notion pertaining to the membrane-EDL capacitances. We demonstrate that, on the basis of the system parameters, the EDL capacitance for both the permeable and semipermeable membranes can be small enough to influence Ceff. For the semipermeable membranes, however, this lowering of the EDL capacitance can be much larger, ensuring a reduction of Ceff by more than 20-25%. Furthermore, for the semipermeable membranes, the reduction in Ceff is witnessed over a much larger range of system parameters. We attribute such an occurrence to the highly nonintuitive electrostatic potential distribution associated with the recently discovered phenomena of charge-inversion-like electrostatics and the attainment of a positive zeta potential at the MCI for charged semipermeable membranes. We anticipate that our findings will impact the quantification and the identification of a large number of biophysical phenomena that are probed by measuring the plasma membrane capacitance.