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Sachar, Harnoor Singh

Publications and source records attributed to Sachar, Harnoor Singh.

18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Anion Selectivities in Zwitterion Grafted Nanopores: Effect of Zwitterion Architecture

The separation of ions of similar charge is a crucial challenge in many applications, from water treatment to precious metal recovery. Membranes with cross-linked zwitterionic amphiphilic copolymer (ZAC-X) selective layers, which feature self-assembled, zwitterion-lined nanodomains for permeation, offer unique permselectivity between monovalent anions (e.g. $\mathrm{Cl^-}$/$\mathrm{F^-}$). This has motivated studies on the mechanisms of transport and selectivity in this family of materials. In this study, we conducted molecular dynamics simulations of aqueous salt solutions within zwitterion-functionalized nanopores to elucidate the influence of dipole orientation of the ZI ligands on anion diffusivities, partitioning, and permeabilities. Here, our model compares systems with contrasting ZI organization: surface-cation-anion ($\mathrm{S\text{-}ZI^{+}\text{-}ZI^{-}}$, Motif A) and surface-anion-cation ($\mathrm{S\text{-}ZI^{-}\text{-}ZI^{+}}$, Motif B). Our results reveal that Motif A exhibits less pronounced ion pairing due to a spatial separation in the radial profiles of cations and anions. Motif B demonstrates prominent ion pairing for smaller anions owing to their overlap with cation distributions. Further, our potential of mean force profiles reveal that anion partitioning increases with anion size in both ligand motifs, whereas Motif B exhibits significantly higher partitioning selectivity towards larger anions compared to Motif A. Our results for ion diffusivities show that the self-diffusivities of both anions and cations are lower for Motif B compared to Motif A. Such trends in anion partitioning and diffusivities can be explained by differences in the interactions and steric hindrance experienced by the anionic species in Motifs A and B. Finally, our results for anion permselectivity, obtained by combining partitioning and diffusivity, indicate that partitioning trends dominate over diffusivity trends. Consequently, anion permeability increases with anion size, and ligand Motif B yields much higher permselectivity towards larger anions compared to ligand Motif A.

anions↗
DOE OSTI2024

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. Here, 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.

anions↗
DOE OSTI2023

Cation–polymer interactions and local heterogeneity determine the relative order of alkali cation diffusion coefficients in PEGDA hydrogels

Current research efforts are focused on endowing polymer membranes with ion–ion selectivity by incorporating ion–polymer interactions into materials to bias the selective partitioning and or diffusivity of one species over another. However, little is known about the impact of such interactions on the mechanisms of ion transport. In this study, we probe the influence of cation–polymer interactions on cation, anion, and salt diffusivity in a model membrane material, poly(ethylene glycol) diacrylate (PEGDA) by modeling concentrated polyethylene oxide solutions via molecular dynamics simulations. These results are compared to published experimental data for LiCl, NaCl, and KCl diffusion in PEGDA. Experimentally, the order of salt and cation diffusion coefficients for LiCl, NaCl, and KCl deviate from the order in aqueous solutions. Here, simulations identify these deviations to arise from cation–polymer coordination in the membrane. Both the fraction of bound cations and the average binding lifetime increases with decreasing cation hydration free energy (moving down the alkali series), leading to different diffusivity trends in the membrane compared to solution. However, to recover the experimentally observed order of diffusivities cations and salt in our simulations, we needed to incorporate membrane heterogeneity explicitly via a polymer charge scaling procedure. Together, our results indicate that cation–polymer interactions, as well as spatial heterogeneity within the membrane, play a critical role in dictating the observed order of alkali cation and salt diffusion coefficients in membranes.

36 MATERIALS SCIENCE↗
DOE OSTI2022

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

The response of polyelectrolytes (PEs) to applied electric fields drive applications in energetics, diagnostics, materials development, and many more. Here we employ all-atom 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. As a result, such bending versus tilting responses is commensurate with the electric-field-driven increase (decrease) in the flexibility of strongly (weakly) charged PEs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

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

Energy generation through nanofluidics is a topic of great nanotechnological relevance. In this work, 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 coions within the PE brush-free bulk, and a coion-driven electroosmotic (EOS) transport in the presence of small to moderate applied axial electric fields. Here, 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 screen 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, coion-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 to the direction of the counterion-driven streaming current. The streaming current and the streaming electric field interact to generate the electrokinetic energy. Equally importantly, this streaming electric field induces an EOS transport, which becomes coion-driven, due to the presence of excess coions 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 to the pressure-driven transport, and hence the coion (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 coion (or cation) driven EOS flow, will again be in the same direction as the pressure-driven flow. Therefore, whenever there occurs the presence of the OS and the resulting coion-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↗
DOE OSTI2021

All-Atom Molecular Dynamics Simulations of the Temperature Response of Densely Grafted Polyelectrolyte Brushes

Water acts a “good” solvent for most polyelectrolyte (PE) systems due to its high dielectric strength. Theoretical and experimental studies have highlighted some remarkably unconventional properties of strongly water-swollen PE brushes such as a reduction in brush height with increase in temperature. However, an understanding of the atomistic-scale response of the grafted PE chains as well as the local arrangement and organization of the brush-supported counterions and water molecules to changes in temperature is missing. Here, we conduct an all-atom molecular dynamics (MD) study to probe the influence of temperature on the “microstructure” of densely grafted, fully ionized polyacrylic acid (PAA) brushes. The atomistic insights obtained from our study shed light on prior experimental observations and elucidate the key role played by changes in hydrogen bonding network in dictating the hydrophilicity of the brushes. In addition to the temperature-mediated reduction in height of the PE brush layer, we study the influence of temperature on several properties of the brush-supported water molecules such as the number distribution and kinetics of water-water and water-PE hydrogen bonds (HBs), mass density, orientational tetrahedral order parameter, and the activation energy associated with the self-diffusion of water. The effect of temperature on the properties of the brush-trapped counterions is also quantified via changes in their solvation structure, counterion-water radial distribution function (RDF), and the translational mobility. Our findings unravel several hitherto unknown phenomena such as the enhanced propensity for attaining PE brush-induced “water-in-salt”-like scenarios at elevated temperatures, and an increase in the activation energy for the self-diffusion of water with increase in the degree of PE brush-induced nanoconfinement (from ‘bulk water’ to ‘sparsely grafted PE brushes’ to ‘densely grafted PE brushes’). Further, we anticipate that this work will be helpful in improving our predictive capabilities of the temperature response of water-swollen PE brushes, which would be instrumental for better design of several PE brush systems used in a multitude of applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Quantification of Mono- and Multivalent Counterion-Mediated Bridging in Polyelectrolyte Brushes

Multivalent counterion-induced bridging interactions have been identified as the key mechanism of drastic collapse of the height of polyelectrolyte (PE) brushes. In this article, we employ all-atom molecular dynamics (MD) simulations to quantify the bridging interactions in PE brushes for counterions of different sizes and valences. We identify that unlike the current notion, bridging interactions are not the sole function of the counterion valence. Rather the bridging interactions depend on the fraction of counterions (of a given type) that get physically condensed on the PE backbone as well as the size of the counterion solvation shell. These mechanisms ensure that certain monovalent counterions demonstrate much stronger bridging interactions than that witnessed for certain divalent and trivalent counterions, while certain counterions of identical valences show drastically different bridging. Here, we argue that these counterion-specific bridging interactions eventually enable not only the significant reduction of the PE brush height in presence of certain multivalent screening counterions, but may also give rise to scenarios where the brush height reduction for certain monovalent counterions is larger than certain divalent and trivalent counterions. This latter observation contradicts the experimental findings where the multivalent counterions invariably led to a larger decrease in the height of the PE brushes: we argue that this discrepancy stems from the fact that in our simulations we only consider densely grafted and short (and hence less flexible) PE brushes that hinder the formation of different laterally inhomogeneous structures (like pinned micelles and cylindrical bundles) that would have led to a larger brush height reduction (in experiments, which invariably consider longer and less densely grafted brushes, the formation of such inhomogeneous structures are primarily responsible for larger brush height reduction in presence of multivalent counterions). Finally, we also probe the dynamic properties of the counterions (i.e., their time-dependent displacements) and their bridging interactions (i.e., lifetime of bridging interactions).

42 ENGINEERING↗
DOE OSTI2021

Thermoosmotic Transport in Nanochannels Grafted with pH-responsive Polyelectrolyte Brushes Modelled Using Augmented Strong Stretching Theory

In this paper, we develop a theory to establish that the thermoosmotic (TOS) effects, induced by the application of an axial temperature gradient, leads to a massive enhancement in liquid transport in nanochannels grafted with charged polyelectrolyte (PE) brushes. Here, we quantify the TOS transport by quantifying the induced electric field and the induced TOS flow field. The different components of the electric field, namely the ionic component, the thermal component, and the osmotic component, as well as the contributions of different ions to these components, are quantified. Furthermore, we express the TOS velocity as a combination of chemiosmotic (COS), thermal, and electroosmotic (EOS) components. The COS and the thermal components augment each other and the overall strength and direction of the TOS flow are dictated by the direction and the relative strength of the EOS component. Most importantly, we compare the cases of brush-grafted nanochannels with those of the brush-free nanochannels of identical surface charge densities: the TOS transport is massively augmented in the brush-grafted nanochannels attributed to the combination of the localization of the EDL (and hence any body force that depends on the EDL charge density) away from the nanochannel wall (i.e., the location of the maximum drag force) and the presence of a possible molecular slip (experienced by the liquid) along the brush surface.

42 ENGINEERING↗
DOE OSTI2021

Overscreening, Coion-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. In this work, the net effect is the reversal of the direction of the EOS transport by merely changing the strength of the electric field.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Hydrogen Bonding and Its Effect on the Orientational Dynamics of Water Molecules inside Polyelectrolyte Brush-Induced Soft and Active Nanoconfinement

Despite being the most ubiquitous compound on Earth, the fundamental properties of water are not fully understood, especially in nanoconfinement. Densely grafted polyelectrolyte (PE) molecules attain the configuration of a “brush”: these PE brushes, due to their ability to form hydrogen bonds (HBs) with water via the PE functional groups, act as a source of soft and active nanoconfinement for the brush-trapped water molecules. In this paper, we study the effects of PE brush-induced confinement on the structure, dynamics and energetics of the water-water and water-PE HBs. Our results indicate a significant weakening of the HBs from bulk to sparsely grafted to densely grafted brushes. i.e., by increasing the degree of brush-induced nanoconfinement. We explain that this weakening of water-water HBs is caused by the disruption of the extended network of water molecules within the brush-induced nanoconfinement. This is confirmed by performing a ring structure analysis of the water molecules, which yields a reduction in the average ring size at higher degrees of brush-induced nanoconfinement (i.e., at higher brush grafting densities). Furthermore, we investigate the role of HB on the orientational dynamics of the water molecules. Here, we observe that the rotational motion of the water molecules becomes sluggish inside the PE brushes. Recent findings have indicated that the water and counterions trapped in brush-induced nanoconfinement demonstrate structures (in combination with the PE functional group) analogous to that in “water-in-salt” electrolytes that have seen extensive recent uses for Li-ion battery applications. However, the rotational dynamics of water molecules inside the brush-induced nanoconfinement is found to be distinctly different from that of conventional “water-in-salt” electrolytes in absence of any confinement; therefore, the present study will provide the necessary platform towards conceptualizing polymer-based nanoconfinement for battery applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

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. Finally, 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.

42 ENGINEERING↗
DOE OSTI2020

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

In this paper, we demonstrate that functionalizing nanoslits with pH-responsive polyelectrolyte (PE) brushes can lead to extremely fast electroosmotic (EOS) water transport, where the maximum centreline velocity and the volume ow 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. Lastly, the calculated flux values for the present brush-grafted nanochannels are found to be signi ficantly 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.

42 ENGINEERING↗
DOE OSTI2020

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

Here, 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. In time, 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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

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

In this paper, we study the diffusioosmotic (DOS) transport in a nanochannel grafted with pH-responsive polyelectrolyte (PE) brushes and establish brush-functionalization-driven enhancement in induced nanofluidic electric field and electrokinetic transport. The PE brushes are modelled using our recently developed augmented strong stretching theory (SST). We consider the generation of the DOS transport due to the imposition of a salt concentration gradient along the length of the nanochannel. The presence of the salt concentration gradient induces an electric field that has an osmotic (associated with the flow-driven migration of the ions in the induced electric double layer) and an ionic (associated with the conduction current) component. These two components evolve in a manner such that the electric field in the brush-grafted nanochannel is larger (smaller) in magnitude than that in the brush-less nanochannels for the case where the electric field is positive (negative). Furthermore, we quantify the DOS flow velocity and establish that for most of the parameter choices, the DOS velocity, which is a combination of the induced pressure-gradient-driven chemiosmotic component and the induced electric field driven electroosmotic transport, is significantly larger for the nanochannels grafted with backbone-charged PE brushes (i.e., brushes where the charge is distributed along the entire length of the brushes) as compared to brush-free nanochannels or nanochannels grafted with PE brushes containing charges on their non-grafted ends.

42 ENGINEERING↗