Counterions In Solution Research Articles

Direct Demonstration of DNA Compaction Mediated by Divalent Counterions.

We unambiguously demonstrated DNA attraction and its regulation mediated by divalent cations Mg2+ and Ca2+ by tethering a DNA single chain at various pH solutions. It is found that DNA is compacted when the pH of the solution containing these divalent counterions is decreased below 5. When the pH of the medium is ∼4, DNA is in an unstable transition state, being able to switch between compact and extensible states. We can also regulate the DNA attraction through a cyclic process of DNA compaction and unraveling by alternating the pH of the solution between 3 and 8. The corresponding change of morphology of DNA modulated by pH is also confirmed by atomic force microscopy (AFM). In the theoretical aspect, the present experimental finding is consistent with the coarse-grained simulation of Langevin dynamics on the effect of pH on DNA in a solution of divalent counterions.

Growth Temperature and Electrochemical Performance in Vapor-Deposited Poly(3,4-ethylenedioxythiophene) Thin Films for High-Rate Electrochemical Energy Storage

Poly(ethylene 3,4-dioxythiophene (PEDOT) films synthesized by oxidative chemical vapor deposition (oCVD) display strong electrochemical activity in the region from 2 to 4.2 V vs Li/Li+. By contrast, the more commonly studied PEDOT:polystyrenesulfonate (PSS) films have negligible electrochemical activity in this region. For the oCVD films, its small dopant anions (Cl–) that can easily enter and exit the polymer structure allow exchange with the Li+ counterion in solution, while for PEDOT:PSS, the poly(styrenesulfonate) dopant is a large macromolecule having substantially lower mobility. Here, we seek to elucidate the relationship between the structural characteristics of oCVD PEDOT thin films and their electrochemical properties, particularly in Li-ion electrolyte systems. Specifically, we seek to rationally design the thin-film properties of oCVD PEDOT for high-rate performance and cycle life by varying the film growth temperature. We observe that the dominant effect of increasing growth temperature is an...

From octopus to dendrite-Semiflexible polyelectrolyte brush condensates in trivalent counterion solution.

Interplay between counterion-mediated interaction and stiffness inherent to polymer chain can bring substantial complexity to the morphology and dynamics of polyelectrolyte brush condensates. Trivalent counterions induce collapse of flexible polyelectrolyte brushes, over a certain range of grafting density, into octopus-like surface micelles; however, if individual chains are rigid enough, the ion-mediated local nematic ordering assembles the brush chains into fractal-like dendritic condensates whose relaxation dynamics is significantly slower than that in the surface micelles. Notably, the trivalent ions condensed in the dendritic condensates are highly mobile displaying quasi-one-dimensional diffusion in parallel along the dendritic branches. Our findings in this study are potentially of great significance to understanding the response of cellular organization such as chromosomes and charged polysaccharides on membranes to the change in ionic environment.

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.

Change of flow structure and heat transfer performance of photosensitive micellar solutions with light irradiation

Heat transfer enhancement in the low Reynolds number condition is increasingly required with the perpetual attempt to integrate and miniaturize electronic devices. The authors have suggested the application of photo-rheological fluid, whose rheological properties change by external photic stimulus, to a coolant of small heat exchangers. High heat transfer state and low pressured drop state can be switched by light irradiation to the fluid according to the demand for heat transfer enhancement. This study investigated how the change of viscoelasticity of photosensitive micellar solutions by light irradiation relates to flow structure and heat transfer performance. Flow visualization and heat transfer measurement were conducted using an aqueous solution of surfactant and counterions (cetyl-trimethyl-ammonium bromide/sodium salicylate/ortho-methoxy cinnamic acid) with different light irradiation time in serpentine channels. The results showed that the decrease of the viscoelasticity with light irradiation caused the decrease in heat transfer rate and pressure drop. In the flow visualization, two types of secondary vortex, i.e., a single large-scaled vortex and a pair of counter-rotating vortices, were observed. As the light irradiation time increased, flow unsteadiness diminished, and the frequency in the appearance of a single vortex and fast flow near wall decreased. The decrease in heat transfer rate and pressure drop with light irradiation was ascribed to the deterioration of fluid mixing by flow unsteadiness, the reduction of momentum and thermal diffusion by the decrease in near-wall velocity gradient, and the decrease in the strength of a pair of vortices.

The Mixing Counterion Effect on DNA Compaction and Charge Neutralization at Low Ionic Strength.

DNA compaction and charge neutralization in a mixing counterion solution involves competitive and cooperative electrostatic binding, and sometimes counterion complexation. At normal ionic strength, it has been found that the charge neutralization of DNA by the multivalent counterion is suppressed when being added extra mono- and di-valent counterions. Here, we explore the effect mixing counterion on DNA compaction and charge neutralization under the condition of low ionic strength. Being quite different from normal ionic strength, the electrophoretic mobility of DNA in multivalent counterion solution (octalysine, spermine) increases the presence of mono- and di-valent cations, such as sodium and magnesium ions. It means that the charge neutralization of DNA by the multivalent counterion is promoted rather than suppressed when introducing extra mono- and di-valent counterions into solution. This conclusion is also supported by the measurement of condensing and unraveling forces of DNA condensates under the same condition by single molecular magnetic tweezers. This mixing effect can be attributed to the cooperative electrostatic binding of counterions to DNA when the concentration of counterions in solution is below a critical concentration.

Interaction of NaOH solutions with silica surfaces

HypothesisSodium adsorption on silica surfaces depends on the solution counter-ion. Here, we use NaOH solutions to investigate basic environments. SimulationsSodium adsorption on hydroxylated silica surfaces from NaOH solutions were investigated through molecular dynamics with a dissociative force field, allowing for the development of secondary molecular species. FindingsAcross the NaOH concentrations (0.01 M − 1.0 M), ∼50% of the Na+ ions were concentrated in the surface region, developing silica surface charges between − 0.01 C/m2 (0.01 M NaOH) and − 0.76 C/m2 (1.0 M NaOH) due to surface site deprotonation. Five inner-sphere adsorption complexes were identified, including monodentate, bidentate, and tridentate configurations and two additional structures, with Na+ ions coordinated by bridging oxygen and hydroxyl groups or water molecules. Coordination of Na+ ions by bridging oxygen atoms indicates partial or complete incorporation of Na+ ions into the silica surface. Residence time analysis identified that Na+ ions coordinated by bridging oxygen atoms stayed adsorbed onto the surface four times longer than the mono/bi/tridentate species, indicating formation of relatively stable and persistent Na+ ion adsorption structures. Such inner-sphere complexes form only at NaOH concentrations of > 0.5 M. Na+ adsorption and lifetimes have implications for the stability of silica surfaces.

Effect of ion exchange in NaAOT surfactant on droplet size and location of dye within Rhodamine B (RhB)-containing microemulsion at low dye concentration

Dioctyl sulfosuccinate sodium salt or Sodium AOT (NaAOT), an ionic surfactants give Na+ counterions in solution, which can interact with a negatively charged surface formed during the formation of nanodroplets in NaAOT/water/oil microemulsion system. In this work, effect of cation exchange in NaAOT surfactant from Na to H on the water droplet size and location of dye of Rhodamine B (RhB) at low concentration within water/decane microemulsions at W=[Water]/[Surfactant]=40 was studied by the dynamic light scattering (DLS) and spectroscopy techniques. The dynamical characterization of nanometer sized micelles by DLS technique indicated that by exchanging cation within NaAOT microemulsion containing dye, the water droplet size increased and the interactions between nanodroplets were less repulsive with the mass fraction of nanodroplets (MFD). It was observed that hydrophilic dye of RhB at low concentration can be localized near tail (Gouy Chapman Layer) of HOT microemulsion in addition to the locating into the water/oil interface (Stern Layer), slightly than NaAOT microemulsion. Furthermore, molecules of RhB at high dye concentration were localized near tail (Gouy Chapman Layer) of NaAOT microemulsion in addition to locating into the water/oil interface (Stern Layer) compared to molecules of dye at low concentration within NaAOT microemulsion.

Counter-ion solution modeling in salt-free polyelectrolytes depicts clustering mystery

Polymer chains modeled in a counter-ion solvation lead to polyelectrolyte clustering that is observed in solution, but has so far been mysterious in origin.

Modulation of wettability, gradient and adhesion on self-assembled monolayer by counterion exchange and pH

In this study, two quaternary ammonium salts derived from l-lipoic acid were applied for self-assembled monolayers formation on rough structured gold surface. The derivatives differ in functionality since one possesses simple quaternary ammonium group whereas the other one is carboxybetaine ester containing quaternary ammonium group with pH hydrolysable ester group as a pendant. The response of surface wettability to ion exchange between Cl− and perfluorooctanoate, kinetics and gradient wettability were examined by water contact angle measurement and confirmed by X-ray photoelectron spectroscopy. Furthermore, adhesion forces related to applied counterion on the entire surface and after hydrolysis were investigated by atomic force microscopy measurement at nanometer scales. A dramatic change in wettability upon counterion exchange from superhydrophilic for Cl− to very or superhydrophobic for perfluorooctanoate in a repeatable manner was observed for both derivatives. Kinetics of counterion exchanges revealed faster hydration of simple quaternary derivate. The wettability gradient could be designed from superhydrophobic to superhydrophilic either in a reversible manner by simple immersion of the modified surface in a counterion solution modulated by ionic strength or in an irreversible manner for carboxybetaine ester derivate by time-controlled hydrolysis to charge balanced carboxybetaine.

DNA Compaction and Charge Inversion Induced by Organic Monovalent Ions.

DNA condensation and charge inversion usually occur in solutions of multivalent counterions. In the present study, we show that the organic monovalent ions of tetraphenyl chloride arsenic (Ph4As+) can induce DNA compaction and even invert its electrophoretic mobility by single molecular methods. The morphology of condensed DNA was directly observed by atomic force microscopy (AFM) in the presence of a low concentration of Ph4As+ in DNA solution. The magnetic tweezers (MT) measurements showed that DNA compaction happens at very low Ph4As+ concentration (≤1 μM), and the typical step-like structures could be found in the extension-time curves of tethering DNA. However, when the concentration of Ph4As+ increased to 1 mM, the steps disappeared in the pulling curves and globular structures could be found in the corresponding AFM images. Electrophoretic mobility measurement showed that charge inversion of DNA induced by the monovalent ions happened at 1.6 mM Ph4As+, which is consistent with the prediction based on the strong hydrophobicity of Ph4As+. We infer that the hydrophobic effect is the main driving force of DNA charge inversion and compaction by the organic monovalent ion.

Heterogeneous Morphology and Dynamics of Polyelectrolyte Brush Condensates in Trivalent Counterion Solution

Recent experiments have shown that trivalent ion, spermidine$^{3+}$, can provoke lateral microphase segregation in DNA brushes. Using molecular simulations and simple theoretical arguments, we explore the effects of trivalent counterions on polyelectrolyte brushes. At a proper range of grafting density, polymer size, and ion concentration, the brush polymers collapse heterogeneously into octopus-like surface micelles. Remarkably, the heterogeneity in brush morphology is maximized and the relaxation dynamics of chain and condensed ion are the slowest at the 1:3 stoichiometric concentration of trivalent ions to polyelectrolyte charge. A further increase of trivalent ion concentration conducive to a charge inversion elicits modest reswelling and homogenizes the morphology of brush condensate. Our study provides a new insight into the origin of the diversity in DNA organization in cell nuclei as well as the ion-dependent morphological variation in polyelectrolyte brush layer of biological membranes.

Strengthening of the Coordination Shell by Counter Ions in Aqueous Th4+ Solutions.

The presence of counterions in solutions containing highly charged metal cations can trigger processes such as ion-pair formation, hydrogen bond breakages and subsequent re-formation, and ligand exchanges. In this work, it is shown how halide (Cl-, Br-) and perchlorate (ClO4-) anions affect the strength of the primary solvent coordination shells around Th4+ using explicit-solvent and finite-temperature ab initio molecular dynamics modeling methods. The 9-fold solvent geometry was found to be the most stable hydration structure in each aqueous solution. Relative to the dilute aqueous solution, the presence of the counterions did not significantly alter the geometry of the primary hydration shell. However, the free energy analyses indicated that the 10-fold hydrated states were thermodynamically accessible in dilute and bromide aqueous solutions within 1 kcal/mol. Analysis of the results showed that the hydrogen bond lifetimes were longer and solvent exchange energy barriers were larger in solutions with counterions in comparison with the solution with no counterions. This implies that the presence of the counterions induces a strengthening of the Th4+ hydration shell.

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.

Specific Ion Effects and pH Dependence on the Interaction Forces between Polystyrene Particles.

Colloidal interactions have been extensively studied due to the wide number of applications where colloids are present. In general, the electric double layer force and the van der Waals interaction dominate the net force acting between two colloids at large separation distances. However, it is well accepted that some other phenomena, especially those acting at short separation distances, might be relevant and induce substantial changes in the force profiles. Within these phenomena, those related to the surface contact angle, the hydration degree of the ions, or the pH, may dominate the force profiles features, not only at short distances. In this paper, we analyzed the effect of the pH and counterion type on the long-range as well as short-range forces between polystyrene colloidal particles by using the colloidal probe technique based on AFM. Our results confirm that the features of the force profiles between polystyrene surfaces are strongly affected by the pH and hydration degree of the counterions in solution. Additionally, we performed a study of the role of the pH on the wettability properties of hydrated and nonhydrated polystyrene sheets to scan the wettability properties of this material with pH. Contact angle measurements confirmed that the polystyrene surface is hydrophobic in aqueous solutions over the entire range of pHs investigated. These results are in good agreement with the features observed in the force profiles at low pH. At high pH, a short-range repulsion similar to the one observed for hydrophilic materials is observed. This repulsion scales with the pH, and it also depends on the hydration degree of the ions in solution. This way, the short-range forces between polystyrene surfaces may be tunable with the pH, and its origin does not seem to be related to the hydrophobicity of the material.

Simple, Reversible, and Fast Modulation in Superwettability, Gradient, and Adsorption by Counterion Exchange on Self-Assembled Monolayer.

A simple fabrication method for preparation of surfaces able to switch from superhydrophobic to superhydrophilic state in a reversible and fast way is described. A self-assembled monolayer (SAM) consisting of quaternary ammonium group with aliphatic tail bearing terminal thiol functionality was created on gold nano/microstructured and gold planar surfaces, respectively. A rough nano/microstructured surface was prepared by galvanic reaction on a silicon wafer. The reversible counterion exchange on the rough surface resulted in a switchable contact angle between <5° and 151°. The prewetted rough surface with Cl(-) as a counterion possesses a superoleophobic underwater character. The kinetics of counterion exchanges suggests a long hydration process and strong electron ion pairing between quaternary ammonium group and perfluorooctanoate counterion. Moreover, a wettability gradient from superhydrophobic to superhydrophilic can be formed on the modified rough gold surface in a robust and simple way by passive incubation of the substrate in a counterion solution and controlled by ionic strength. Furthermore, adsorption of gold nanoparticles to modified plain gold surface can be controlled to a high extent by counterions present on the SAM layer.

Effect of Divalent Counterions on Polyelectrolyte Multilayer Properties

When exposed to divalent counterion solutions, polyelectrolyte multilayer (PEM) films of poly(diallyldimethylammonium chloride) and sodium poly(styrenesulfonate) (NaPSS) prepared in the presence of monovalent salt, or equilibrated with such a salt, are physically cross-linked by divalent counterion incorporation, altering PEM properties significantly. The rapid cross-linking was monitored by the quartz crystal microbalance with dissipation (QCM-D) method, which finds PEM deswelling and rigidification after exposures to a low concentration of Cu(NO3)2; at higher concentration, deswelling is countered by increased PEM uptake of the salt, which disrupts polyelectrolyte–polyelectrolyte ion pairs. Divalent ion incorporation into PEMs has the character of ion exchange, and incorporated divalent ions are quickly and completely removed when presented with monovalent salt solution but not with water. While counterion cross-linking extends across the bulk of the PEM, the fraction of exchanged counterions remains lo...

The effect of pH on charge inversion and condensation of DNA.

Charge inversion and condensation of DNA in solutions of trivalent and quadrivalent counterions are significantly influenced by the pH value of the solution. We systematically investigated the condensation and charge compensation of DNA by spermidine, hexammine cobalt(iii) (cohex, [Co(NH3)6](3+)) and spermine in solutions of a wide range of pH values from 3 to 9.3 by dynamic light scattering, magnetic tweezers, and atomic force microscopy. In trivalent counterion solution, we found that there is a critical concentration (0.75 mM for cohex and 0.5 mM for spermidine), under which the electrophoresis mobility of DNA initially increases, reaches a maximum, and finally decreases when the pH value is decreased. In contrast, above the critical concentration, the electrophoretic mobility of DNA increases monotonously with decreasing pH value of the solution. The corresponding condensing force has the same dependence on the pH value. However, for the case of quadrivalent counterions, the electrophoretic mobility of DNA is monotonously promoted by lowering the pH value of the solution at any concentration of counterions in which charge inversion of DNA may occur. In atomic force microscopy images and force spectroscopy of magnetic tweezers, we found that maximal charge neutralization and condensation force correspond to the most compact DNA condensation. We propose a mechanism of promoting DNA charge neutralization: small and highly mobile hydrogen ions tend to attach to the DNA-counterion complex to further neutralize its remaining charge, which is related to the surface area of the complex. Therefore, this further neutralization is prominent when the complex is toroidal which corresponds to the case of mild ion concentration while it is less prominent for more compact globules or rod complexes at high counterion concentration.

Electrostatic potential of mean force between two curved surfaces in the presence of counterion connectivity.

In this paper, we investigate effects of counterion connectivity (i.e., association of the counterions into a chain molecule) on the electrostatic potential of mean force (EPMF) between two similarly charged cylinder rods in a primitive model electrolyte solution by solving a classical density functional theory. The main findings include the following: (i) The counterion connectivity helps in inducing a like-charge-attractionlike (LCA-like) phenomenology even in a monovalent counterion solution wherein the LCA-like observation generally does not occur without the counterion connectivity. (ii) For divalent counterion solutions, the counterion connectivity can reinforce or weaken the LCA-like observation depending on the chain length N, and simply increases the equilibrium nearest surface separation of the rods corresponding to the minimum EPMF to nearly three times the counterion site diameter, whether N is large or small. (iii) If N is large enough, the LCA-like strength tends to be negatively correlated with the electrolyte concentration c over the entire range of the rod surface charge magnitude |σ*| considered; whereas if N drops, the correlation tends to become positive with decrease of the |σ*| value, and particularly for modest |σ*| values, the correlation relationship exhibits an extreme value phenomenon. (iv) In the case of a 1:1 electrolyte, the EPMF effects of the diameters of counterion and coion sites are similar in both situations with and without the counterion connectivity. All of these findings can be explained self-consistently by a recently proposed hydrogen-bonding style mechanism reinforced by one additional concept: flexibility of the counterion chain and the factors affecting it, like N and counterion site valence.

Effective interactions between fluid membranes.

A self-consistent theory is proposed for the general problem of interacting undulating fluid membranes subject to the constraint that they do not interpenetrate. We implement the steric constraint via an exact functional integral representation and, through the use of a saddle-point approximation, transform it into a novel effective steric potential. The steric potential is found to consist of two contributions: one generated by zero-mode fluctuations of the membranes and the other by thermal bending fluctuations. For membranes of cross-sectional area S, we find that the bending fluctuation part scales with the intermembrane separation d as d-2 for d≪√S but crosses over to d-4 scaling for d≫√S, whereas the zero-mode part of the steric potential always scales as d-2. For membranes interacting exclusively via the steric potential, we obtain closed-form expressions for the effective interaction potential and for the rms undulation amplitude σ, which becomes small at low temperatures T and/or large bending stiffnesses κ. Moreover, σ scales as d for d≪√S but saturates at √kBTS/κ for d≫√S. In addition, using variational Gaussian theory, we apply our self-consistent treatment to study intermembrane interactions subject to different types of potentials: (i) the Moreira-Netz potential for a pair of strongly charged membranes with an intervening solution of multivalent counterions, (ii) an attractive square well, (iii) the Morse potential, and (iv) a combination of hydration and van der Waals interactions.