Presence Of Multivalent Counterions Research Articles

Unveiling the Role of Electrostatic Forces on Attraction between Opposing Polyelectrolyte Brushes.

Electrostatic interaction and molecular excluded-volume effects are responsible for a plethora of nonintuitive phenomena in soft-matter systems, including local charge inversion and attraction between similar charges. In the current work, we study the surface forces and swelling behavior of opposing polyelectrolyte brushes using a classical density functional theory that accounts for electrostatic and excluded-volume correlations. We observe that the detachment pressure between similarly charged brushes is sensitive to salt concentration in both the osmotic and salted regimes and can be negative in the presence of multivalent counterions. A comparison of the theoretical results with the mean-field predictions unravels the role of correlation effects in determining the surface forces and brush structure. For systems containing multivalent counterions, the detachment pressure attains negative values at an intermediate brush-brush separation, and the attractive region in the pressure vs distance plot is magnified in terms of both the depth and width of attraction with increasing counterion valency. However, the interbrush attraction vanishes when the size-induced correlations are switched off. We also investigated the role of counterion size and polymer chain length on the detachment pressure. It is found that smaller counterions are more effective in neutralizing the polymer charge than bigger counterions, leading to a reduced interbrush repulsion and, in some cases, attraction between like-charged brushes at intermediate distances. Meanwhile, varying the chain length of the grafted polymers only shifts the location of the attraction basin, with little influence on the interaction strength. The theoretical predictions show qualitative agreement with experimental observations and offer valuable insights into the interaction between similarly charged polymer brushes in the presence of multivalent ions.

Conformational Behavior of Single Circular Semiflexible Polyelectrolyte in the Presence of Multivalent Counterions

Conformational Behavior of Single Circular Semiflexible Polyelectrolyte in the Presence of Multivalent Counterions

The aggregation and charging of natural clay allophane: Critical coagulation ionic strength in the presence of multivalent counter-ions

Allophane is an amorphous aluminosilicate clay mineral which is typically found in young volcanic ash soils. The aggregation-dispersion and charging behaviors of allophane may affect the usability and protection of the volcanic ash soils and surrounding environment. To understand the aggregation-dispersion and charging of natural allophane, dynamic and electrophoretic light scattering experiments were carried out at pH 5 as a function of potassium salt concentration and counter-ion (Cl - , SO 4 2- , Fe(CN) 6 3- , and Fe(CN) 6 4- ) valence. The temporal increase of particle hydrodynamic diameters and electrophoretic mobilities (EPM) were measured, and the stability ratio, critical coagulation concentration (CCC), critical coagulation ionic strength (CCIS), zeta potential, and surface charge density were further obtained. The experimental results of stability ratio and EPM of allophane particles were affected by the counter-ion valence, and the CCCs strongly depended on the counter-ion valence. The behavior was well described by the well-known empirical Schulze-Hardy rule. The allophane particles showed a low surface charge density in the presence of multivalent counter-ions. Thus, Derjaguin-Landau and Verwey-Overbeek (DLVO) theory with the Debye-Huckel (DH) approximation was applied to consider the CCIS. The experimental relationship between CCIS and surface charge density was well explained by DLVO theory by taking account of particle size. We conclude that, in the presence of multivalent counter-ions, the DLVO theory with DH approximation has a good applicability to the aggregation-dispersion of natural clay allophane particles. • Counter-ion valence affects the aggregation and charging of allophane particles. • Tetravalent counter-ion induces charge reversal and re-stabilization of allophane. • Critical coagulation ionic strength vs. surface chare follows DLVO theory. • DLVO theory has a good applicability to the aggregation and dispersion of allophane.

Unusual stability of protein molecules in the presence of multivalent counterions

Proteins are known to undergo denaturation and form different phases with varying physicochemical parameters. We report unusual stability of bovine serum albumin protein against commonly used denaturants (temperature and surfactant) in the charged reversal reentrant phase, caused by the multivalent counterions. Unlike monovalent counterions, which promote the denaturants' induced protein unfolding, the unfolding is restricted in the presence of multivalent ions. The observations are beyond the scope of general understanding of protein unfolding and are believed to be governed by ion-ion correlations driven strong condensation of the multivalent ions.

Modifications in the nanoparticle-protein interactions for tuning the protein adsorption and controlling the stability of complexes

We report the pathways to suppress or enhance the protein adsorption on nanoparticles and thereby control the stability of the nanoparticle-protein complexes with the help of selective additives. This has been achieved by tuning the electrostatic interaction between the nanoparticles and proteins, in the presence of surfactant and multivalent counterions. The preferential binding of the proteins with the surfactant and multivalent ions induced charge reversibility of nanoparticles can lead to adsorption of an otherwise non-adsorbing protein and vice versa. The findings are demonstrated for anionic silica nanoparticles and two globular proteins [lysozyme (cationic) and bovine serum albumin (BSA) (anionic)] as model systems, in the presence of two ionic surfactants [anionic sodium dodecyl sulfate (SDS) and cationic dodecyltrimethylammonium bromide (DTAB)], and ZrCl4 as multivalent salt. Small-angle neutron scattering with the unique advantage of contrast variation has been used to probe the role of individual components in the multi-component system. It is shown that the non-adsorbing behavior of BSA with silica nanoparticles changes into adsorbing in the presence of oppositely charged DTAB surfactant, whereas the strong adsorbing behavior of lysozyme on nanoparticles modifies to be non-adsorbing in the presence of oppositely charged SDS surfactant. The presence of multivalent counterions (ZrCl4) leads the charge reversal of the nanoparticles, transforming the lysozyme from adsorbing to non-adsorbing, and no significant change in the behavior of BSA. The results presented can find potential applications in the field of nanobiotechnology.

α-Sulfo alkyl ester surfactants: Impact of changing the alkyl chain length on the adsorption, mixing properties and response to electrolytes of the tetradecanoate

Hypothesis: The α-sulfo alkyl ester, AES, surfactants are a class of anionic surfactants which have potential for improved sustainable performance in a range of applications, and an important feature is their enhanced tolerance to precipitation in the presence of multivalent counterions. It is proposed that their adsorption properties can be adjusted substantially by changing the length of the shorter alkyl chain, that of the alkanol group in the ester.Experiments: Surface tension and neutron reflectivity have been used to investigate the variation in the adsorption properties with the shorter alkyl chain length (methyl, ethyl and propyl), the impact of NaCl on the adsorption, the tendency to form surface multilayer structures in the presence of AlCl3, and the effects of mixing the methyl ester sulfonate with the ethyl and propyl ester sulfonates on the adsorption.Findings: The variations in the critical micelle concentration, CMC, the adsorption isotherms, the saturation adsorption values, and the impact of NaCl illustrate the subtle influence of varying the shorter alkyl chain length of the surfactant. The non-ideal mixing of pairs of AES surfactants with different alkanol group lengths of the ester show that the extent of the non-ideality changes as the difference in the alkanol length increases. The surface multilayer formation observed in the presence of AlCl3 varies in a complex manner with the length of the short chain and for mixtures of surfactants with different chains lengths.

Bridges of Calcium Bicarbonate Tightly Couple Dipolar Lipid Membranes.

The interaction between lipid membranes and ions is associated with a range of key physiological processes. Most earlier studies have focused on the interaction of lipids with cations, while the specific effects of the anions have been largely overlooked. Owing to dissolved atmospheric carbon dioxide, bicarbonate is an important ubiquitous anion in aqueous media. In this paper, we report on the effect of bicarbonate anions on the interactions between dipolar lipid membranes in the presence of previously adsorbed calcium cations. Using a combination of solution X-ray scattering, osmotic stress, and molecular dynamics simulations, we followed the interactions between 1,2-didodecanoyl-sn-glycero-3-phosphocholine (DLPC) lipid membranes that were dialyzed against CaCl2 solutions in the presence and absence of bicarbonate anions. Calcium cations adsorbed onto DLPC membranes, charge them, and lead to their swelling. In the presence of bicarbonate anions, however, the calcium cations can tightly couple one dipolar DLPC membrane to the other and form a highly condensed and dehydrated lamellar phase with a repeat distance of 3.45 ± 0.02 nm. Similar tight condensation and dehydration has only been observed between charged membranes in the presence of multivalent counterions. Bridging between bilayers by calcium bicarbonate complexes induced this arrangement. Furthermore, in this condensed phase, lipid molecules and adsorbed ions were arranged in a two-dimensional oblique lattice.

Interfacial solvation can explain attraction between like-charged objects in aqueous solution.

Over the past few decades, the experimental literature has consistently reported observations of attraction between like-charged colloidal particles and macromolecules in aqueous solution. Examples include nucleic acids and colloidal particles in the bulk solution and under confinement, and biological liquid-liquid phase separation. This observation is at odds with the intuitive expectation of an interparticle repulsion that decays monotonically with distance. Although attraction between like-charged particles can be rationalized theoretically in the strong-coupling regime, e.g., in the presence of multivalent counterions, recurring accounts of long-range attraction in aqueous solution containing monovalent ions at low ionic strength have posed an open conundrum. Here, we show that the behavior of molecular water at an interface-traditionally disregarded in the continuum electrostatics picture-provides a mechanism to explain the attraction between like-charged objects in a broad spectrum of experiments. This basic principle will have important ramifications in the ongoing quest to better understand intermolecular interactions in solution.

Multivalent electrolyte induced surface ordering and solution self-assembly in anionic surfactant mixtures: Sodium dodecyl sulfate and sodium diethylene glycol monododecyl sulfate

The formation of surface multilayer structures, with the addition of multivalent electrolytes, has been observed in a range of different anionic surfactants; and notably the sodium oxyethylene glycol alkyl sulfate, SAES, and alkyl ester sulfonate, AES, surfactants. The addition of increasing amounts of AlCl3 results in increasing surface layering, with a transition from monolayer to bilayer to ultimately more extended multilayer structures at the interface.The headgroup structures of these SAES and AES surfactants and their hydrophilic / hydrophobic balance give a degree of tolerance to the precipitation induced by multivalent counterions. This was considered to be important factor associated with the multivalent counterion induced surface layering. In this paper the impact of sodium dodecyl sulfate, SDS, an anionic surfactant more susceptible to precipitation in the presence of multivalent counterions, on the surface multilayer formation and solution self-assembly of sodium diethylene glycol monododecyl sulfate, SLES, is explored using surface tension, neutron reflectivity and small angle neutron scattering.The results show that SDS exhibits a similar progressive evolution in surface structures with increasing AlCl3 concentrations, as observed in SLES and related SAES surfactants, and in MES, sodium methyl ester dodecyl sulfonate surfactant. However in the SLES / SDS mixtures the structural evolution is different, and more complex pattern with increasing AlCl3 concentration is observed. The initial transition from monolayer to bilayer / trilayer structures exists, but the surface at higher AlCl3 concentration reverts to monolayer adsorption before extended multilayer structures are formed. Complementary small angle neutron scattering measurements indicate a more complex evolution in the micelle structure which broadly correlates with the surface behaviour.The results illustrate how subtle changes in headgroup structure and packing affect relative counterion binding and hence the surface and solution structures. The results reinforce and extend the observations of related structures on different SAES and AES surfactants, and highlight the opportunity for manipulating surface adsorption behaviour with surfactant mixtures.

Impact of molecular structure, headgroup and alkyl chain geometry, on the adsorption of the anionic ester sulfonate surfactants at the air-solution interface, in the presence and absence of electrolyte

The transition from monolayer to multilayer adsorption at the air-water interface in the presence of multivalent counterions has been demonstrated for a limited range of anionic surfactants which exhibit increased tolerance to precipitation in the presence of multivalent counterions. Understanding the role of molecular structure in determining the transition to surface ordering is an important aspect of the phenomenon. The focus of the paper is on the alkyl ester sulfonate, AES, surfactants; a promising group of anionic surfactants, with the potential for improved performance and biocompatibility. Neutron reflectivity measurements were made in aqueous solution and in the presence of NaCl, CaCl2, MgCl2 and AlCl3, for a range of alkyl ester sulfonate surfactants, in which the headgroup and alkyl chain geometries were manipulated.In the regions of monolayer adsorption changing the AES headgroup and alkyl chain geometries results in an increased saturation adsorption and in a more gradual decrease in the adsorption at low concentrations, consistent with a greater adsorption efficiency. Changing the AES headgroup and alkyl chain geometries also results in changes in the transition from monolayer adsorption to more ordered surface structures with the addition of AlCl3 and mixed multivalent electrolytes. A more limited surface layering is observed for the ethyl ester sulfonate, EES, with a C14 alkyl chain. Replacing the C14 alkyl chain with a C18 isostearic chain results in only monolayer adsorption.The results demonstrate the role and importance of the surfactant molecular structure in determining the nature of the surface adsorption in the presence of different electrolytes, and in the tendency to form extended surface multilayer structures.

Self-assembly of polyelectrolyte diblock copolymers at monovalent and multivalent counterions.

In this work, the self-assembly behaviors of diblock copolymers consisting of one hydrophobic block and one ionizable polyelectrolyte (PE) block in the presence of monovalent and multivalent counterions are systematically discussed through molecular dynamics simulation. Copolymers are molded as bead-spring chains and the ions are explicitly considered. First, the self-assembled structures of symmetrical block copolymers at different charge fractions are analyzed in detail. Spherical hydrophobic cores are favored by all of the micelles. The effect of counterion valence is much more noticeable at high values of charge fraction. When the PE blocks are fully charged, the presence of multivalent counterions preferably provokes the formation of macroscopic structures. A precipitant spherical micelle is generated in the presence of divalent counterions. Special shapes of coronas are created in the presence of trivalent ions, and a remarkable one dimensional macroscopic cylindrical aggregation of micelles forms; the whole assembly is not typical core-shell micelles, but rather a cylinder with alternating spherical micelles arranged perpendicular to the cylinder axis. The self-assemblies with different lengths of fully charged PE blocks are also discussed. Surprisingly, in the presence of divalent counterions, two dimensional in-plane macroscopic aggregation of micelles is realized when the proportion of PE blocks is larger than 1/2; the self-assembled spherical micelles locate approximately in the same plane to form an inter-linked network. One dimensional aggregation of micelles in the presence of trivalent counterions is maintained with an increased proportion of the PE block. Owing to the dominant intra- and inter-condensation of divalent and trivalent counterions, respectively, two and one dimensional macroscopic aggregation of the micelles is achieved. Our findings indicate that varying the counterion valence is a powerful mechanism to tune the properties of self-assemblies, and the bridging effect introduced by multivalent counterions is the key parameter for the aggregation of the micelles.

Charged nanorods at heterogeneously charged surfaces.

We study the spatial and orientational distribution of charged nanorods (rodlike counterions) as well as the effective interaction mediated by them between two plane-parallel surfaces that carry fixed (quenched) heterogeneous charge distributions. The nanorods are assumed to have an internal charge distribution, specified by a multivalent monopolar moment and a finite quadrupolar moment, and the quenched surface charge is assumed to be randomly distributed with equal mean and variance on the two surfaces. While equally charged surfaces are known to repel within the traditional mean-field theories, the presence of multivalent counterions has been shown to cause attractive interactions between uniformly charged surfaces due to the prevalence of strong electrostatic couplings that grow rapidly with the counterion valency. We show that the combined effects due to electrostatic correlations (caused by the coupling between the mean surface field and the multivalent, monopolar, charge valency of counterions) as well as the disorder-induced interactions (caused by the coupling between the surface disorder field and the quadrupolar moment of counterions) lead to much stronger attractive interactions between two randomly charged surfaces. The interaction profile turns out to be a nonmonotonic function of the intersurface separation, displaying an attractive minimum at relatively small separations, where the ensuing attraction can exceed the maximum strong-coupling attraction (produced by multivalent monopolar counterions between uniformly charged surfaces) by more than an order of magnitude.

Multivalent counterions diminish the lubricity of polyelectrolyte brushes.

Polyelectrolyte brushes provide wear protection and lubrication in many technical, medical, physiological, and biological applications. Wear resistance and low friction are attributed to counterion osmotic pressure and the hydration layer surrounding the charged polymer segments. However, the presence of multivalent counterions in solution can strongly affect the interchain interactions and structural properties of brush layers. We evaluated the lubrication properties of polystyrene sulfonate brush layers sliding against each other in aqueous solutions containing increasing concentrations of counterions. The presence of multivalent ions (Y3+, Ca2+, Ba2+), even at minute concentrations, markedly increases the friction forces between brush layers owing to electrostatic bridging and brush collapse. Our results suggest that the lubricating properties of polyelectrolyte brushes in multivalent solution are hindered relative to those in monovalent solution.

Monovalent and divalent ion sorption in a cation exchange membrane based on cross-linked poly (p-styrene sulfonate-co-divinylbenzene)

Many fundamentals of ion sorption in ion exchange membranes (IEMs) are not fully understood, and few modern studies focus on multivalent ion sorption, despite the importance of divalent ion transport properties in water purification applications. Here, MgCl2 and CaCl2 sorption in a commercial cross-linked, styrene-divinylbenzene cation exchange membrane (CEM) with fixed sulfonate groups was measured and compared with NaCl, LiCl and KCl sorption in the same material. The data were correlated with counter-ion size and valence, as well as membrane water content. Divalent salts had higher sorption coefficients than monovalent salts, even though membrane water content significantly decreased in the presence of divalent counter-ions. The Donnan potential is weaker in the presence of multivalent counter-ions, which increases co-ion sorption, contributing to the observed increase in salt sorption coefficients for divalent salts. Ion activity coefficients in the membrane were calculated and compared to predictions of the recently proposed Manning-Donnan model. This model showed much better agreement with the experimental data than the ideal Donnan model, suggesting that ion non-ideality in the membrane is important when analyzing ion sorption and transport in IEMs.

Interactions between charged particles with bathing multivalent counterions: experiments vs. dressed ion theory.

We compare the recent experimentally measured forces between charged colloidal particles, as well as their effective surface potentials (surface charge) in the presence of multivalent counterions in a bathing monovalent salt solution, with the predictions of the dressed ion theory of strongly charged colloidal systems. The benchmark for comparison is provided by the DLVO theory and the deviations from its predictions at small separations are taken as an indication of the additional non-DLVO attractions that can be fitted by an additional phenomenological exponential term. The parameters characterizing this non-DLVO exponential term as well as the dependencies of the effective potential on the counterion concentration and valency predicted by the dressed ion theory are well within the experimental values. This suggests that the deviations from the DLVO theory are probably caused by ion correlations as formalized within the dressed ion theory.

Single Molecular Demonstration of Modulating Charge Inversion of DNA.

Charge inversion of DNA is a counterintuitive phenomenon in which the effective charge of DNA switches its sign from negative to positive in the presence of multivalent counterions. The underlying microscopic mechanism is still controversial whether it is driven by a specific chemical affinity or electrostatic ion correlation. It is well known that DNA shows no charge inversion in normal aqueous solution of trivalent counterions though they can induce the conformational compaction of DNA. However, in the same trivalent counterion condition, we demonstrate for the first time the occurrence of DNA charge inversion by decreasing the dielectric constant of solution to make the electrophoretic mobility of DNA increase from a negative value to a positive value. In contrast, the charge inversion of DNA induced by quadrivalent counterions can be canceled out by increasing the dielectric constant of solution. The physical modulation of DNA effective charge in two ways unambiguously demonstrates that charge inversion of DNA is a predominantly electrostatic phenomenon driven by the existence of a strongly correlated liquid (SCL) of counterions at the DNA surface. This conclusion is also supported by the measurement of condensing and unraveling forces of DNA condensates by single molecular MT.

Structure of Polyelectrolyte Brushes in the Presence of Multivalent Counterions

Polyelectrolyte brushes are of great importance to a wide range of fields, ranging from colloidal stabilization to responsive and tunable materials to lubrication. We synthesized high-density polystyrenesulfonate (PSS) brushes using surface initiated atom transfer radical polymerization and performed neutron reflectivity (NR) and surface force measurements using a surface forces apparatus (SFA) to investigate the effect of monovalent Na+, divalent Ca2+, Mg2+, and Ba2+, and trivalent Y3+ counterions on the structure of the PSS brushes. NR and SFA results demonstrate that in monovalent salt solution the behavior of the PSS brushes agrees with scaling theory well, exhibiting two distinct regimes: the osmotic and salted brush regimes. Introducing trivalent Y3+ cations causes an abrupt shrinkage of the PSS brush due to the uptake of Y3+ counterions. The uptake of Y3+ counterions and shrinkage of the brush are reversible upon increasing the concentration of monovalent salt. Divalent cations, Mg2+, Ca2+, and Ba2...

The effect of multivalent counterions to the structure of highly dense polystyrene sulfonate brushes

Surface tethered polyelectrolyte brushes are scientifically interesting and technologically relevant to many applications, ranging from colloidal stabilization to responsive and tunable materials to lubrication. Many applications operate in environments containing multi-valent ions, media in which our scientific understanding is not yet well-developed. We synthesized high-density polystyrene sulfonate (PSS) brushes via surface initiated atom-transfer radical polymerization, and performed neutron reflectivity (NR) measurements to investigate and compare the effects of mono-valent Rb+ and tri-valent Y3+ counterions to the structure of the densely tethered PSS brushes. Our NR results show that in mono-valent RbNO3 solution, the dense PSS brush retained its full thickness up to a salt concentration of 1 M, whereas it immediately collapsed upon adding 1.67 mM of tri-valent Y3+. Increasing the concentration of Y3+ beyond this level did not lead to any significant further structure change of the PSS brush. Our findings demonstrate that the presence of multi-valent counterions can significantly alter the structure of polyelectrolyte brushes, in a manner different from mono-valent ions, which has implications for the functionality of the brushes.

Ionic current inversion in pressure-driven polymer translocation through nanopores.

We predict streaming current inversion with multivalent counterions in hydrodynamically driven polymer translocation events from a correlation-corrected charge transport theory including charge fluctuations around mean-field electrostatics. In the presence of multivalent counterions, electrostatic many-body effects result in the reversal of the DNA charge. The attraction of anions to the charge-inverted DNA molecule reverses the sign of the ionic current through the pore. Our theory allows for a comprehensive understanding of the complex features of the resulting streaming currents. The underlying mechanism is an efficient way to detect DNA charge reversal in pressure-driven translocation experiments with multivalent cations.

Charge Inversion Effects in Electrophoresis of Polyelectrolytes in the Presence of Multivalent Counterions and Transversal Electric Fields

By molecular dynamics simulations we investigate the transport of charged polymers in confinement, under externally applied electric fields, in straight cylinders of uniform diameter and in the presence of monovalent or multivalent counterions. The applied electric field has two components; a longitudinal component along the axis of the cylinder and a transversal component perpendicular to the cylinder axis. The direction of electrophoretic velocity depends on the polyelectrolyte length, valency of the counterions present in solution and transversal electric field value. A statistical model is put forward in order to explain these observations.