Like-charged Surfaces Research Articles

Debye–Hückel theory for mixtures of rigid rodlike ions and salt

Like-charged surfaces are able to attract each other if they are embedded in an electrolyte solution of multivalent rodlike ions, even if the rods are long. To reproduce this ability the Poisson-Boltzmann model has recently been extended so as to account for the rodlike structure of the mobile ions. Our model properly accounts for intraionic correlations but still neglects correlations between different rodlike ions. For sufficiently long rods, the model shows excellent agreement with Monte Carlo simulations and exhibits two minima - a depletion and a bridging minimum - in the interaction free energy. In the present work, we generalize the Poisson-Boltzmann model to systems with polydisperse rod lengths and arbitrary charge distributions along the rods, including the presence of salt. On the level of the linearized Debye-Hückel model we derive a general criterion for whether an electrolyte with given distribution of rodlike ions is able to mediate attraction between like-charged surfaces. We numerically analyze two special cases, namely the influence of salt on symmetric and asymmetric mixtures of monodisperse rodlike ions. The symmetric mixture is characterized by the presence of both negatively and positively charged (but otherwise identical) rodlike ions. For the asymmetric mixture, the system contains rodlike ions of only one type. We demonstrate that the addition of salt retains the depletion minimum but tends to eliminate the bridging minimum.

Attraction between Like Charged Surfaces Mediated by Uniformly Charged Spherical Colloids in a Salt Solution

Like-charged macromolecules repel in electrolyte solutions that contain small (i.e. point-like) monovalent co- and counterions. Yet, if the mobile ions of one species are spatially extended instead of being point-like, the interaction may turn attractive. This effect can be captured within the mean-field Poisson-Boltzmann framework if the charge distribution within the spatially extended ions is accounted for. This has been demonstrated recently for rod-like ions. In the present work, we consider an electrolyte solution that is composed of monovalent point-like salt ions and uniformly charged spherical colloids, sandwiched between two planar like-charged surfaces. Minimization of the mean-field free energy yields an integral-differential equation for the electrostatic potential that we solve numerically within the linear Debye-Huckel limit. The free energy, which we calculate from the potential, indeed predicts attractive in- teractions for sufficiently large spherical colloids. We derive an approximate analytical expression for the critical colloid size, above which attraction between like-charged surfaces starts to emerge. (doi: 10.5562/cca1824)

Electrostatic origins of polyelectrolyte adsorption: Theory and Monte Carlo simulations

Electrostatic interactions may promote or abate polyelectrolyte adsorption onto a charged surface depending on a number of interrelated factors including the surface and polymer charge densities, the salt concentration, and nonelectrostatic interactions such as van der Waals and hydrophobic forces. Even without the nonelectrostatic interactions, the electrostatic behavior of polyelectrolyte systems is often counterintuitive and cannot be explained with conventional theories of polymers or simple electrolytes. In this work, a nonlocal density functional theory (NLDFT) and Monte Carlo simulations are used together to investigate polyelectrolyte adsorption at both oppositely charged and like-charged surfaces (one due to the direct electrostatic attraction and the other due to counterion correlations). The simulation results provide a stringent test of the numerical performance of the NLDFT, in particular for systems containing multivalent counterions where electrostatic correlations are important. A systematic study of the effects of ion valence, salt concentration, and polyion chain length reveals that polyelectrolyte attraction to an oppositely charged surface is nearly a neutralization effect, little influenced by the polyion chain length and counterion valence. Neither the surface mean electrostatic potential nor the integrated local charge density shows no significant sign of charge inversion. Both theory and simulation predict polyelectrolyte adsorption onto a like-charged surface when the system contains multivalent counterions. In that case, the surface excess is sensitive to the surface charge density, the counterion valence, and the salt concentration. The surface mean electrostatic potential shows a clear evidence of charge inversion when two layers of like charges are mediated by multivalent counterions. The theoretical investigations indicate that most likely, the electrostatic correlation mediated by multivalent counterions is responsible for the layer-by-layer assembly of oppositely charged polyelectrolyte films.

The properties of dimers confined between two charged plates

We consider two like-charged planar surfaces immersed in solution of oppositely charged dimer counterions with a bond length l. To analyze this system, we extend and employ a self-consistent field theory that has been shown to be accurate from the weak to the intermediate through to the strong coupling regimes. In the limit of very short dimers, the results converge to the results for pointlike divalent ions. Near the surfaces, the dimers lie parallel to the charged plates. In the intermediate coupling regime, the dimers are aligned perpendicularly to the surface when they are a distance l from a surface. In the weak coupling regime, the interactions are only repulsive. At slightly higher couplings, there is a minimum in the variation of the free energy with distance at approximately the bond length of the dimers, which arises from bridging conformations of the dimers. In the intermediate coupling regime, an additional minimum in the free energy is observed at much smaller distances, which is due to the correlations between the dimers. For large dimer bond lengths, this minimum is metastable with respect to the previous minimum. However, as the bond length decreases, this minimum becomes the stable, while the minimum associated with the dimer bond length becomes metastable and eventually disappears. For shorter dimer bond length the attractive interaction is the result of correlations between counterions and charges on the surfaces. We find that dimers can mediate attractive interaction between like-charged surfaces in the intermediate coupling regime. The analysis of orientations confirms the bridging mechanism for sufficiently long dimers, whereas at high electrostatic couplings charge correlations contribute to the attraction.

Interaction between Charged Surfaces Mediated by Rodlike Counterions: The Influence of Discrete Charge Distribution in the Solution and on the Surfaces

The interaction between two charged surfaces, with discrete or uniform charge distributions, embedded in a solution of rodlike counterions has been studied. Monte Carlo simulations and density functional theory have been applied to study the concentration profiles of counterions and the force between the charged surfaces. We show that for low surface charge densities the repulsive force between like-charged surfaces is observed regardless of the rod length. Where high surface charge densities are present, attractive forces at surface separations related to the rod length are observed.

Encapsulation of small spherical liposome into larger flaccid liposome induced by human plasma proteins

We show that human plasma can induce the encapsulation of small spherical liposomes into larger flaccid liposomes. To explain the observed phenomena, it is proposed that the orientational ordering of charged plasma proteins induces attractive interaction between two like-charged liposome surfaces in close contact. It is observed that the encapsulation of the spherical liposome is possible only if the membrane of the target liposome is flexible enough to adapt its shape to the shape of the spherical liposome. In the theoretical model, the shapes of the two agglutinated liposomes are determined by minimisation of the sum of the adhesion energy and the membrane elastic energy. In the simulations, the membrane of liposomes is considered as an elastic structure and discretised via the finite element method using spring elements. It is shown that the observed agglutination of liposomes and encapsulation of smaller spherical liposomes into larger flaccid liposomes may be explained as a competition between the membrane deformation energy and the membrane adhesion energy.

Attraction between like-charged surfaces induced by orientational ordering of divalent rigid rod-like counterions: theory and simulations

We consider the interaction between two equally charged surfaces in an electrolyte solution composed of long divalent rigid rod-like counterions of arbitrary length. Further, we study the influence of orientational ordering of rigid rod-like counterions on the interaction between charged surfaces. Density functional theory is introduced, where the spatial distribution of charge within the divalent rod-like counterions is represented by two effective charges at a fixed distance. The result of variational procedure gives an integral differential equation for the electrostatic potential which was solved numerically. From the electrostatic potential and the concentration of counterions, the free energy of two charged surfaces interacting in a solution of rod-like counterions is calculated. For large surface charge densities and for long enough divalent rod-like counterions the minimum of the free energy is obtained at a distance between the surfaces which equals the counterion length. This indicates that a bridging mechanism might be responsible for the attraction between like-charged surfaces. The analysis of the orientational distribution function confirms that, at the minimum of the free energy, the rod-like counterions are oriented perpendicularly and thus connect the like-charged surfaces. Finally, canonical Monte-Carlo simulations confirm the theoretical calculations of the osmotic pressure between like-charged surfaces for long enough rod-like counterions.

Attractions between Like-Charged Surfaces with Dumbbell-Shaped Counterions

We study the effect of dumbbell-like counterions on the interactions between similarly charged surfaces. Via a systematic study using Monte Carlo simulations and field theory, we fully consider electrostatic correlations and ion structure and find that their intricate coupling determines the equilibrium phase behaviors. In particular, an energetic bridging mechanism is revealed to cause surface attractions for a finite range of surface separations, even in the Poisson-Boltzmann limit.

Attraction between negatively charged surfaces mediated by spherical counterions with quadrupolar charge distribution

We observed monoclonal antibody mediated coalescence of negatively charged giant unilamellar phospholipid vesicles upon close approach of the vesicles. This feature is described, using a mean field density functional theory and Monte Carlo simulations, as that of two interacting flat electrical double layers. Antibodies are considered as spherical counterions of finite dimensions with two equal effective charges spatially separated by a fixed distance l inside it. We calculate the equilibrium configuration of the system by minimizing the free energy. The results obtained by solving the integrodifferential equation and by performing the Monte Carlo simulation are in excellent agreement. For high enough charge densities of the interacting surfaces and large enough l, we obtain within a mean field approach an attractive interaction between like-charged surfaces originating from orientational ordering of quadrupolar counterions. As expected, the interaction between surfaces turns repulsive as the distance between charges is reduced.

Agglutination of like-charged red blood cells induced by binding of β2-glycoprotein I to outer cell surface

Plasma protein-mediated attractive interaction between membranes of red blood cells (RBCs) and phospholipid vesicles was studied. It is shown that β 2 -glycoprotein I ( β 2 -GPI) may induce RBC discocyte–echinocyte–spherocyte shape transformation and subsequent agglutination of RBCs. Based on the observed β 2 -GPI-induced RBC cell shape transformation it is proposed that the hydrophobic portion of β 2 -GPI molecule protrudes into the outer lipid layer of the RBC membrane and increases the area of this layer. It is also suggested that the observed agglutination of RBCs is at least partially driven by an attractive force which is of electrostatic origin and depends on the specific molecular shape and internal charge distribution of membrane-bound β 2 -GPI molecules. The suggested β 2 -GPI-induced attractive electrostatic interaction between like-charged RBC membrane surfaces is qualitatively explained by using a simple mathematical model within the functional density theory of the electric double layer, where the electrostatic attraction between the positively charged part of the first domains of bound β 2 -GPI molecules and negatively charged glycocalyx of the adjacent RBC membrane is taken into account.

Attachment of β2-glycoprotein I to negatively charged liposomes may prevent the release of daughter vesicles from the parent membrane

The temperature-induced budding of POPC-cardiolipin-cholesterol, POPC-POPS-cholesterol and POPC-POPG-cholesterol giant lipid vesicles in the presence of beta 2-glycoprotein I (beta 2-GPI) in the outer solution was studied experimentally and theoretically. The observed budding transition of vesicles was continuous which can be explained by taking into account the orientational ordering and direct interactions between oriented lipids. The attachment of positively charged beta 2-GPI to the negatively charged outer surface of POPC-cardiolipin-cholesterol, POPC-POPS-cholesterol and POPC-POPG-cholesterol giant vesicles caused coalescence of the spheroidal membrane bud with the parent vesicle before the bud could detach from the parent vesicle, i.e. vesiculate. Theoretically, the protein-mediated attraction between the membrane of a bud and the parent membrane was described as an interaction between two electric double layers. It was shown that the specific spatial distribution of charge within beta 2-GPI molecules attached to the negatively charged membrane surface may explain the observed attraction between like-charged membrane surfaces.

Surface sliding friction of negatively charged polyelectrolyte gels

The friction between two polyelectrolyte gels carrying the same or opposite sign of charges has been investigated using a rheometer. It is found that the friction was strongly dependent on the interfacial interaction between two gel surfaces. In the repulsive interaction case, especially, the friction was extremely low. The friction behavior is attempted to be described in terms of the hydrodynamic lubrication of the solvent layer between two like-charged gel surfaces, which is formed due to the electrostatic repulsion of the two gel surfaces. From the theoretical analysis (hydrodynamic mechanism), the friction behaviors were explained qualitatively, all of the experimental results, nevertheless, could not be understood well. The viscoelastic feature of the gel and the non-Newtonian behavior of water at the friction interface are considered to be important to elucidate the gel friction.

DLVO theory for like-charged polyelectrolyte and surface interactions

The mechanism of adsorption of charged polymers onto like-charged surfaces has been a matter of debates in recent years. We propose a mean-field theory for the interactions of like-charged polyelectrolyte and a plane surface in the presence of 1:1 and 2:1 electrolyte, which takes into account long-range van der Waals and double-layer forces. The theory is able to predict the optimum conditions, such as solvent composition, pH and temperature, for polyelectrolyte adsorption on like-charged surfaces as well as to account for the conformational properties of a polymer at the surfaces. An excellent agreement of the predictions of the theory with the available experimental data suggests that it can be used as a simple tool for predicting the optimum conditions for studying various polyelectrolyte properties, including their biological activity, at the surfaces. Particularly simple expressions for the polymer/surface interaction energy have been obtained in the Debye–Huckel limit. The applicability of this approximation to the biological molecules at the surfaces is discussed.

Observed and Simulated Fluid Drag Effects on Colloid Deposition in the Presence of an Energy Barrier in an Impinging Jet System

The deposition and re-entrainment behaviors of carboxylate-modified latex microspheres (0.2-, 0.5-, 1.0-, and 2.0-microm diameter) were examined in an impinging jet system on both glass and quartz substrata under a variety of environmentally relevant fluid velocities (2.12 x 10(-4) to 1.06 x 10(-3) m sec(-1)) in both the absence and the presence of an energy barrier to deposition. In the absence of an energy barrierto deposition, deposition fluxes onto glass and quartz substrata increased with increasing fluid velocity for all four microsphere sizes, in accordance with expectations from theory. In contrast, in the presence of an energy barrier to deposition, deposition efficiencies onto glass and quartz substrata decreased with increasing fluid velocity for all four microsphere sizes. Lack of re-entrainment and observed strong attachment were consistent with the expectation that deposition occurs via the primary energy minima where nanoscale surface heterogeneity locally reduces or eliminates the energy barrier to deposition. Colloid deposition onto an overall opposite-charged surface was simulated using a particle transport model with randomly distributed hetero domains that were like-charged relative to the colloid. Varying the size and number of the hetero domains showed that simulated colloid deposition efficiencies decreased with increasing fluid velocity when the hetero domains were small relative to the colloid. The simulations thereby demonstrate that observed decreases in colloid deposition efficiencies with increasing fluid velocity are consistent with the hypothesis that colloid deposition onto overall like-charged surfaces occurs at nanoscale hetero domains where repulsion is locally reduced or eliminated.

Anionic Polyelectrolyte Adsorption on Mica Mediated by Multivalent Cations: A Solution to DNA Imaging by Atomic Force Microscopy under High Ionic Strengths

Adsorption of DNA molecules on mica, a highly negatively charged surface, mediated by divalent or trivalent cations is considered. By analyzing atomic force microscope (AFM) images of DNA molecules adsorbed on mica, phase diagrams of DNA molecules interacting with a mica surface are established in terms of concentrations of monovalent salt (NaCl) and divalent (MgCl2) or multivalent (spermidine, cobalt hexamine) salts. These diagrams show two transitions between nonadsorption and adsorption. The first one arises when the concentration of multivalent counterions is larger than a limit value, which is not sensitive to the monovalent salt concentration. The second transition is due to the binding competition between monovalent and multivalent counterions. In addition, we develop a model of polyelectrolyte adsorption on like-charged surfaces with multivalent counterions. This model shows that the correlations of the multivalent counterions at the interface between DNA and mica play a critical role. Furthermore, it appears that DNA adsorption takes place when the energy gain in counterion correlations overcomes an energy barrier. This barrier is induced by the entropy loss in confining DNA in a thin adsorbed layer, the entropy loss in the interpenetration of the clouds of mica and DNA counterions, and the electrostatic repulsion between DNA and mica. The analysis of the experimental results provides an estimation of this energy barrier. We then discuss some important issues, including DNA adsorption under physiological conditions.

Structuring of Macroions Confined between Like-Charged Surfaces

We investigate the structuring of charged spherical nanoparticles and micelles (i.e., "macroions") between two surfaces as a function of bulk macroion concentration. Structuring is deduced from measured force profiles between a silica particle and a silica plate in the presence of an aqueous macroion (Ludox silica nanoparticle or sodium dodecyl sulfate micelle) solution, obtained with an atomic force microscope. We observe oscillatory force profiles that decay with separation. We find that the wavelength of the force profiles scales with the bulk number density as rho(-)(1/3), rather than with the effective macroion size. Only at very high silica nanoparticle concentration (above 10 vol %) in a low ionic strength solution does the wavelength become smaller than that predicted by the simple rho(-)(1/3) scaling; however, the original scaling is recovered upon the addition of a small amount of electrolyte. A comparison between the measured wavelength and the predicted spacing between the macroions in the bulk shows that the two variables differ in both magnitude and bulk density scaling. This finding suggests that confined macroions are more ordered than those in the bulk and the nature of this ordering is maintained over a relatively wide range of bulk concentration.

Direct Observation of Cations and Polynucleotides Explains Polyion Adsorption to Like-Charged Surfaces

We show an experimental approach for directly observing the condensation of polynucleotides and their electrolyte counterions at a liquid/solid interface. X-ray standing waves (XSW) generated by Bragg diffraction from a d = 20 nm Si/Mo multilayer substrate are used to measure the distinct distribution profiles of the polyanions and simple cations along the surface normal direction with subnanometer resolution. The 1D spatial sensitivity of this approach is enhanced by observing the XSW induced fluorescence modulations over multiple orders of Bragg peaks. We study the interesting divalent cation driven adsorption of anionic polynucleotides to anionic surfaces by exposing a hydroxyl-terminated silica surface to an aqueous solution with ZnCl2 and mercurated poly-uridylic acid (a synthetic RNA molecule). The in situ long-period XSW measurements are used to follow the evolution of both the Zn and Hg distribution profiles during the adsorption process. The conditions and physical mechanisms that govern the observed divalent cation adsorption and subsequent polynucleotide adsorption to an anionic surface are explained by a thermodynamic model that incorporates nonlinear electrostatic effects.

Electrostatically Tuned Interactions in Silica Microsphere−Polystyrene Nanoparticle Mixtures

We explore the generality of nanoparticle haloing as a novel colloidal stabilization mechanism in binary mixtures of silica microspheres and polystyrene nanoparticles. By selectively tuning their electrostatic interactions, both the initial microsphere stability and the role of nanoparticle additions are varied. Adsorption isotherm and zeta potential measurements indicate that highly charged nanoparticles exhibit a weak (haloing) association with negligibly charged microspheres, whereas they either strongly adsorb onto oppositely charged or are repelled by like-charged microsphere surfaces, respectively. Bulk sedimentation and confocal scanning fluorescence microscopy reveal that important differences in system stability emerge depending on whether the added nanoparticles serve as haloing, bridging, or depletant species.

A dynamic fountain model for lunar dust

There is much evidence to show that lunar “horizon glow” and “streamers” observed at the terminator are caused by sunlight scattered by dust grains originating from the surface. The dust grains and lunar surface are electrostatically charged by the Moon’s interaction with the local plasma environment and the photoemission of electrons due to solar UV and X-rays. This effect causes the like-charged surface and dust particles to repel each other, and creates a near-surface electric field. Previous models have explained micron-sized dust observed at ∼10 cm above the surface, by suggesting that charged grains “levitate” in the local electric field; however this cannot account for observations of 0.1 μm-scale grains at ∼100 km altitude. In order to explain the high-altitude dust observations, we propose a dynamic “fountain” model in which charged dust grains follow ballistic trajectories, subsequent to being accelerated upward through a narrow sheath region by the surface electric field. These dust grains could affect the optical quality of the lunar environment for astronomical observations and interfere with exploration activities.

Friction of Gels. 7. Observation of Static Friction between Like-Charged Gels

An extensive static friction has been observed when two like-charged polyelectrolyte gels are slid over each other in water. The two like-charged gel surfaces could not slip with each other at the initial shearing until the shear stress acting on the interface exceeded a certain critical value. The critical yield shear stress (static friction) and strain did not show a distinct dependence on the shearing rate but decreased with increasing temperature. The value of the static friction also increased with the increase in the normal pressure and effect of pressure becomes more substantial at a higher temperature. Furthermore, the static friction decreased 3−4 times when the counterions of the polyelectrolyte gels were changed from Na+ to Cs+. The observation of the static friction indicates that our previously proposed repulsion−adsorption model, which predicts a hydrodynamic mechanism with no static friction for two like-charged hydrogels in pure water, requires modification. Possible origins of the static ...