Like-charged Surfaces Research Articles

Electric double layer of electrons: Attraction between two like-charged surfaces induced by Fermi–Dirac statistics

Recently we reported that Fermi–Dirac statistics of electrons contained between two oppositely charged surfaces separated by the order of nanometers create overlapping electric double layers with repulsive total force between the surfaces. Here, we present a new branch of solutions to the same variational problem resulting in higher energy densities and identify regions of the phase space where the force between two like-charged surfaces is attractive.

Electrostatic Adsorption of Uniformly Charged Electrolytes within Like-charged Electrodes

The classical-fluids density functional theory has been developed for studying the structural and the electrical properties of electrolyte solutions containing uniformly charged hard-spherical ions. The modified fundamental-measure theory has been used to evaluate the hard-sphere contribution. The mean-field approximation has been employed to calculate the cross correlation between the hard sphere contribution and the Coulomb interaction. The Poisson equation for ions carrying charges that are spatially separated has been solved. The present theory shows reasonably good agreement with the corresponding Monte Carlo simulation results. The calculated results show that the attraction between like-charged planar surfaces is the result of the intra-ionic correlation and depends strongly on the ion size, valence, mole fraction, and charge distribution of electrolytes.

Charged Surface Regulates the Molecular Interactions of Electrostatically Repulsive Peptides by Inducing Oriented Alignment.

Regulation of molecular orientation of charged dipeptides and involved interactions by electrostatic repulsion from like-charged surfaces were studied using all-atom molecular dynamics simulations. It was found that a charged surface can induce oriented alignment of like-charged peptides, and the oriented alignment leads to enhanced electrostatic repulsion between the peptide molecules. The findings are consistent with previous experimental results about the inhibition of charged protein aggregation using like-charged ion-exchange resin. Furthermore, the simulations provided molecular insights into this process, and demonstrated the distinct regulation effect of like-charged surfaces on the molecular interactions between peptides that possess an electric dipole structure. Both the charged surface and the electric dipole structure of peptides were confirmed to be crucial for the regulation. The research is expected to facilitate the rational design of surfaces or devices to regulate the behavior of amphoteric molecules such as proteins for both in vivo and in vitro applications, which would contribute to the regulation of protein-protein interactions and its application in life science and biotechnology.

Spermidine-Induced Attraction of Like-Charged Surfaces Is Correlated with the pH-Dependent Spermidine Charge: Force Spectroscopy Characterization.

The ubiquitous molecule spermidine is known for its pivotal roles in the contact mediation, fusion, and reorganization of biological membranes and DNA. In our model system, borosilicate beads were attached to atomic force microscopy cantilevers and used to probe mica surfaces to study the details of the spermidine-induced attractions. The negative surface charges of both materials were largely constant over the measured pH range of pH 7.8 to 12. The repulsion observed between the surfaces turned into attraction after the addition of spermidine. The attractive force was correlated with the degree of spermidine protonation, which changed from +3 to +1 over the measured pH range. The force was maximal at pH 7.8. To explain the observed pH and spermidine concentration dependence, two different theoretical approaches were used: a chemical model of the charge equilibrium of spermidine and Monte-Carlo simulations of the orientation of the rodlike spermidine molecules in the gap between the borosilicate and mica surfaces. Monte-Carlo simulations of the orientational ordering of the rodlike spermidine molecules suggested the induction of attractive interactions between the surfaces if the gap was bridged by the molecules. For larger gaps, the orientational distribution function of the spermidine molecules predicted a considerable degree of parallel attachment of the molecules to the surfaces, resulting in reduced effective surface charge densities of both surfaces, which reduced their electrostatic repulsion.

Note: Density functional theory for uniformly charged hard-sphere ions

The density function theory has been proposed for studying the structural properties of electrolytes containing uniformly charged hard-spherical ions. The calculated result shows good agreement with the corresponding Monte Carlo simulation data of Bohinc et al. [J. Chem. Phys. 145, 234901 (2016)]. The results confirm that the attraction between like-charged planar surfaces is the results of the intra-ionic correlation and depends strongly on the charge distribution of hard-sphere ions.

Adsorption of silica colloids onto like-charged silica surfaces of different roughness

Particle adsorption was explored in a model optical polishing system, consisting of silica colloids and like-charged silica surfaces. The adsorption was monitored in situ under various suspension conditions, in the absence of surfactants or organic modifiers, using a quartz crystal microbalance with dissipation monitoring (QCM-D). Changes in surface coverage with particle concentration, particle size, pH, ionic strength and ionic composition were quantified by QCM-D and further characterized ex situ by atomic force microscopy (AFM). A Monte Carlo model was used to describe the kinetics of particle deposition and provide insights on scaling with particle concentration. Transitions from near-zero adsorption to measurable adsorption were compared with equilibrium predictions made using the Deraguin-Verwey-Landau-Overbeek (DLVO) theory. In addition, the impact of silica surface roughness on the propensity for particle adsorption was studied on various spatial scale lengths by intentionally roughening the QCM sensor surface using polishing methods. It was found that a change in silica surface roughness at the AFM scale from 1.3nm root-mean-square (rms) to 2.7nmrms resulted in an increase in silica particle adsorption of 3-fold for 50-nm diameter particles and 1.3-fold for 100-nm diameter particles—far exceeding adsorption observed by altering suspension conditions alone, potentially because roughness at the proper scale reduces the total separation distance between particle and surface.

Poisson-Boltzmann model of electrolytes containing uniformly charged spherical nanoparticles.

Like-charged macromolecules typically repel each other in aqueous solutions that contain small mobile ions. The interaction tends to turn attractive if mobile ions with spatially extended charge distributions are added. Such systems can be modeled within the mean-field Poisson-Boltzmann formalism by explicitly accounting for charge-charge correlations within the spatially extended ions. We consider an aqueous solution that contains a mixture of spherical nanoparticles with uniform surface charge density and small mobile salt ions, sandwiched between two like-charged planar surfaces. We perform the minimization of an appropriate free energy functional, which leads to a non-linear integral-differential equation for the electrostatic potential that we solve numerically and compare with predictions from Monte Carlo simulations. Nanoparticles with uniform surface charge density are contrasted with nanoparticles that have all their charges relocated at the center. Our mean-field model predicts that only the former (especially when large and highly charged particles) but not the latter are able to mediate attractive interactions between like-charged planar surfaces. We also demonstrate that at high salt concentration attractive interactions between like-charged planar surfaces turn into repulsion.

Simulations of Polyelectrolyte Adsorption to a Dielectric Like-Charged Surface.

We explore, using the recently developed efficient Monte Carlo simulation method, the interaction of an anionic polyelectrolyte solution with a like-charged dielectric surface. In addition to polyions, the solution also contains salt with either monovalent, divalent, or trivalent counterions. In agreement with recent experimental observations, we find that multivalent counterions can lead to strong adsorption of polyions onto the surface. On the other hand, addition of a 1:1 electrolyte diminishes the adsorption induced by the multivalent counterions. Dielectric discontinuity at the interface is found to play only a marginal role in polyion adsorption.

Image-induced overcharging in the weakly charged surfaces

Electrostatic interactions at dielectric boundaries can bring about novel properties which are vital not only to understand the complex behavior at biological interfaces but also to design desirable interfacial processes for a broad variety of practical applications. Here, we aim to provide comprehensive information by Monte Carlo simulations about the annihilation and formation of the image-induced overcharging effect, a new class of charge accumulation phenomena resulting from an excess of adsorbed ions on the like-charged surfaces. It is observed that whether the image-charge interactions are repulsive or attractive, the buildup of overcharging depends critically on both the concentration and the charge-asymmetry of electrolytes. On the other hand, overcharging is gradually cancelled with an increase in the size of ions and the concentration of monovalent salt as well as in the magnitude of surface charge density. Also, our simulations show that the image-induced overcharging effect cannot promote charge reversal. These findings are especially relevant to colloidal stability and dispersibility, and will hopefully serve as a foundation for the development of adequate theories.

Atomic force microscopy as a tool to study the adsorption of DNA onto lipid interfaces.

The Atomic Force Microscopy (AFM) technique appears as a central tool for the characterization of DNA adsorption onto lipid interfaces. Regardless of the huge number of surveys devoted to this issue, there are still fascinating phenomena in this field that have not been explored in detail by AFM. For instance, adsorption of DNA onto like-charged lipid surfaces mediated by cations is still not fully understood even though it is gaining popularity nowadays in gene therapy and nanotechnology. Studies related to the complexation of DNA with anionic lipids as a non-viral gene delivery vehicle as well as the formation of self-assembled nanoscale DNA constructs (DNA origami) are two of the most attractive systems. Unfortunately, molecular mechanisms underlying the adsorption of DNA onto anionic lipid interfaces remain unclear so far. In view of that, AFM becomes an appropriate technique to provide valuable information to understand the adsorption of DNA to anionic lipid surfaces. As a second part of this review we provide an illustrative example of application of the AFM technique to probe the DNA adsorption onto a model lipid monolayer negatively charged. Microsc. Res. Tech. 80:11-17, 2017. © 2016 Wiley Periodicals, Inc.

On the electrostatics of DNA in chromatin

We examine the interaction between DNA molecules immersed in an aqueous solution of oppositely charged, trivalent spermidine molecules. The DNA molecules are modeled as planar, likecharged surfaces immersed in an aqueous solution of multivalent, rod-like ions consisting of rigidly bonded point charges. An approximate field theory is used to determine the properties of this system from the weak to the intermediate through to the strong coupling regimes. In the weak coupling limit, the interaction between the charged surfaces is only repulsive, whereas in the intermediate coupling regime, the rod-like ions with spatial charge distribution can induce attractive force between the charged surfaces. In the strong coupling limit, the inter-ionic charge correlations induce attractive interaction at short separations between the surfaces. This theoretical study can give new insights in the problem of interaction between DNA molecules mediated by trivalent spermidine molecules.

Polyelectrolyte adsorption onto like-charged surfaces mediated by trivalent counterions: a Monte Carlo simulation study.

Both experiments and theory have evidenced that multivalent cations can mediate the interaction between negatively charged polyelectrolytes and like-charged objects, such as anionic lipoplexes (DNA-cation-anionic liposome complexes). In this paper, we use Monte Carlo simulations to study the electrostatic interaction responsible for the trivalent-counterion-mediated adsorption of polyelectrolytes onto a like-charged planar surface. The evaluation of the Helmholtz free energy allows us to characterize both the magnitude and the range of the interaction as a function of the polyelectrolyte charge, surface charge density, [3:1] electrolyte concentration, and cation size. Both polyelectrolyte and surface charge favor the adsorption. It should be stressed, however, that the adsorption will be negligible if the surface charge density does not exceed a threshold value. The effect of the [3:1] electrolyte concentration has also been analyzed. In certain range of concentrations, the counterion-mediated attraction seems to be independent of this parameter, whereas very high concentrations of salt weaken the adsorption. If the trivalent cation diameter is doubled the adsorption moderates due to the excluded volume effects. The analysis of the integrated charge density and ionic distributions suggests that a delicate balance between charge inversion and screening effects governs the polyelectrolyte adsorption onto like-charged surfaces mediated by trivalent cations.

Physicochemical Factors Controlling the Activity and Energy Coupling of an Ionic Strength-gated ATP-binding Cassette (ABC) Transporter

Cells control their volume through the accumulation of compatible solutes. The bacterial ATP-binding cassette transporter OpuA couples compatible solute uptake to ATP hydrolysis. Here, we study the gating mechanism and energy coupling of OpuA reconstituted in lipid nanodiscs. We show that anionic lipids are essential both for the gating and the energy coupling. The tight coupling between substrate binding on extracellular domains and ATP hydrolysis by cytoplasmic nucleotide-binding domains allows the study of transmembrane signaling in nanodiscs. From the tight coupling between processes at opposite sides of the membrane, we infer that the ATPase activity of OpuA in nanodiscs reflects solute translocation. Intriguingly, the substrate-dependent, ionic strength-gated ATPase activity of OpuA in nanodiscs is at least an order of magnitude higher than in lipid vesicles (i.e. with identical membrane lipid composition, ionic strength, and nucleotide and substrate concentrations). Even with the chemical components the same, the lateral pressure (profile) of the nanodiscs will differ from that of the vesicles. We thus propose that membrane tension limits translocation in vesicular systems. Increased macromolecular crowding does not activate OpuA but acts synergistically with ionic strength, presumably by favoring gating interactions of like-charged surfaces via excluded volume effects.

Testing Theories of DNA Interaction and Condensation

By measuring and comparing DNA force curves under different temperature and ionic conditions, we investigate DNA condensation and interaction in dense aggregates. Oriented or unoriented bundles of parallel hexagonally arranged B-form DNA double helices are osmotically stressed in solutions of multivalent condensing counterions and simple monovalent salts. Helices are pushed together by known osmotic pressures. At each pressure, interaxial spacings are measured by x-ray diffraction. We observe osmotically induced phase transitions (DNA condensation) in force curves over a critical range of counterion concentrations that depend on the valence and type of the condensing counterion as well as the type and concentration of monovalent salt. We also determine critical osmotic pressures required to condense DNA. At certain concentrations of specific counterions, critical pressures can be temperature dependent. We investigate this dependence to distinguish entropic and enthalpic components of interaction. This allows determination of the binding properties of specific counterions to the DNA under osmotic pressures that induce phase transitions. In this way we can evaluate theories of DNA interaction in the presence of multivalent cations. A range of theories use diverse approximations to model and predict DNA interaction. Their different parameters are examined through controlled experimental variables. Mean-field electrostatic theories model DNA as a uniformly charged rod and neglect counterion correlations. They always predict repulsion between like-charged homogeneous surfaces and therefore cannot explain attractive forces. In the opposite case, under the assumption that attractive counterion correlations are very strong, counterions might form a Wigner crystal in a bundle of DNA helices. Formulation of electrostatic interaction that includes helical structure and distribution patterns of adsorbed counterions also predicts attraction and rationalizes counterion specificity of DNA condensation. These models can be distinguished, critically tested, and compared with the hydration force interpretation of counterion-induced condensation.

Interactions between charged surfaces mediated by molecules with spatially distributed charges

A short review of recent theoretical advances in studies of the interaction between highly charged systems is presented. Such a system could not be described by the mean field theory. More advanced methods have to be used in order to introduce the correlations between highly charged particles. In this work I focus on the system of highly charged surfaces, separated by a solution of molecules with spatially distributed charge. Two different representations of the molecular shape will be considered: rod-like and spherical. The system will be theoretically described by the density functional theory. For sufficiently long molecules and large surface charge densities, an attractive force between like-charged surfaces arises due to the spatially distributed charges within the molecules. The added salt has influence on the condition for the attractive force between like-charged surfaces. The theoretical results will be compared with Monte Carlo (MC) simulations. Recent measurements with multivalent rigid rod-like particles will be discussed.

Effect of the Degree of Dissociation of Molecules in a Monolayer at an Air/Water Interface on the Force Between the Monolayer and a Like-Charged Particle in the Subphase

We used the monolayer particle interaction apparatus to measure the force between a monolayer of stearic acid or octadecanol at the air/water interface and a colloidal silica sphere. The silica sphere approached the monolayer from the aqueous subphase. The aim was to analyze how the magnitude of the charge of a deformable interface affects the interaction between that interface and a like-charged hard particle. The charge density of the stearic acid monolayer was controlled by adjusting the pH (5.8-9) and the surface pressure. The octadecanol monolayer acted as a reference; the alcohol headgroup did not dissociate between pH 5.8-9.0. Stable monolayers of dissociated stearic acid molecules were formed at the air/water interface by dissolving stearic acid into the subphase to give a saturated concentration at each pH value studied. The approach force curve showed that the electrostatic repulsion increased with an increasing degree of dissociation and therefore the charge of the monolayer. The strength of the repulsion corresponded to that measured between two like-charged hard surfaces, but the apparent range of the repulsion was larger for a deformable interface. Retracting force curves displayed a significant adhesion, whose magnitude and range depended on the surface pressure and subphase pH.

Adsorption of Spherical Polyelectrolyte Brushes: from Interactions to Surface Patterning

Abstract Adsorption of colloidal particles constitutes an attractive route to tailor the properties of surfaces. However, for efficient material design full control over the particle-substrate interactions is required. We investigate the interaction of spherical polyelectrolyte brushes (SPB) with charged substrates based on adsorption studies and atomic force spectroscopy. The brush layer grafted from the colloidal particles allows a precise adjustment of their adsorption behavior by varying the concentration of added salt. We find a pronounced selectivity between oppositely and like-charged surfaces for ionic strengths up to 10 mM. Near the transition from the osmotic to the salted brush regime at approximately 100 mM attractive secondary interactions become dominant. In this regime SPB adsorb even to like-charged surfaces. To determine the adhesion energy of SPB on charged surfaces directly, we synthesize micrometer-sized SPB. These particles are used in colloidal probe AFM studies. Measurements on oppositely charged surfaces show high forces of adhesion for low ionic strengths that can be attributed to an entropy gain by counterion release. Transferring our observations to charge patterned substrates, we are able to direct the deposition of SPB into two-dimensional arrays. Considering that numerous chemical modifications have been reported for SPB, our studies could open exiting avenues for the production of functional materials with a hierarchical internal organization.

Computational evidence of two driving mechanisms for overcharging in an electric double layer near the point of zero charge

We have adopted an ensemble Monte Carlo simulation method to systematically verify two physical driving mechanisms responsible for overcharging which refers to the adsorption of an effective charge onto a like-charged planar surface around the point of zero charge within the primitive model of mixed electrolytes with varying salt concentrations. One is electrostatic in character dominated by dielectric images and the other is purely entropic in origin by ionic size asymmetry effects, of which the former has never been reported both theoretically and experimentally and the latter could be interpreted satisfactorily in terms of available theoretical approaches. The electrostatically driven mechanism is found to critically depend on the ionic sizes while the entropically driven mechanism occurs with almost the same efficiency in a relative wide range of surface charge density. Depending on the delicate interplay between charge and steric correlations, the two distinct driving mechanisms may cooperatively give rise to a more pronounced overcharging process.

Calcium Mediated Polyelectrolyte Adsorption on Like-Charged Surfaces

Monte Carlo simulations within the primitive model of calcium-mediated adsorption of linear and comb polyelectrolytes onto like-charged surfaces are described, focusing on the effect of calcium and polyion concentrations as well as on the ion pairing between polymers and calcium ions. We use a combination of Monte Carlo simulations and experimental data from titration and calcium binding to quantify the ion pairing. The polymer adsorption is shown to occur as a result of surface overcharging by Ca(2+) and ion pairing between charged monomers and Ca(2+). In agreement with experimental observations, the simulations predict that the polymer adsorption isotherm goes through a maximum as the calcium or the polymer concentration is increased. The non-Langmuir isotherms are rationalized in terms of charge-charge correlations.

The Poisson-Helmholtz-Boltzmann model

We present a mean-field model of a one-component electrolyte solution where the mobile ions interact not only via Coulomb interactions but also through a repulsive non-electrostatic Yukawa potential. Our choice of the Yukawa potential represents a simple model for solvent-mediated interactions between ions. We employ a local formulation of the mean-field free energy through the use of two auxiliary potentials, an electrostatic and a non-electrostatic potential. Functional minimization of the mean-field free energy leads to two coupled local differential equations, the Poisson-Boltzmann equation and the Helmholtz-Boltzmann equation. Their boundary conditions account for the sources of both the electrostatic and non-electrostatic interactions on the surface of all macroions that reside in the solution. We analyze a specific example, two like-charged planar surfaces with their mobile counterions forming the electrolyte solution. For this system we calculate the pressure between the two surfaces, and we analyze its dependence on the strength of the Yukawa potential and on the non-electrostatic interactions of the mobile ions with the planar macroion surfaces. In addition, we demonstrate that our mean-field model is consistent with the contact theorem, and we outline its generalization to arbitrary interaction potentials through the use of a Laplace transformation.