Monovalent Salt Concentration Research Articles

Fragile-to-strong crossover, growing length scales, and dynamic heterogeneity in Wigner glasses.

Colloidal particles, which are ubiquitous, have become ideal testing grounds for the structural glass transition theories. In these systems glassy behavior arises as the density of the particles is increased. Thus, soft colloidal particles with varying degree of softness capture diverse glass-forming properties, observed normally in molecular glasses. Brownian dynamics simulations for a binary mixture of micron-sized charged colloidal suspensions show that tuning the softness of the interaction potential, achievable by changing the monovalent salt concentration results in a continuous transition from fragile to strong behavior. Remarkably, this is found in a system where the well characterized interaction potential between the colloidal particles is isotropic. We also show that the predictions of the random first-order transition (RFOT) theory quantitatively describes the universal features such as the growing correlation length, ξ∼(ϕ_{K}/ϕ-1)^{-ν} with ν=2/3 where ϕ_{K}, the analog of the Kauzmann temperature, depends on the salt concentration. As anticipated by the RFOT predictions, we establish a causal relationship between the growing correlation length and a steep increase in the relaxation time and dynamic heterogeneity as the system is compressed. The broad range of fragility observed in Wigner glasses is used to draw analogies with molecular and polymer glasses. The large variations in the fragility are normally found only when the temperature dependence of the viscosity is examined for a large class of diverse glass-forming materials. In sharp contrast, this is vividly illustrated in a single system that can be experimentally probed. Our work also shows that the RFOT predictions are accurate in describing the dynamics over the entire density range, regardless of the fragility of the glasses.

Charge regulation mechanism in end-tethered weak polyampholytes.

Weak polyampholytes, containing oppositely charged dissociable groups, are expected to be responsive to changes in ionic conditions. Here, we determine structural and thermodynamic properties, including the charged groups' degrees of dissociation, of end-tethered weak polyampholyte layers as a function of salt concentration, pH, and the solvent quality. For diblock weak polyampholytes grafted by their acidic blocks, we find that the acidic monomers increase their charge while the basic monomers decrease their charge with decreasing salt concentration for pH values less than the pKa value of both monomers and vice versa when the pH > pKa. This complex charge regulation occurs because the electrostatic attraction between oppositely charged blocks is stronger than the repulsion between monomers with the same charge in both good and poor solvents when the screening by salt ions is weak. This is evidenced by the retraction of the top block into the bottom layer. In the case of poor solvent conditions to the basic block (the top block), we find lateral segregation of basic monomers into micelles, forming a two-dimensional hexagonal pattern on the surface at intermediate and high pH values for monovalent salt concentrations from 0.01 to 0.1 M. When the solvent is poor to both blocks, we find lateral segregation of the grafted acidic block into lamellae with longitudinal undulations of low and high acidic monomer density. By exploiting weak block polyampholytes, our work expands the parameter space for creating responsive surfaces stable over a wide range of pH and salt concentration.

Study of the polyribosyl-ribitol-phosphate precipitation mechanism by salts and organic solvents

Precipitation has been widely applied to purification and fractionation of biological macromolecules. Several physical-chemical factors contribute to the destabilization of those solutions, such as the nature of solvent employed, presence of salts, temperature, and concentration of the macromolecule. In the case of charged biopolymers, electrostatic forces are the major contributors to their stability in solution. However, the role of each variable and the exact mechanism of precipitation are not completely understood yet. The aim of this work was to study the precipitation of polyribosyl-ribitol-phosphate (PRP, a linear homogeneous anionic biopolymer) in presence of salts and non-solvents, in order to contribute to the elucidation of its precipitation mechanism. The solvents tested (acetone, ethanol, and isopropanol) presented distinct dielectric constants. The salts used (NH4Cl, NaCl, KCl, MgCl2, and CaCl2) differ by their cations. For each salt concentration, the solvent fraction that induces precipitation was identified and the dielectric constant of the bulk solution was calculated. Precipitation always occurred at well-defined combinations of solvents and salts. At low concentration of monovalent salts, there was a linear correlation between the logarithm of the salt concentration and the inverse of the medium dielectric constant at a defined precipitation point. This is a strong indication that the stability of the solution depends almost exclusively on the balance of electrostatic forces. This behavior is compatible with the DLVO modeling of colloidal systems. When divalent salts were used, low concentrations of the counterion were sufficient to induce precipitation, due to a phenomenon called ionic condensation. Apparently, PRP precipitates when around 90% of its charges are neutralized, value that is similar to charge neutralization for DNA precipitation.

Effect of salt valency and concentration on shear and extensional rheology of aqueous polyelectrolyte solutions for enhanced oil recovery

The injection of polymer solutions into an oil basin can lead to enhanced oil recovery (EOR) by increasing the microscopic sweep of the reservoir, improving the water-oil motility ratio, and thus leading to greater yield from oil fields. In this contribution, we characterize both shear and extensional rheological response of aqueous solutions of partially hydrolyzed polyacrylamide (HPAM), the most commonly used polymer for EOR, for velocity gradients in both the flow direction (extensional) and perpendicular to flow (shear) arise in EOR applications. As HPAM is a charged polymer, to better emulate the environment in oil basins, the rheological response was investigated in presence of salt, sodium chloride, and calcium chloride, with concentrations 3.7 × 10−4 − 1.5 M, as a function of polymer molecular weight (2–10 million g/mol) and concentration (0.005–0.3 wt%). The extensional relaxation times and extensional viscosity are measured using dripping-onto-substrate (DoS) rheometry protocols, and a commercial shear rheometer was utilized for characterizing the shear rheology response. The polyelectrolyte solutions formed by HPAM exhibit shear thinning in steady shear, but show strain hardening in response to extensional flow. Even though an increase in monovalent salt concentration leads to a decrease in both shear viscosity and extensional relaxation times, an increase in divalent salt concentration leads to an increase in extensional viscosity and relaxation time, implying that ion coordination can play a role in the presence of multivalent ions.

Interactions of Monovalent and Divalent Cations with Cardiolipin Monolayers.

Cardiolipin is a mitochondrial phospholipid with four alkyl chains and two phosphate moieties. Tetramyristoyl cardiolipin (TMCL, (14:0)4CL) monolayers at the air-water interface are characterized by compression isotherms, which show a liquid expanded/liquid condensed phase transition. The phase transition surface pressure πc depends on the composition of the aqueous solution. In a calculation, this is attributed to the electrostatic double layer, which is induced by the head groups of the model membrane, and competitive ion binding. The intrinsic binding constant is large for protons ( KH = 10 L/mol) and small for monovalent cations ( KM (Na+, K+, Cs+) = 10-3 L/mol). The different intrinsic binding constants explain the non-monotonic behavior of πc on increasing the salt concentration: raising the monovalent salt concentration increases πc by charging the TMCL monolayer until 0.1 mol/L, then screening effects dominate and decrease πc by reducing the electrostatic repulsion between lipid head groups. When at fixed 0.15 mol/L NaCl concentration, the concentration of divalent cations is increased, πc decreases. The intrinsic binding constants of divalent cations follow the sequence Sr2+ < Mg2+ < Mn2+ ≈ Zn2+ ≈ Ca2+ ( KD,Ca = 1.2 L/mol). The predictive power of the calculations was tested with different solutions.

Resolving solution conformations of the model semi-flexible polyelectrolyte homogalacturonan using molecular dynamics simulations and small-angle x-ray scattering.

The conformation of polyelectrolytes in the solution state has long been of interest in polymer science. Herein we utilize all atom molecular dynamics simulations (MD) and small-angle x-ray scattering experiments (SAXS) to elucidate the molecular structure of the model polyelectrolyte homogalacturonan. Several degrees of polymerization were studied and in addition partial methylesterification of the otherwise charge-carrying carboxyl groups was used in order to generate samples with varying intra-chain charge distributions. It is shown that at length scales above around 1nm the conformation of isolated chains has surprisingly little dependence on the charge distribution or the concentration of attendant monovalent salts, reflective of the intrinsic stiffness of the saccharide rings and the dynamical constraints of the glycosidic linkage. Indeed the conformation of isolated chains over all accessible length scales is well described by the atomic coordinates available from fibre diffraction studies. Furthermore, in more concentrated systems it is shown that, after careful analysis of the SAXS data, the form of the inter-particle effects heralded by the emergence of a so-called polyelectrolyte peak, can be extracted, and that this phenomena can be reproduced by multiple chain MD simulations.

Coupling Transport of Water and Ions through a Carbon Nanotube in a Pressure Difference: The Relation between Dynamics and Ion Structures

Understanding the coupling relation between water and ions in their transport through nanochannels is relevant to many physicochemical process and biological activities and should be the preliminary knowledge that nanofluidic devices depend on. This work is devoted to revealing the coupling transport relation between water and ions through a (10,10) carbon nanotube under the drive of pressure difference. By using extensive molecular dynamics simulations, we compare different salts from monovalent to trivalent cations as well as the monovalent salt concentration. An interesting phenomenon is that the distinct cation valence will result in different channel blockage that ultimately leads to the dynamics bifurcation of water and ions. The strong coupling relation between water and ions can be uncovered by their dynamical similarity, such as the linear flux and power law translocation time behaviors with the change of pressure difference. Furthermore, with the increase of salt concentration, the dynamics of w...

Effects of Design Choices on the Stiffness of Wireframe DNA Origami Structures.

DNA origami is a powerful method for the creation of 3D nanoscale objects, and in the past few years, interest in wireframe origami designs has increased due to their potential for biomedical applications. In DNA wireframe designs, the construction material is double-stranded DNA, which has a persistence length of around 50 nm. In this work, we study the effect of various design choices on the stiffness versus final size of nanoscale wireframe rods, given the constraints on origami designs set by the DNA origami scaffold size. An initial theoretical analysis predicts two competing mechanisms limiting rod stiffness, whose balancing results in an optimal edge length. For small edge lengths, the bending of the rod's overall frame geometry is the dominant factor, while the flexibility of individual DNA edges has a greater contribution at larger edge lengths. We evaluate our design choices through simulations and experiments and find that the stiffness of the structures increases with the number of sides in the cross-section polygon and that there are indications of an optimal member edge length. We also ascertain the effect of nicked DNA edges on the stiffness of the wireframe rods and demonstrate that ligation of the staple breakpoint nicks reduces the observed flexibility. Our simulations also indicate that the persistence length of wireframe DNA structures significantly decreases with increasing monovalent salt concentration.

Intrastrand Base Pair Formation in Repetitive DNA Sequences

Repetitive trinucleotide DNA motifs at distinct genetic loci are responsible for triggering a broad group of hereditary, neurodegenerative diseases. The occurrence of non-helical DNA structure within these motif-harboring regions has long been identified as a structural precursor that interferes with normal DNA processing and ultimately leads to pathogenic states. The precise role of these structures in this interference remains unresolved; therefore, an improved understanding of these structures is critical for developing targeted therapies. As a first step, we characterized the formation and dynamics of CAG and CTG repeat DNA hairpins, a specific non-helical structure that can form within these trinucleotide sequences, which are associated with numerous forms of spinocerebellar ataxia and Huntington's disease. DNA constructs were designed that contain a specific number of either CAG or CTG repeats, and they were labeled at specific sites with the fluorescent dyes Cy3 and Cy5. Using single-molecule fluorescence microscopy, we directly imaged nanoscale changes in distance between the fluorescence dyes for individual DNA molecules in real time with sub-second resolution. The changes in interdye distance arise due to continuous conformational transitions between two hairpin states (open and closed). The time traces revealed that the CTG repeats form more stable hairpins than the CAG repeats. Strikingly, both sequences exhibit highly dynamic behavior with rapid open and closing rates. In addition, increases in the monovalent salt concentration increased the likelihood of the hairpin in the closed conformation. Analysis of the open and closing behavior revealed two reasons for this observation: 1) the closed form of the hairpin was longer lived at higher salt concentrations and 2) the transition from the open to the closed state occurred faster.

Strong attractions and repulsions mediated by monovalent salts

Controlling interactions between proteins and nanoparticles in electrolyte solutions is crucial for advancing biological sciences and biotechnology. The assembly of charged nanoparticles (NPs) and proteins in aqueous solutions can be directed by modifying the salt concentration. High concentrations of monovalent salt can induce the solubilization or crystallization of NPs and proteins. By using a multiscale coarse-grained molecular dynamics approach, we show that, due to ionic correlations in the electrolyte, NPs pairs at high monovalent salt concentrations interact via remarkably strong long-range attractions or repulsions, which can be split into three regimes depending on the surface charge densities of the NPs. NPs with zero-to-low surface charge densities interact via a long-range attraction that is stronger and has a similar range to the depletion attraction induced by polymers with radius of gyrations comparable to the NP diameter. On the other hand, moderately charged NPs with smooth surfaces as well as DNA-functionalized NPs with no possibility of hybridization between them interact via a strong repulsion of range and strength larger than the repulsion predicted by models that neglect ionic correlations, including the Derjaguin-Landau-Vervey-Overbeek (DLVO) model. Interactions between strongly charged NPs (>2 e/nm2), both types smooth and DNA-functionalized NPs, show an attractive potential well at intermediate-to-high salt concentrations, which demonstrates that electrolytes can induce aggregation of strongly charged NPs. Our work provides an improved understanding of the role of ionic correlations in NP assembly and design rules to utilize the salting-out process to crystallize NPs.

Ion-Induced Hydrogel Formation and Nematic Ordering of Nanocrystalline Cellulose Suspensions.

Nanocrystalline cellulose (NCC) is a promising material for formation of hydrogels and nematic liquid crystals. While salt addition is known to facilitate hydrogel formation, it remains unclear whether this originates from cationic bridging or charge screening effects. Herein, we demonstrate the effect of mono- and divalent salts on NCC gelation and nematic ordering. A strong correlation of NCC suspension zeta-potential and rheological behavior was found. Lower concentrations of divalent cations were needed to decrease NCC zeta-potential and form hydrogels. The same zeta-potentials and gel strengths were achieved at higher concentrations of monovalent salts. Salt-induced NCC aggregation is thus caused by intermolecular attractive forces rather than cationic bridging. Against excluded volume argumentation, salt addition was found to promote NCC nematic phase formation. Increased nematic ordering was observed in a transition regime of moderate salt addition before complete aggregation occurs. This regime is governed by an equilibrium of repulsive and attractive forces. Small angle neutron scattering suggests lateral orientation of NCC. Hence, NCC gelation and nematic ordering can be modulated via its zeta-potential by targeted salt addition.

Salt Promotes Protonation of Amine Groups at Air/Water Interface.

Interfacial water reorientation caused by charged Langmuir monolayers consisting of primary fatty amine (ODA) and cationic lipid having quaternary amine headgroup (DPTAP) were investigated by interface-selective vibrational sum-frequency generation spectroscopy. For DPTAP monolayer, initially large sum-frequency intensity from interfacial water OH band decreased steadily by increasing monovalent salt (NaCl, NaI) concentration due to counterion adsorption. On the other hand, ODA/water exhibited significantly smaller sum-frequency intensity than DPTAP/water, implying only small portion of protonated amine group (-NH3+) initially existed. By increasing the ionic strength, however, SF intensity of water OH band was enhanced markedly up to ∼1 mM, and then decreased in both NaCl and NaI solutions. By measuring the phase of the sum-frequency spectra, it was found that water dipoles under the ODA headgroup point downward, indicating that the surfaces were always positively charged. This demonstrated that increasing ionic strength facilitates protonation of primary amine headgroups. A simple model based on Poisson-Boltzmann (PB) theory explained this protonation behavior of primary amines.

A simple model for electrical charge in globular macromolecules and linear polyelectrolytes in solution.

We present a model for calculating the net and effective electrical charge of globular macromolecules and linear polyelectrolytes such as proteins and DNA, given the concentration of monovalent salt and pH in solution. The calculation is based on a numerical solution of the non-linear Poisson-Boltzmann equation using a finite element discretized continuum approach. The model simultaneously addresses the phenomena of charge regulation and renormalization, both of which underpin the electrostatics of biomolecules in solution. We show that while charge regulation addresses the true electrical charge of a molecule arising from the acid-base equilibria of its ionizable groups, charge renormalization finds relevance in the context of a molecule’s interaction with another charged entity. Writing this electrostatic interaction free energy in terms of a local electrical potential, we obtain an “interaction charge” for the molecule which we demonstrate agrees closely with the “effective charge” discussed in charge renormalization and counterion-condensation theories. The predictions of this model agree well with direct high-precision measurements of effective electrical charge of polyelectrolytes such as nucleic acids and disordered proteins in solution, without tunable parameters. Including the effective interior dielectric constant for compactly folded molecules as a tunable parameter, the model captures measurements of effective charge as well as published trends of shifts in globular proteins. Our results suggest a straightforward general framework to model electrostatics in biomolecules in solution. In offering a platform that directly links theory and experiment, these calculations could foster a systematic understanding of the interrelationship between molecular 3D structure and conformation, electrical charge and electrostatic interactions in solution. The model could find particular relevance in situations where molecular crystal structures are not available or rapid, reliable predictions are desired.

Mathematical modeling the formation of a histone octamer

A physical model of the interaction of protein molecules and their ability to form complex biological systems for the in vitro case in a solution of monovalent salt has been developed. Their reactive abilities using the methods of electrostatics based on the example of the step-by-step formation of the histone octamer from the H2A, H2B, H3, and H4 proteins have been studied. To analyze the ability of protein molecules to form compounds the matrix of potential energy of interactions between protein molecules in solutions with different concentrations of monovalent salt has been examined.

Kinetics of Swelling and Collapse of a Single Polyelectrolyte Chain

Kinetics of swelling and collapse of a single, isolated, flexible polyelectrolyte (PE) is described by a theoretical model considering uniform spherical expansion along with charge regularization. The equation of motion resulting from the osmotic and viscous forces is first solved numerically to obtain the temporal profiles for size and charge of the PE for swelling and deswelling in good solvent and collapse in poor solvent. Further, simpler analytical expressions for the equation of motion are obtained in the high- and low-salt limits. Finally, asymptotic analytical expressions for the time evolution of the size of the PE are derived. Of three major effects, like-charge repulsion of monomers, entropic force, and two-body attraction dominate respectively the processes of swelling, deswelling, and collapse. In general, the profiles show that the chain swells faster and farther for higher temperatures, lower dielectric mismatch, and lower concentrations of monovalent salt and deswells faster to smaller sizes for reverse trends of the same parameters. The kinetics is faster for lower molecular weights. Deswelling, followed by collapse, along with condensation of counterions, occurs in sufficiently poor solvents. For all configurational changes, charge of the PE chain decreases with decreasing size. Our results are in qualitative agreement with those available from simulations and experiments.

Synthesis and Measurement of Cohesive Mechanics in Polydopamine Nanomembranes

Polydopamine (PDA) is a complex biomimetic material exhibiting advantageous properties of both melanin polymers and biological adhesives. These concomitant features have prompted an increasing interest in exploiting PDA for bioelectronics, smart coatings, and functional membranes. The ability to apply the rich biochemistry of melanins in structurally durable contexts will help accelerate their practical implementation and the design of next‐generation materials. Here, this paper presents approaches for manipulating the adhesion and cohesive mechanics of PDA nanomembranes. Visual recording of PDA film delamination from SiO2 substrates reveals accelerated delamination (spanning orders of magnitude from 12 h to 1 min) with increasing pH and monovalent salt concentration. Delamination is retarded for films synthesized from higher dopamine concentrations, and it is completely prevented in the presence of Ca2+. Delaminated nanomembranes exhibit orientation‐dependent underwater adhesion to polydimethylsiloxane elastomer. Elastic moduli of PDA nanomembranes are quantified by compressive thin film buckling, and the measured value of 2.0 ± 0.9 GPa supports compositional simulations of PDA. Crosslinking of primary amines within the nanomembranes by genipin is successful in increasing the modulus to 7.9 ± 2.5 GPa. These results demonstrate that the adhesive stability and elastic modulus of PDA films/nanomembranes can be controlled by synthesis and postprocessing techniques.

Ion-Specific Modulation of Interfacial Interaction Potentials between Solid Substrates and Cell-Sized Particles Mediated via Zwitterionic, Super-Hydrophilic Poly(sulfobetaine) Brushes.

Zwitterionic polymer brushes draw increasing attention not only because of their superhydrophilic, self-cleaning capability but also due to their excellent antifouling capacity. We investigated the ion-specific modulation of the interfacial interaction potential via densely packed, uniform poly(sulfobetaine) brushes. The vertical Brownian motion of a cell-sized latex particle was monitored by microinterferometry, yielding the effective interfacial interaction potentials V(Δh) and the autocorrelation function of height fluctuation. The potential curvature V″(Δh) exhibited a monotonic increase according to the increase in monovalent salt concentrations, implying the sharpening of the potential confinement. An opposite tendency was observed in CaCl2 solutions, suggesting that the ion specific modulation cannot be explained by the classical Hofmeister series. When the particle fluctuation was monitored in the presence of free sulfobetaine molecules, the increase in [sulfobetaine] resulted in a distinct increase in hydrodynamic friction. This was never observed in all the other salt solutions, suggesting the interference of zwitterionic pairing of sulfobetaine side chains by the intercalation of sulfobetaine molecules into the brush layer. Furthermore, poly(sulfobetaine) brushes exhibited a very low V″(Δh) and hydrodynamic friction to human erythrocytes, which seems to explain the excellent blood repellency of zwitterionic polymer materials.

Particle Zeta Potentials Remain Finite in Saturated Salt Solutions.

The zeta potential of a particle characterizes its motion in an electric field and is often thought to be negligible at high ionic strength (several moles per liter) due to thinning of the electrical double layer (EDL). Here, we describe zeta potential measurements on polystyrene latex (PSL) particles at monovalent salt concentrations up to saturation (∼5 M NaCl) using electrophoresis in sinusoidal electric fields and high-speed video microscopy. Our measurements reveal that the zeta potential remains finite at even the highest concentrations. Moreover, we find that the zeta potentials of sulfated PSL particles continue to obey the classical Gouy-Chapman model up to saturation despite significant violations in the model's underlying assumptions. By contrast, amidine-functionalized PSL particles exhibit qualitatively different behaviors such as zero zeta potentials at high concentrations of NaCl and KCl and even charge inversion in KBr solutions. The experimental results are reproduced and explained by Monte Carlo simulations of a simple lattice model of the EDL that accounts for effects due to ion size and ion-ion correlations. At high salt conditions, the model suggests that quantitative changes in the magnitude of surface charge can result in qualitative changes in the zeta potential-most notably, charge inversion of highly charged surfaces. These findings have important implications for electrokinetic phenomena such as diffusiophoresis within salty environments such as oceans, geological reservoirs, and living organisms.

On the kinetics of chitosan/tripolyphosphate micro- and nanogel aggregation and their effects on particle polydispersity

Submicron chitosan/tripolyphosphate (TPP) particles are widely investigated as nanocarriers for drugs, genes and vaccines. One of the key particle properties that requires control is their size distribution, which depends on the extent of chitosan/TPP primary nanoparticle aggregation into higher-order submicron colloids. To provide a better understanding of this higher-order aggregation process, this study analyzes the factors that control chitosan/TPP particle aggregation kinetics in the presence of free TPP (such as present during particle formation). The aggregation rates exhibit a sharp power-law decrease with the monovalent salt concentration and a power-law increase with the free TPP concentration. Moreover, the aggregation rates increase with the pH and with the chitosan degree of deacetylation (DD). These variations in aggregation rates reflect the effects of monovalent salt, TPP concentration, pH and chitosan DD on particle bridging by the surface-bound TPP. Furthermore, these aggregation rates are much faster than those predicted based on Derjaguin and Landau, Verwey and Overbeek (DLVO) interaction potentials, which might reflect nonuniformities in particle shape and charge, and/or complications caused by particle softness. Finally, implications of the above aggregation kinetics on the uniformity of chitosan/TPP micro- and nanogel size are analyzed, where we: (1) show how particle polydispersity can be diminished by lowering the chitosan DD; and (2) explain the opposing results on how chitosan/TPP particle polydispersity is affected by monovalent salt.

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...