Surface Charge Heterogeneity Research Articles

Mechanically activated and deactivated ion transport across nanopores with heterogeneous surface charge distributions

To mimic the intricate and adaptive functionalities of biological ion channels, electrohydrodynamic ion transport has been studied extensively, albeit mostly, across uniformly charged nanochannels. Here, we analyze the ion transport under coupled electric field and pressure across heterogeneously charged nanopores with oppositely charged sections on their lateral surface. We only consider such pores with symmetric hourglass-like and cylindrical shapes to focus on the effects of the non-uniform surface charge distribution. Finite-element simulations of a continuum model demonstrate that a pressure applied in either direction of the pore-axis equally suppresses or amplifies the ionic conductance, depending on the electric field polarity, by distorting the quasi-static distribution of ions in the pore. The resulting anomalous mechanical deactivation and activation of ionic current under opposite voltage biases exhibit the functional modularity of our setup, while their intensities are highly tunable, substantially greater than those of analogous behaviors in other nanochannels, and fundamentally correlated to ionic current rectification (ICR) in our pores. A detailed study of ICR subsequently reveals its counterintuitive non-monotonous variations, in the pores, with the magnitude of applied voltage and the pore length, that can help optimize their diode-like behavior. We further illustrate that while the hourglass-shaped nanopores yield the more efficient mechanical suppressors of ion transport, their cylindrical analogs are the superior rectifiers and mechanical amplifiers of ion conduction. Therefore, this article provides a blueprint for the strategic design of nanofluidic circuits to attain a robust, modular, and tunable control of ion transport under external electrical and mechanical stimuli.

Nanoplastics enhance the stability of kaolinite and affect the sorption of Pb2+ in aquatic environments

The heteroaggregation of nanoplastics with minerals has been widely investigated. However, few studies have focused on the interaction between similar negatively charged clay minerals and nanoplastics, especially how their interactions affect the migration of heavy metals. This study investigated heteroaggregation of kaolinite (a model of natural mineral) with polystyrene nanoplastics (PSNPs) and the effect of heteroaggregates on lead (Pb2+) sorption. Dynamic light scattering (DLS) measurement highlighted that PSNPs owned higher colloidal stability than kaolinite in aquatic environments. Due to the surface charge heterogeneity of kaolinite, the heteroaggregation behavior between kaolinite and PSNPs was highly pH-dependent. At pH 3.0 and 6.0, kaolinite particles with positively charged edge surfaces favored the association of PSNPs. PSNPs were found to be primarily attached on the kaolinite edge (010) surface, followed by Al-terminated (001) and Si-terminated (001¯) surfaces based on density functional theory (DFT) calculations. At pH 10.0, they had negligible interaction due to the strong electrostatic repulsion. Interestingly, kaolinite carried more negative charges after heteroaggregation with PSNPs, resulting in slower edge-to-face homoaggregation and higher dispersibility, and thus increasing the sorption capacity for Pb2+. This investigation provided a new understanding of nanoplastics interactions with negatively charged minerals and highlighted the vital role of nanoplastics in mineral stability and contaminant migration.

Dual roles of goethite coating on the transport of plastic nanoparticles in heterogeneous porous media: The significance of collector surface roughness

This study systematically examines the roles of positive goethite on the retention and release of negative plastic nanoparticles (PSNPs) with different surface functional groups (Blank, -COOH, and -NH2). It provides the first evidence for the dual roles of goethite coatings on colloid transport; e.g., increased transport caused by surface morphology modification or decreased transport due to increased surface roughness and charge heterogeneity. Although previous work has shown that goethite-coated sand increases the retention of negative colloids, this work demonstrates that collector surface roughness can also reduce the retention of PSNPs due to increased interaction energy profiles. Nonmonotonic retention of all the different functionalized PSNPs was observed in goethite-coated rough sand, and the magnitude of variations was contingent on the PSNP functionalization, the solution ionic strength (IS), and the goethite coating. The release of PSNPs with IS decrease (phase I) and pH increase (phase II) varied significantly due to differences in energy barriers to detachment, e.g., release in phase I was inhibited in both goethite-coated sands, whereas release in phase II was enhanced in coated smooth sand but completely inhibited in rough sand. The findings of this study provide innovative insight into transport mechanisms for colloidal and colloid-associated contaminants.

Molecular simulation on CO2 adsorption heterogeneity in montmorillonite nanopores with different surface charges in presence of water

Understanding the CO2 adsorption mechanism in clay nanopores is crucial for enhancing carbon capture and storage in shale reservoirs. Nonetheless, the impact of surface charge on the heterogeneity of CO2 adsorption and its subsequent effects on adsorption capacity within clay nanopores are not fully understood. By employing molecular dynamics simulations and grand canonical Monte Carlo techniques, we examine the effects of surface charge, pressure, and nanopore water on the adsorption capacity, density distribution, and free energy profile of CO2 in montmorillonite (Mt) nanopore. Utilizing density entropy as an innovative metric for adsorption heterogeneity, we shed light on how isomorphous substitution, which results in a non-uniform distribution of negative charges within Mt layers, triggers heterogeneous distribution of CO2 and water in Mt nanopore. Our findings highlight a unique correlation between the density entropy of CO2 and its adsorption capacity. The maximum adsorption capacity is strongly associated with surface charge heterogeneity in dry Mt nanopores and is governed by the heterogeneity of water distribution in wet Mt nanopores. We find that a greater heterogeneity in the CO2 distribution within the Mt nanopore system aligns with an increased maximum adsorption energy for CO2, consequently resulting in an enhanced overall adsorption capacity.

Centrifuge fractionation during purification of cellulose nanocrystals after acid hydrolysis and consequences on their chiral self-assembly

The inherent colloidal dispersity (due to length, aspect ratio, surface charge heterogeneity) of CNCs, when produced using the typical traditional sulfuric acid hydrolysis route, presents a great challenge when interpreting colloidal properties and linking the CNC film nanostructure to the helicoidal self-assembly mechanism during drying. Indeed, further improvement of this CNC preparation route is required to yield films with better control over the CNC pitch and optical properties. Here we present a modified CNC-preparation protocol, by fractionating and harvesting CNCs with different average surface charges, rod lengths, aspect ratios, already during the centrifugation steps after hydrolysis. This enables faster CNC fractionation, because it is performed in a high ionic strength aqueous medium. By comparing dry films from the three CNC fractions, discrepancies in the CNC self-assembly and structural colors were clearly observed. Conclusively, we demonstrate a fast protocol to harvest different populations of CNCs, that enable tailored refinement of structural colors in CNC films.

Investigating the influence of induced charge and surface charge heterogeneity on electrokinetic micromixing: A comparative analysis

This paper investigates the performance of a micromixer by considering the combined effects of surface charge heterogeneity and the induced charge electrokinetic (ICEK) phenomenon. With the applied electric field, surface charge heterogeneity generates a recirculation zone within the microchannel, while conductive links induce micro-vortices. The main objective is to explore the influence of various control parameters on the mixing efficiency, including zeta potential, the number and orientation of conductive links, and applied electrical potential. The generated recirculation zone and induced micro vortices introduce chaotic perturbations within the microchannel, improving mixing performance. The results demonstrate that increasing the zeta potential on heterogeneously charged patches enhances the mixing performance, albeit at the expense of a reduced average outlet velocity. Additionally, a higher number of conductive links leads to increased mixing efficiency. The combination of mounting a conductive link inside the channel and applying a heterogeneous zeta potential patch on the wall substantially increased mixing efficiency. The influence of fluid rheology on mixing performance is also investigated, revealing that pseudoplastic fluids exhibit better mixing than dilatant fluids. Finally, the combined effects of all parameters are evaluated using the Taguchi method, resulting in an optimized mixing performance for the proposed micromixer. Micromixing efficiency exceeding 98% is achieved for all types of non-Newtonian fluids under optimal parametric conditions.

Trimetallic NiZr/CoOx catalyst for the selective hydrogenation of furfural into furfuryl alcohol

Catalytic transfer hydrogenation has attracted great interest as a safe and green method to achieve biomass-based hydrogenation. In this study, a NiZr1/CoOx-400 catalyst was fabricated by in-situ structure topological transformation of layered double hydroxides (LDHs) precursors, which exhibited excellent performance in the in-situ hydrogenation of furfural (FF) using isopropanol as the hydrogen donor, with a maximum FF conversion of 83.5% in only 2 h. According to XRD, XPS and H2-TPR analyses, there was a strong interaction between different metal species in NiZr/CoOx, which increased the surface charge heterogeneity and promoted the phase transition of metal oxides, thereby favoring the generation of oxygen vacancies. Among them, LDHs structure of NiZr1/CoOx-400 catalyst was conducive to expose oxygen vacancies, which could facilitate its interaction with FF. And the Co in the catalyst similarly promoted the adsorption of FF through the coordination of the catalyst surface with the –CHO group in FF, thus contributing to its conversion. The electron-rich Zr repelled the furan ring and prevented the excessive hydrogenation, which improved its selectivity. In addition, the FF hydrogenation followed the quasi-primary kinetics with a low apparent activation energy of 18.4 kJ/mol, which was comparable to that of noble metal catalysts.

Predicting Outcomes of Nanoparticle Attachment by Connecting Atomistic, Interfacial, Particle, and Aggregate Scales.

Predicting nanoparticle aggregation and attachment phenomena requires a rigorous understanding of the interplay among crystal structure, particle morphology, surface chemistry, solution conditions, and interparticle forces, yet no comprehensive picture exists. We used an integrated suite of experimental, theoretical, and simulation methods to resolve the effect of solution pH on the aggregation of boehmite nanoplatelets, a case study with important implications for the environmental management of legacy nuclear waste. Real-time observations showed that the particles attach preferentially along the (010) planes at pH 8.5 and the (101) planes at pH 11. To rationalize these results, we established the connection between key physicochemical phenomena across the relevant length scales. Starting from molecular-scale simulations of surface hydroxyl reactivity, we developed an interfacial-scale model of the corresponding electrostatic potentials, with subsequent particle-scale calculations of the resulting driving forces allowing successful prediction of the attachment modes. Finally, we scaled these phenomena to understand the collective structure at the aggregate-scale. Our results indicate that facet-specific differences in surface chemistry produce heterogeneous surface charge distributions that are coupled to particle anisotropy and shape-dependent hydrodynamic forces, to play a key role in controlling aggregation behavior.

Quantifying the Molecular Polarization Response of Liquid Water Interfaces at Heterogeneously Charged Surfaces.

The hydration shells of proteins mediate interactions, such as small molecule binding, that are vital to their biological function or in some cases their dysfunction. However, even when the structure of a protein is known, the properties of its hydration environment cannot be easily predicted due to the complex interplay between protein surface heterogeneity and the collective structure of water's hydrogen bonding network. This manuscript presents a theoretical study of the influence of surface charge heterogeneity on the polarization response of the liquid water interface. We focus our attention on classical point charge models of water, where the polarization response is limited to molecular reorientation. We introduce a new computational method for analyzing simulation data that is capable of quantifying water's collective polarization response and determining the effective surface charge distribution of hydrated surfaces over atomistic length scales. To illustrate the utility of this method, we present the results of molecular dynamics simulations of liquid water in contact with a heterogeneous model surface and the CheY protein.

Dynamics of electrostatic interaction and electrodiffusion in a charged thin film with nanoscale physicochemical heterogeneity: Implications for low-salinity waterflooding

The slow kinetics of wettability alteration toward a more water-wetting state by low-salinity waterflooding (LSWF) in oil-brine-rock (OBR) systems is conjectured to be pertinent to the electrokinetic phenomena in the thin brine film. We hypothesize that the nanoscale physicochemical heterogeneities such as surface roughness and surface charge heterogeneity at the rock/brine interface control further the dynamics of electrodiffusion and electrostatic disjoining pressure (Πel), thus the time-scale and the magnitude of the low salinity effect (LSE). In this regard, film-scale computational fluid dynamics (CFD) simulations were performed. The coupled Poisson-Nernst-Planck (PNP) equations were solved numerically in a thin water film confined between a solid surface and oil, both negatively charged. The solid surface is representative of sandstone (quartz/kaolinite) with patchwise physicochemical heterogeneity. The electrical properties of the oil are representative of a crude-oil sample. The OBR system was initially under chemical equilibrium with high salinity (HS) brine, then was exposed to low salinity (LS) brine. The time-scale of reaching chemical equilibrium under LS, and the spatio-temporal evolution of electric potential were investigated. We find that surface roughness (introduced by quartz patches on quartz surface) increases the diffusion time up to 3-fold due to increased tortuosity. However the effect of surface roughness and surface charge heterogeneity (introduced by kaolinite patches on quartz surface) on the effective diffusion coefficient (Deff) is minor. While surface roughness and surface charge heterogeneity affect the disjoining pressure (Πel) significantly, the influence of surface roughness on Πel is more pronounced under HS than LS condition. In contrast, the effect of surface charge heterogeneity is more appreciable under LS than HS. Our findings imply that the LS effect can be enhanced in rough, heterogeneously charged systems like clayey sandstone, although its magnitude depends on the charge density of the roughness and its variation with salinity. We introduce two scaling factors, namely the effective diffusion coefficient (Deff) and the retardation coefficient (ω), to upscale the nanoscale results to pore-scale and beyond.

Electroosmotic mixing in a microchannel with heterogeneous slip dependent zeta potential

We investigate the electroosmotic mixing characteristics for flow through a hydrophobic microchannel with interfacial slip dependent heterogeneous surface charge. A comprehensive theoretical framework is developed to solve the Poisson–Boltzmann equation for the induced potential within the electrical double layer, mass and momentum conservation equations for fluid flow, and species transport equation with appropriate boundary conditions. We identify two different flow regimes based on diffusive Peclet number such that in the first regime the value of mixing efficiency is almost 100% as the recirculation zones formed due to the non-uniform surface potential provide sufficient convection mixing and the flow rate enhances with the slip length. The critical value of Peclet number increases with both the slip length and Debye parameter. In the next regime the effect of interfacial slip is significant in altering the mixing performance and the mixing efficiency decreases both with the slip and Debye parameter. The patch surface potential modulates the flow rate and mixing performance and, in this context, different range of patch surface potential for the considered physicochemical parameters is identified for which both the flow rate and mixing efficiency enhances due to the interfacial slip.

Enhanced Electroosmotic Mixing in a Wavy Micromixer Using Surface Charge Heterogeneity

Enhanced Electroosmotic Mixing in a Wavy Micromixer Using Surface Charge Heterogeneity

Detecting Individual Proteins and Their Surface Charge Variations in Solution by the Potentiometric Nanoimpact Method.

Label-free detection and analysis of proteins in their natural form and their dynamic interactions with substrates at the single-molecule level are important for both fundamental studies and various applications. Herein, we demonstrate a simple potentiometric method to achieve this goal by detecting the native charge of protein in solution by utilizing the principle of single-entity electrochemistry techniques. When a charged protein moves near the vicinity of a floating carbon nanoelectrode connected to a high-impedance voltage meter, the distinct local electrostatic potential changes induced by the transient collision event of protein, also called the "nanoimpact" event, can be captured by the nanoelectrode as a potential probe. This potentiometric method is highly sensitive for charged proteins, and low-molecular-weight proteins less than 10 kDa can be detected in low-salt-concentration electrolytes. By analyzing the shape and magnitude of the recorded time-resolved potential change and its time derivative, we can reveal the charge and motion of the protein in the nonspecific protein-surface interaction event. The charge polarity variations of the proteins at different pH values were also successfully probed. Compared with synthetic spherical nanoparticles, the statistical analysis of many single-molecule nanoimpact events revealed a large variation in the recorded transient potential signals, which may be attributed to the intrinsic protein dynamics and surface charge heterogeneity, as suggested by the finite element method and molecular dynamic simulations.

Partition and selectivity of electrolytes in cylindrical nanopores with heterogeneous surface charge

In this work, ion partitioning and selectivity in cylindrical nanopores with heterogeneous surface charges at equilibrium with reservoirs are investigated by a two-dimensional (2D) classical density functional theory (DFT). We present an efficient numerical method for the large 2D system in which the fast Hankel transform and fast Fourier transform are used to calculate convolution integrals, and a hybrid method of Picard iteration and Anderson mixing is used to solve the Euler-Lagrange equations. The performance of the 2D DFT is tested by calculating the profiles of a model electrolyte in long homogeneous cylindrical nanopores. The profiles from the 2D DFT model matches well with those from a 1D DFT, and the computing time of the hybrid iteration algorithm is six times shorter than that of pure Picard iteration. We apply the model to electrolytes in cylindrical nanopores with heterogeneous surface charges. It is found that the ion adsorption and selectivity are strongly affected by the surface charge pattern, the magnitude of the surface charge, the size of charged domains on the surface, and the pore size.

Visible-light-driven graphene supported Cu/Pd alloy nanoparticle-catalyzed borylation of alkyl bromides and chlorides in air

A highly efficient photocatalytic protocol for borylation of alkyl bromides and chlorides with graphene supported Cu/Pd alloy nanoparticles as a heterogeneous catalyst is reported. This photocatalytic system operates with visible light in air, providing a wide range of primary and secondary alkyl halides with B2pin2 or B2neop2 in high yields at low temperatures, thereby demonstrating its broad utility and functional group tolerance. The high performance is attributed to a synergistic effect of localized surface plasmon resonance (LSPR) of Cu and charge transfer from Cu to Pd due to the alloy surface charge heterogeneity. Transfer of energetic electrons from Pd to electrophilic alkyl halides lead to the formation of the alkyl radicals, which quickly react with a nucleophilic adduct of a diboron compound with base adsorbed on the positively charged Cu sites to form the corresponding borylation product.

Transport of Colloidal Particles in Microscopic Porous Medium Analogues with Surface Charge Heterogeneity: Experiments and the Fundamental Role of Single-Bead Deposition.

Understanding colloid transport in subsurface environments is challenging because of complex interactions among colloids, groundwater, and porous media over several length scales. Here, we report a versatile method to assemble bead-based microfluidic porous media analogues with chemical heterogeneities of different configurations. We further study the transport of colloidal particles through a family of porous media analogues that are randomly packed with oppositely charged beads with different mixing ratios. We recorded the dynamics of colloidal particle deposition at the level of single grains. From these, the maximum surface coverage (θmax = 0.051) was measured directly. The surface-blocking function and the deposition coefficient (kpore = 3.56 s-1) were obtained. Using these pore-scale parameters, the transport of colloidal particles was modeled using a one-dimensional advection-dispersion-deposition equation under the assumption of irreversible adsorption between oppositely charged beads and colloids, showing very good agreement with experimental breakthrough curves and retention profiles at the scale of the entire porous medium analogue. This work presents a new approach to fabricate chemically heterogeneous porous media in a microfluidic device that enables the direct measurement of pore-scale colloidal deposition. Compared with the conventional curve-fitting method for deposition constant, our approach allows quantitative prediction of colloidal breakthrough and retention via coupling of direct pore-scale measurements and an advection-dispersion-deposition model.

Different surface charged plastic particles have different cotransport behaviors with kaolinite ☆particles in porous media

The wide utilization of plastic related products leads to the ubiquitous presence of plastic particles in natural environments. Plastic particles could interact with kaolinite (one type of typical clay particles abundant in environments) and form plastic-kaolinite heteroaggregates. The fate and transport of both plastic particles and kaolinite particles thus might be altered. The cotransport and deposition behaviors of micron-sized plastic particles (MPs) with different surface charge (both negative and positive surface charge) with kaolinite in porous media in both 5 and 25 mM NaCl solutions were investigated in present study. Both types of MPs (negatively charged carboxylate-modified MPs (CMPs) and positively charged amine-modified MPs (AMPs)) formed heteroaggregates with kaolinite particles under both solution conditions examined, however, CMPs and AMPs exhibited different cotransport behaviors with kaolinite. Specifically, the transport of both CMPs and kaolinite was increased under both ionic strength conditions when kaolinite and CMPs were copresent in suspensions. While, when kaolinite and positively charged AMPs were copresent in suspensions, negligible transport of both kaolinite and AMPs were observed under examined salt solution conditions. The competition deposition sites by kaolinite (the portion suspending in solution) with CMPs-kaolinite heteroaggregates led to the increased transport both CMPs and kaolinite when both types of colloids were copresent. In contrast, the formation of larger sized AMPs-kaolinite heteroaggregates with surface charge heterogeneity led to the negligible transport of both kaolinite and AMPs when they were copresent in suspensions. The results of this study show that when plastic particles and kaolinite particles are copresent in natural environments, their interaction with each other will affect their transport behaviors in porous media. The alteration in the transport of MPs or kaolinite (either increased or decreased transport) is highly correlated with the surface charge of MPs.

Importance of surface roughness on perfluorooctanoic acid (PFOA) transport in unsaturated porous media

Understanding the subsurface transport of perfluorooctanoic acid (PFOA) is of considerable interest for evaluating its potential risks to humans and ecosystems. In this study, packed-column experiments were conducted to examine the influence of surface roughness on PFOA transport in unsaturated glass beads, quartz sand and limestone porous media. Results showed decreasing moisture content significantly increased the air-water interfacial adsorption of PFOA and led to greater retardation in all three types of porous media. Particularly, rougher surface (limestone > quartz sand > glass beads) and smaller grain size (i.e. a larger solid specific surface area, SSSA) significantly enhanced PFOA retardation under unsaturated conditions. These results were further supported by bubble column experiments and SSSA analysis of porous media, which demonstrate that except for the factors affecting PFOA transport in solid-water interface (e.g. surface charge and chemical heterogeneity), the greater retardation of PFOA during transport is attributed to the larger air-water interfacial areas associated with rougher surface and smaller grain size and hence greater interfacial adsorption of PFOA. Our results indicated the importance of surface roughness on the retention and transport of PFOA in the unsaturated zone.

Topological-Defect-Induced Surface Charge Heterogeneities in Nematic Electrolytes.

We show that topological defects in an ion-doped nematic liquid crystal can be used to manipulate the surface charge distribution on chemically homogeneous, charge-regulating external surfaces, using a minimal theoretical model. In particular, the location and type of the defect encodes the precise distribution of surface charges and the effect is enhanced when the liquid crystal is flexoelectric. We demonstrate the principle for patterned surfaces and charged colloidal spheres. More generally, our results indicate an interesting approach to control surface charges on external surfaces without changing the surface chemistry.

Heterogeneous surface charge confining an electrolyte solution.

The structure of dilute electrolyte solutions close to a surface carrying a spatially inhomogeneous surface charge distribution is investigated by means of classical density functional theory within the approach of fundamental measure theory. For electrolyte solutions, the influence of these inhomogeneities is particularly strong because the corresponding characteristic length scale is the Debye length, which is large compared to molecular sizes. Here, a fully three-dimensional investigation is performed, which accounts explicitly for the solvent particles, and thus provides insight into effects caused by ion-solvent coupling. The present study introduces a versatile framework to analyze a broad range of types of surface charge heterogeneities even beyond the linear response regime. This reveals a sensitive dependence of the number density profiles of the fluid components and of the electrostatic potential on the magnitude of the charge as well as on the details of the surface charge patterns at small scales.