Like-charged Surfaces Research Articles

Early-age hydration of tricalcium aluminate in chloride solutions

Understanding the kinetics and mechanisms involved in early-age hydration of tricalcium aluminate (C3A) in chloride solutions holds promise for implementing seawater-mixed concrete in the marine environment, as C3A remains the most reactive component of Portland cement (PC), affecting both PC and concrete's early-age hardening and long-term durability. Herein, we conducted a series of meticulously designed ex-situ and in-situ experiments to elucidate the intricate hydration behaviors of C3A in various chloride solutions. The results reveal that C3A exhibits distinct hydration kinetics and structural evolution processes in different solutions. The rapid precipitation of alumino-ferrite-mono (AFm) and C3AH6 phases contributes to the swift development of hydration heat and storage modulus in water and NaCl solutions, with a slight acceleration observed in the later one. Conversely, the formation of C3AH6 is delayed in CaCl2 and MgCl2 solutions before 20 min, with the subsequent precipitation of Cl-AFm enhancing its later production, particularly in CaCl2 solutions. Ab-initio calculations further elucidate that the acceleration effect of Cl ions originates from the ionization and structurization of hydrated surface Ca ions. However, this positive effect is significantly offset by Cl pairing with counterions, resulting in a dramatic adverse effect of solution Ca ions originated from the negative entropy effect of structuralized water molecules and electrostatic repulsion with like-charged surface Ca ions and solvent dipoles. Our findings provide valuable insights for sustainable and durable designs on cement-based materials mixed with seawater.

Toward Modeling the Structure of Electrolytes at Charged Mineral Interfaces Using Classical Density Functional Theory.

The organization of water molecules and ions between charged mineral surfaces determines the stability of colloidal suspensions and the strength of phase-separated particulate gels. In this article, we assemble a density functional that measures the free energy due to the interaction of water molecules and ions in electric double layers. The model accounts for the finite size of the particles using fundamental measure theory, hydrogen-bonding between water molecules using Wertheim's statistical association theory, long-range dispersion interactions using Barker and Henderson's high-temperature expansion, electrostatic correlations using a functionalized mean-spherical approximation, and Coulomb forces through the Poisson equation. These contributions are shown to produce highly correlated structures, aptly rendering the layering of counterions and co-ions at highly charged surfaces and permitting the solvation of ions and surfaces to be measured by a combination of short-range associations and long-ranged attractions. The model is tested in a planar geometry near soft, charged surfaces to reproduce the structure of water near graphene and mica. For mica surfaces, explicitly representing the density of the outer oxygen layer of the exposed silica tetrahedra allows water molecules to hydrogen-bond to the solid. When electrostatic interactions are included, water molecules assume a hybrid character, being accounted for implicitly in the dielectric constant but explicitly otherwise. The disjoining pressure between approaching like-charged surfaces is calculated, demonstrating the model's ability to probe pressure oscillations that arise during the expulsion of ions and water layers from the interfacial gap and predict strong interattractive stresses that form at narrow interfacial spacing when the surface charge is overscreened. This interattractive stress arises not due to in-plane correlations under strong electrostatic coupling but due to the out-of-plane structuring of associating ions and water molecules.

Origin of Anomalously Large Depletion Zones in Like-Charged Colloid-Polyelectrolyte Mixtures.

Depletion zones in polyelectrolyte solutions in contact with like-charged flat surfaces are investigated. Using a coupled self-consistent field and Debye-Hückel approach, an explicit expression for the thickness δ of the depletion layer is derived. It is found that δ∼δ_{n}+cκ^{-1}, where δ_{n} is the depletion thickness at a neutral surface, c is a function of the electrostatic characteristics of the system, and κ^{-1} is the Debye length. It is argued that the theory still holds beyond the mean-field approximation, which is confirmed by quantitative agreement between our theoretical results and experiments.

Ion correlation-driven like-charge attraction in multivalent salt solutions.

The electrostatic double layer force is key to determining the stability and self-assembly of charged colloids and many other soft matter systems. Fully understanding the attractive force between two like-charged surfaces remains a great challenge. Here, we apply the modified Gaussian renormalized fluctuation theory to study ion correlation-driven like-charge attraction in multivalent salt solutions. The effects of spatially varying ion correlations on the structure of overlapping double layers and their free energy are self-consistently accounted for. In the presence of multivalent salts, increasing surface charge or counterion valency leads to a short-range attraction. We demonstrate that although both overcharging and like-charge attraction are outcomes of ion correlation, there is no causal relationship between them. Our theory also captures the non-monotonic dependence of like-charge attraction on multivalent salt concentration. The reduction of attraction at high salt concentrations could be a contributing factor toward the reentrant stability of charged colloidal suspensions. Our theoretical predictions are consistent with the observations reported in experiments and simulations.

Voltage and concentration gradients across membraneless interface generated next to hydrogels: relation to glycocalyx.

Next to many hydrophilic surfaces, including those of biological cells and tissues, a layer of water that effectively excludes solutes and particles can be generated. This interfacial water is the subject of research aiming for practical applications such as removal of salts, pathogens or manipulation of biomolecules. However, the exact mechanism of its creation is still elusive because its persistence and extension contradict hydrogen-bond dynamics and electric double layer predictions. The experimentally recorded negative voltage of this interfacial water remains to be properly explained. Even less is known about the nature of such water layers in biological systems. We present experimental evidence for ion and particle exclusion as a result of separation of ionic charges with distinct diffusion rates across a liquid junction at the gel/water interface and the subsequent repulsion of ions of a given sign by a like-charged gel surface. Gels represent features of biological interfaces (in terms of functional groups and porosity) and are subject to biologically relevant chemical triggers. Our results show that gels with -OSO3- and -COO- groups can effectively generate ion- and particle-depleted regions of water reaching over 100 μm and having negative voltage up to -30 mV. Exclusion distance and electric potential depend on the liquid junction potential at the gel/water interface and on the concentration gradient at the depleted region/bulk interface, respectively. The voltage and extension of these ion- and particle-depleted water layers can be effectively modified by CO2 (respiratory gas) or KH2PO4 (cell metabolite). Possible implications pertain to biologically unstirred water layers and a cell's bioenergetics.

Single-Molecule Trapping and Measurement in a Nanostructured Lipid Bilayer System.

The repulsive electrostatic force between a biomolecule and a like-charged surface can be geometrically tailored to create spatial traps for charged molecules in solution. Using a parallel-plate system composed of silicon dioxide surfaces, we recently demonstrated single-molecule trapping and high precision molecular charge measurements in a nanostructured free energy landscape. Here we show that surfaces coated with charged lipid bilayers provide a system with tunable surface properties for molecular electrometry experiments. Working with molecular species whose effective charge and geometry are well-defined, we demonstrate the ability to quantitatively probe the electrical charge density of a supported lipid bilayer. Our findings indicate that the fraction of charged lipids in nanoslit lipid bilayers can be significantly different from that in the precursor lipid mixtures used to generate them. We also explore the temporal stability of bilayer properties in nanofluidic systems. Beyond their relevance in molecular measurement, such experimental systems offer the opportunity to examine lipid bilayer formation and wetting dynamics on nanostructured surfaces.

Interaction between surfaces decorated with like-charged pendants: Unravelling the interplay between energy and entropy leading to attraction

HypothesisThe stronger motional coupling between monovalent counterions neutralizing homogeneously like-charged surfaces induced by an increase in charge density is known to foster inter–surface attraction. Compared to a uniformly distributed charge, point-like charges generate locally more intense fields, so that the correlation induced between counterions may be even stronger despite an identical total charge. It should thus be possible to induce surface attraction at lower charge densities than commonly expected. ExperimentsMonte Carlo simulations on primitive electrolyte models have been exploited to compute potential of mean force profiles and mobile ion densities for systems composed of two parallel surfaces bearing surface-tethered monovalent like-charged pendants as a function of the surface distance and pendant densities. FindingsSurfaces bearing like-charged pendants are found to attract each other over a wide range of distances despite the presence of very low charge densities. Notwithstanding the attractive contribution to the inter-surface forces provided by electrostatic interactions, the entropic component of the system Helmholtz energy is found to play the key role in defining the overall magnitude. The latter finding appears justified by an increase in the relative delocalization of counterions upon decreasing the surface distance.

Calcium Ion-Induced Structural Changes in Carboxymethylcellulose Solutions and Their Effects on Adsorption on Cellulose Surfaces.

The adsorption of carboxymethylcellulose (CMC) on cellulose surfaces is one of the most studied examples of the adsorption of an anionic polyelectrolyte on a like-charged surface. It has been suggested that divalent ions can act as a bridge between CMC chains and the surface of cellulose and enhance the CMC adsorption: they can, however, also alter the structure of CMCs in the solution. In previous investigations, the influence of cations on solution properties has been largely overlooked. This study investigates the effect of Ca2+ ions on the properties of CMC solutions as well as the influence on cellulose nanofibers (CNFs), which was studied by dynamic light scattering and correlated with the adsorption of CMC on a cellulose surface probed using QCM-D. The presence of Ca2+ facilitated the multichain association of CMC chains and increased the hydrodynamic diameter. This suggests that the adsorption of CMCs at high concentrations of CaCl2 is governed mainly by changes in solution properties rather than by changes in the cellulose surface. Furthermore, an entropy-driven mechanism has been suggested for the adsorption of CMC on cellulose. By comparing the adsorption of CMC from H2O and D2O, it was found that the release of water from the cellulose surface is driving the adsorption of CMC.

Ionic screening in bulk and under confinement.

Recent experiments have shown that the repulsive force between atomically flat, like-charged surfaces confining room-temperature ionic liquids or concentrated electrolytes exhibits an anomalously large decay length. In our previous publication [J. Zeman, S. Kondrat, and C. Holm, Chem. Commun. 56, 15635 (2020)], we showed by means of extremely large-scale molecular dynamics simulations that this so-called underscreening effect might not be a feature of bulk electrolytes. Herein, we corroborate these findings by providing additional results with more detailed analyses and expand our investigations to ionic liquids under confinement. Unlike in bulk systems, where screening lengths are computed from the decay of interionic potentials of mean force, we extract such data in confined systems from cumulative charge distributions. At high concentrations, our simulations show increasing screening lengths with increasing electrolyte concentration, consistent with classical liquid state theories. However, our analyses demonstrate that-also for confined systems-there is no anomalously large screening length. As expected, the screening lengths determined for ionic liquids under confinement are in good quantitative agreement with the screening lengths of the same ionic systems in bulk. In addition, we show that some theoretical models used in the literature to relate the measured screening lengths to other observables are inapplicable to highly concentrated electrolytes.

Elastic gas/water interface for highly stable foams with modified anionic silica nanoparticles and a like-charged surfactant

Hypothesis. Surface active anionic nanoparticles (NPs) with strategically designed covalent ligands may be combined with a liked-charged surfactant to form a highly elastic gas–water interface leading to highly stable gas/water foams.Experiments. The colloidal stability of the NPs was determined by dynamic light scattering, and the surface elastic dilational modulus E’ of the interface by sinusoidal oscillation of a pendant droplet at 0.1 Hz, which was superimposed on large-amplitude compression-expansion cycles. The foam stability was measured with optical microscopy of the bubble size distribution and from the macroscopic foam height.Findings. The NPs played the key role the formation of a highly elastic air–water interface with a high E’ despite a surfactant level well above the critical micelle concentration. Unlike the case for most previous studies, the NP amphiphilicity was essentially independent of the surfactant given the very low adsorption of the surfactant on the like-charged NP surfaces. With high E’ values, both coalescence and coarsening were reduced leading to highly foam up to 80 °C. However, the surfactant facilitated foam generation at much lower shear rates than with NPs alone. The tuning of NP surfaces with ligands for colloidal stability in brine and simultaneously high amphiphilicity at the gas–water interface, over a wide range in surfactant concentration, is of broad interest for enabling the design of highly stable foams.

Magnesium-Free Immobilization of DNA Origami Nanostructures at Mica Surfaces for Atomic Force Microscopy.

DNA origami nanostructures (DONs) are promising substrates for the single-molecule investigation of biomolecular reactions and dynamics by in situ atomic force microscopy (AFM). For this, they are typically immobilized on mica substrates by adding millimolar concentrations of Mg2+ ions to the sample solution, which enable the adsorption of the negatively charged DONs at the like-charged mica surface. These non-physiological Mg2+ concentrations, however, present a serious limitation in such experiments as they may interfere with the reactions and processes under investigation. Therefore, we here evaluate three approaches to efficiently immobilize DONs at mica surfaces under essentially Mg2+-free conditions. These approaches rely on the pre-adsorption of different multivalent cations, i.e., Ni2+, poly-l-lysine (PLL), and spermidine (Spdn). DON adsorption is studied in phosphate-buffered saline (PBS) and pure water. In general, Ni2+ shows the worst performance with heavily deformed DONs. For 2D DON triangles, adsorption at PLL- and in particular Spdn-modified mica may outperform even Mg2+-mediated adsorption in terms of surface coverage, depending on the employed solution. For 3D six-helix bundles, less pronounced differences between the individual strategies are observed. Our results provide some general guidance for the immobilization of DONs at mica surfaces under Mg2+-free conditions and may aid future in situ AFM studies.

DNA adsorption on like-charged surfaces mediated by polycations

In gene delivery applications, one of the first steps to ensure transfection of the genetic material is promoting the interaction with the negatively charged cell membranes. Due to the like-charged character of DNA and cell membranes, polycations are often used to mediate this interaction. The behaviour of these systems is sometimes counterintuitive and dependent on a number of interrelated factors, including membrane charge and composition, that are, in turn, dependent on cell type and pathology. For instance, upon tumorogenesis the membrane charge becomes more negative. Other features such as polycation structure, charge and concentration also impact on the polyplex-membrane interaction.Due to the inherent complexity of the system, much remains to clarify, especially because most previous work was focused on the cation-mediated adsorption with surfaces with charge densities much higher than those of cell membranes.In this work Monte Carlo simulations within the primitive model allows to establish the optimal conditions for the adsorption of DNA-polyplexes, ranging from undercharged to overcharged, upon responsive surfaces of different charge density mimicking healthy and tumoral cell membranes. Briefly, it was found that polyplex adsorption is favoured in the more negatively charged membranes, for polyplexes formed with longer polycations and at intermediate polycation concentrations.

Nonelectrostatic Adsorption of Polyelectrolytes and Mediated Interactions between Solid Surfaces.

Polymer-mediated interaction between two solid surfaces is directly connected to the properties of the adsorbed polymer layers. Nonelectrostatic interactions with a surface can significantly impact the adsorption of polyelectrolytes to charged surfaces. We use a classical density functional theory to study the effect of various polyelectrolyte solution properties on the adsorption and interaction between two like-charged surfaces. Our results show that nonelectrostatic interactions not only enhance polyelectrolyte adsorption but can also result in qualitatively different salt effects with respect to the adsorbed amount. In particular, we observe decreasing, increasing, and a previously unreported nonmonotonic behavior in the adsorbed amount of polymer with added salt under the conditions studied, although the nonmonotonic regime only occurs for a narrow range in the parameter space. With sufficient nonelectrostatic adsorption, the adsorbed polymer layers produce a long-range repulsive barrier that is strong enough to overcome dispersive interactions that cause surfaces to attract. Concurrently, a short-range bridging attraction is observed when the two polyelectrolyte layers span both the surfaces. Both the repulsive barrier and bridging attraction depend on the charge density of the polymer backbone and the bulk salt concentration but not on the chain length in the semidilute regime studied.

Enhanced Adsorption of Anionic Polymer on Montmorillonite by Divalent Cations and the Effect of Salinity.

Adsorption of polymers from an aqueous solution onto clay minerals is of great interest to many applications such as water purification and soil conditioning. Molecular dynamics simulations were performed to study the adsorption of anionic polyacrylamide (APAM) on anionic montmorillonite, in an aqueous solution containing monovalent or divalent salts. Compared with monovalent salts (NaCl), the enhancement of APAM adsorption brought by divalent salts (CaCl2) was significant, which could not be explained by the Poisson-Boltzmann theory alone. Each solvated Ca2+ was coordinated by 4-6 water oxygens in its first coordination shell. One to two of these water molecules were displaced when APAM formed a complex with Ca2+. Ca2+ ions in the adsorbed Ca2+-APAM complexes did not serve as bridges sandwiched between APAM and Mt; instead, the complexes carried a residual positive charge and were subsequently attracted to montmorillonite. The number of adsorbed Ca2+-APAM complexes changed with salinity in a nonmonotonic manner, due to the modulation of apparent charges of montmorillonite and APAM by Ca2+. Increasing adsorption of Ca2+-APAM complexes also promoted APAM adsorption through direct hydrogen bonding with montmorillonite. The findings provided new molecular insights into the long-standing debates on the role of divalent ions in promoting polymer adsorption on like-charged solid surfaces.

Structural and Double Layer Forces between Silica Surfaces in Suspensions of Negatively Charged Nanoparticles.

Direct force measurements between negatively charged silica microparticles are carried out in suspensions of like-charged nanoparticles with atomic force microscopy (AFM). In agreement with previous studies, oscillatory force profiles are observed at larger separation distances. At smaller distances, however, soft and strongly repulsive forces are present. These forces are caused by double layer repulsion between the like-charged surfaces and can be quantitatively interpreted with the Poisson-Boltzmann (PB) model. However, the PB model must be adapted to a strongly asymmetric electrolyte to capture the nonexponential nature of these forces. Thereby, the nanoparticles are modeled as highly charged co-ions, while the counter ions are monovalent. This model permits extraction of the effective charge of the nanoparticles, which is well comparable to the one obtained from electrophoresis. The PB model also explains the presence of a particle-free layer close to the interface.

Interaction between like-charged surfaces in water: does mean-field (Poisson Boltzmann) always apply for monovalent counterions?

Interaction between like-charged surfaces in water: does mean-field (Poisson Boltzmann) always apply for monovalent counterions?

Modeling hydration-mediated ion-ion interactions in electrolytes through oscillating Yukawa potentials.

Classical Poisson-Boltzmann theory represents a mean-field description of the electric double layer in the presence of only Coulomb interactions. However, aqueous solvents hydrate ions, which gives rise to additional hydration-mediated ion-ion interactions. Experimental and computational studies suggest damped oscillations to be a characteristic feature of these hydration-mediated interactions. We have therefore incorporated oscillating Yukawa potentials into the mean-field description of the electric double layer. This is accomplished by allowing the decay length of the Yukawa potential to be complex valued. Ion specificity emerges from assigning individual strengths and phases to the Yukawa potential for anion-anion, anion-cation, and cation-cation pairs as well as for anions and cations interacting with an electrode or macroion. Excluded volume interactions between ions are approximated by replacing the ideal gas entropy by that of a lattice gas. We derive mean-field equations for the Coulomb and Yukawa potentials and use their solutions to compute the differential capacitance for an isolated planar electrode and the pressure that acts between two planar, like-charged macroion surfaces. Attractive interactions appear if the surface charge density of the macroions is sufficiently small.

Influence of added salt on the surface induced ordering of nanoparticles with discretely distributed charges

The formation of an electric double layer composed of spherical nanoparticles is analyzed by means of a generalized Poisson-Boltzmann model and Monte Carlo simulations. We study a solution of symmetric and asymmetric mixtures of spherical particles that reside between two planar like-charged surfaces. Each spherical particle carries two elementary charges that are attached at its poles. The electrolyte solution also contains monovalent point-like salt ions. Our theoretical model properly accounts for intra-particle correlations - that is correlations between the spatially separated charges belonging to a single multivalent spherical particle. Correlations between different spherical particles are neglected. It is shown that added salt decreases the number density close to the charged surface and influences the orientation of spherical particles. Increasing salt concentration decreases the order parameter of the spherical particles. Generalized Poisson-Boltzmann results, obtained by solving an integral differential equation, and predictions from Monte Carlo (MC) simulations are in excellent agreement.

Interaction between like-charged surfaces mediated by uniformly charged counter-nanoparticles

An electric interfacial layer appears when the mobile ions or nanoparticles of an electrolyte solution interact with an extended, charged surface. The distribution of ions or nanoparticles is driven by electrostatic interactions and entropy. We consider continuously charged spherical nanoparticles of finite size. At thermodynamic equilibrium, the spatial profile of the concentration is obtained by deriving the appropriate Euler–Lagrange equations. We discuss how various model parameters of the nanoparticles influence structural properties of the electric interfacial layer. We calculate the pressure between two like-charged surfaces embedded in a water solution of continuously charged spherical nanoparticles.

Predicting collision efficiencies of colloidal nanoparticles in single spherical and fibrous collectors using an individual particle tracking method

We investigate the deposition of colloids onto granular and fibrous collectors by computational fluid dynamics (CFD) simulations. In particular the collision efficiency under unfavorable conditions, i.e., like-charged surfaces, was in focus. Particle trajectories were analyzed in a Lagrangian reference frame using a discrete phase model (DPM). By user-defined functions (UDFs) we incorporated interception as important deposition mechanism and calculated interaction energies between particle and collector surfaces utilizing the extended Derjaguin-Landau-Verwey-Overbeek (xDLVO) theory. Adhesive and hydrodynamic torques acting on deposited particles were compared through the developed UDFs to consider particle detachment. Within each DPM process, all abovementioned calculations on every particle are performed continuously, allowing to understand particle deposition under different physico-chemical conditions. Simulated data on collision efficiencies for the granular collector were in good agreement with theory and experiments. Simulations for the fibrous collector showed that with increasing fluid velocity the hydrodynamic torque acting on particles attached to smaller fibers was increased. This enhanced the detachment and significantly lowered the collision efficiency, especially for larger particles. In conclusion, the developed CFD methods for predicting the collision efficiency on granular and fibrous collectors provide a powerful tool for examining the deposition behaviors of colloidal particles in porous media.