Presence Of Monovalent Salt Research Articles

Phase Behaviors of Charged Macromolecules in Aqueous Solutions

AbstractCompared to the charge–charge interaction, the role of the dipole‐dipole interaction has long been ignored in the phase behaviors of charged macromolecules in solutions. Charged macromolecules in solutions exhibit rich phase behaviors due to their complexity and they have been studied extensively. Phase separation can happen for charged macromolecules in the presence of monovalent salt, multivalent salt, and oppositely charged polymers, surfactants, etc., and for more advanced charged macromolecules such as polyzwitterions and polyampholytes, the phase diagram is even richer. In this perspective, the unacknowledged role of dipole‐dipole interaction in the phase behaviors of charged macromolecular solutions will be introduced. Dipolar polymers can form complex, self‐regulating structures which can be employed in various fields from drug‐delivery systems to next‐generation polymers. More importantly, it will shed light on how some of the life's basic and coherent structures such as biomolecular condensates and membrane‐less organelles are assembled and built by charged biomacromolecules such as DNA, RNA, and proteins.

Probing stability of the charge-reversed nanoparticles in electrolyte and surfactant solutions

Anionic silica nanoparticles show an interesting re-entrant phase behavior, where the system transforms from stable one-phase to two-phase (aggregated state) and returns back to one-phase as varying concentration of multivalent counterions. The re-entrant phase arises due to charge inversion of nanoparticles via excessive condensation of multivalent counterions. The nanoparticles in the re-entrant regime are found to have contrasting stability in the presence of monovalent salt and non-ionic surfactant, when compared to original phase. The charge-reversed nanoparticles show very high stability against NaCl, whereas very low stability against surfactant-induced depletion attraction. These observations are explained based on modifications in inter-particle interactions.

Interactions between zwitterionic membranes in complex electrolytes.

We investigate the electrostatic interactions of zwitterionic membranes immersed in mixed electrolytes composed of mono- and multivalent ions. We show that the presence of monovalent salt is a necessary condition for the existence of a finite electrostatic force on the membrane. As a result, the mean-field membrane pressure originating from the surface dipoles exhibits a nonuniform salt dependence, characterized by an enhancement for dilute salt conditions and a decrease at intermediate salt concentrations. On addition of multivalent cations to the submolar salt solution, the separate interactions of these cations with the opposite charges of the surface dipoles makes the intermembrane pressure more repulsive at low membrane separation distances and strongly attractive at intermediate distances, resulting in a discontinuous like-charge binding transition followed by the membrane binding transition. By extending our formalism to account for correlation corrections associated with large salt concentrations, we show that membranes of high surface dipole density immersed in molar salt solutions may undergo a membrane binding transition even without the multivalent cations. Hence, the tuning of the surface polarization forces by membrane engineering can be an efficient way to adjust the equilibrium configuration of dipolar membranes in concentrated salt solutions.

Influence of pH and Salt Concentration on Pickering Emulsions Stabilized by Colloidal Peanuts.

Solid-stabilized emulsions commonly known as Pickering emulsions offer unique benefits such as superior stability and controlled permeability compared to conventional surfactant stabilized emulsions. In this article, the effect of pH, the electrolyte and particle concentration, homogenization speed, and volume fraction of oil on the formation, stability, and the microstructure of emulsion droplets stabilized by micron-size peanut-shaped hematite particles are investigated. The influence of surface charge of particles on emulsification is studied by varying the pH of the dispersing medium, the addition of an electrolyte or a combination of both. Stable O/W emulsions are formed only when the aqueous dispersions at intermediate pH between 4 and 11, and decane (2:1 volume ratio) are vigorously mixed. However, emulsions are not formed when the particles are highly charged that is, at pH 2 and 12. The presence of monovalent salt or high-speed homogenization assists the emulsion formation at pH 3, whereas their combination helps in emulsification at pH 2. However, neither the addition of an electrolyte nor the high-speed homogenization or their combination facilitates the formation of emulsions at pH 12. We show that the image-charge repulsion and the surface charge induced wettability change can explain the influence of both pH and salt concentrations on the formation of Pickering emulsions. Although oil-in-water emulsions typically cream because of the density difference, microscopy observations revealed the presence of a large number of small particle-covered oil droplets in the sediments of the emulsified samples. These drops are observed to be entrapped in dense-particle networks. This leads to a considerable reduction in the number of particles available for the stabilization of floating emulsion droplets and thus influences their size and surface coverage. The possibility of tailoring the stability, droplet size and, the surface coverage discussed in this article can play a crucial role in situations that demand controlled release of active components.

Effect of ethanol on structures and interactions among globular proteins

Structures and interactions among globular proteins BSA and lysozyme are explored by small angle neutron scattering (SANS) technique at pD≈7.0 by varying ethanol concentration. Interaction behaviours are also obtained in presence of monovalent salt (NaCl). SANS analysis shows that for both lower and higher BSA concentrations and in presence of NaCl, combination of intermediate-range repulsion and weak long-range attraction is responsible for the effective interaction behaviours with the variation of ethanol concentration. For lysozyme, interaction nature is same as BSA in absence of NaCl but in presence of NaCl, fractal structure factor explains the interaction behaviours.

Molecular dynamics simulations of cylindrical polyelectrolyte brushes in monovalent and multivalent salt solutions

Molecular dynamics simulations are applied to investigate the cylindrical polyelectrolyte brushes in monovalent and multivalent salt solutions. By varying the salt valence and concentration, the brush thickness, shape factor of grafted chains, and distributions of monomers and ions in the solutions are studied. The simulation results show that the single osmotic pressure effect in the brush leads to changes in conformation in the presence of monovalent salt, while the ion exchange effect induces the collapse of the brushes in the multivalent salt solutions. Furthermore, the snapshots combined with the distributions of the end-monomers and the mean bond angles demonstrate a nonuniform stretching picture of the grafted chains, which is different with the chains tethered on the planar surface. The charge ratios between the ions trapped in the brush and the monomers are also calculated to elucidate the details of ion exchange process.

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...

Thermo‐responsive gels in the presence of monovalent salt at physiological concentrations: A Monte Carlo simulation study

ABSTRACTIn this work, slightly charged thermo‐responsive gels in the presence of salt at concentrations close to physiological conditions have been simulated within a coarse‐grained model widely used in the last decade. These simulations allow differentiate charge and salt effects, which are antagonist and coupled in many real systems because the degree of ionization might depend on the electrolyte concentration. An analysis in terms of the different contributions to osmotic pressure is also presented, which highlights the role played by excluded volume effects. In addition, our results also permit us to test some predictions based on the ideal Donnan equilibrium, a common assumption made to justify the swelling behavior of gels and microgels in the presence of salt. More specifically, simulations show that, for the slightly charged gels simulated here, such an assumption overestimates the concentration of salt inside collapsed gels and underestimates the excess of osmotic pressure associated to the additional electrolyte. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1403–1411

Ion permeation inside microgel particles induced by specific interactions: from charge inversion to overcharging.

In this work we have performed a theoretical study of a system formed by ionic microgels in the presence of monovalent salt with the help of Ornstein-Zernike integral equations within the hypernetted-chain (HNC) approximation. We focus in particular on analysing the role that the short-range specific interactions between the polymer fibres of the microgel and the incoming ions have on the equilibrium ion distribution inside and outside the microgel. For this purpose, a theoretical model based on the equilibrium partitioning effect is developed to determine the interaction between the microgel particle and a single ion. The results indicate that when counterions are specifically attracted to the polymer fibres of the microgel, an enhanced counterion accumulation occurs that induces the charge inversion of the microgel and a strong increase of the microgel net charge (or overcharging). In the case of coions, the specific attraction is also able to provoke the coion adsorption even though they are electrostatically repelled, and so increasing the microgel charge (true overcharging). Moreover, we show that ion adsorption onto the microgel particle is very different in swollen and shrunken states due to the competition between specific attraction and steric repulsion. In particular, ion adsorption occurs preferentially in the internal core of the particle for swollen states, whereas it is mainly concentrated in the external shell for de-swollen configurations. Finally, we observe the existence of a critical salt concentration, where the net charge of the microgels vanishes; above this inversion point the net charge of the microgels increases again, thus leading to reentrant stability of microgel suspensions.

Effective charge of ionic microgel particles in the swollen and collapsed states: The role of the steric microgel-ion repulsion

In this work the system formed by charged (ionic) microgels in the presence of monovalent salt is investigated by solving numerically the Ornstein-Zernike integral equations within the Hypernetted-Chain approximation. The ionic density profiles, effective interaction between microgel particles, and the effective charge of the particles are calculated. In addition to the electrostatic interaction, the excluded-volume repulsion between the microgel particle and the ions is also explicitly taken into account. Although this steric interaction is irrelevant in the swollen state (when the packing fraction of the polymer network is low), it becomes a very important contribution close to the de-swollen state, hindering the counterion penetration inside the microgel mesh. The theoretical predictions indicate that the ionic density profiles are strongly affected by the degree of swelling, going from a volumetric distribution of counterions in the swollen state to a surface accumulation outside the particle that becomes more important as the particle shrinks. The electrostatic effective interaction between pairs of microgel particles is shown to be the result of a complex interplay between electrostatic and depletion effects that strongly depend on the bare charge density of the particle. For sufficiently charged microgel particles, the steric exclusion leads to a less efficient screening of the microgel charge near the de-swollen configuration, and so to a significant increase of the effective charge of the microgel particle.

Stability of Chitosan Nanoparticles Cross-Linked with Tripolyphosphate

The physical stability of chitosan nanoparticles cross-linked with sodium tripolyphosphate (TPP) was investigated over a period of 1 month. Special emphasis was placed on changes in the particle size and the particle compactness, which are two important physicochemical parameters of nanoparticulate drug delivery systems. The chitosan-TPP particles were prepared at different ionic strengths, chitosan chloride concentrations, and TPP-to-chitosan ratios. In the presence of monovalent salt, the positive ζ potential of the particles was reduced. In spite of this, the particles were more stable when prepared and stored under saline conditions compared to water. This could be attributed to the smaller particle sizes found in the presence of sodium chloride. Most of the particles prepared in saline solvents were stable with respect to changes in the size and the compactness of the particles. However, instability was observed at the highest cross-linker-to-polymer ratios. Generally, a reduction in the ζ potential and an increase in the particle compactness were observed at increasing TPP-to-chitosan ratios. This combined with the size increase induced by a high concentration of chitosan, increased the aggregation and sedimentation tendency of the particles and reduced the colloidal stability of the particles.

A physico-chemical model for polyelectrolyte enhanced ultrafiltration

A physico-chemical model for the polyelectrolyte enhanced ultrafiltration (PEUF) was developed. Experimental data available for removal of cadmium using poly-ammonium acrylate (PAmA) was used to validate the model. The model was based on counter ion condensation theory of Oosawa. Intensity of binding of cadmium to polyelectrolyte was estimated. Solution osmotic pressure was quantified by computing the activity of the polyelectrolyte–counter ion complex. Using osmotic pressure controlling model, the permeate flux was quantified. Variation of polyelectrolyte diffusivity and viscosity with concentration within the mass transfer boundary layer was included in the calculation. Various parameters of the model were estimated by comparing the experimental data with the calculated ones. The model was also extended and developed for PEUF system in presence of monovalent salt, variation of pH and for pure polyelectrolyte solution.

A tale of tails: how histone tails mediate chromatin compaction in different salt and linker histone environments.

To elucidate the role of the histone tails in chromatin compaction and in higher-order folding of chromatin under physiological conditions, we extend a mesoscale model of chromatin (Arya, Zhang, and Schlick. Biophys. J. 2006, 91, 133; Arya and Schlick. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16236) to account for divalent cations (Mg(2+)) and linker histones. Configurations of 24-nucleosome oligonucleosomes in different salt environments and in the presence and absence of linker histones are sampled by a mixture of local and global Monte Carlo methods. Analyses of the resulting ensembles reveal a dynamic synergism between the histone tails, linker histones, and ions in forming compact higher-order structures of chromatin. In the presence of monovalent salt alone, oligonucleosomes remain relatively unfolded, and the histone tails do not mediate many internucleosomal interactions. Upon the addition of linker histones and divalent cations, the oligonucleosomes undergo a significant compaction triggered by a dramatic increase in the internucleosomal interactions mediated by the histone tails, formation of a rigid linker DNA "stem" around the linker histones' C-terminal domains, and reduction in the electrostatic repulsion between linker DNAs via sharp bending in some linker DNAs caused by the divalent cations. Among all histone tails, the H4 tails mediate the most internucleosomal interactions, consistent with experimental observations, followed by the H3, H2A, and H2B tails in decreasing order. Apart from mediating internucleosomal interactions, the H3 tails also contribute to chromatin compaction by attaching to the entering and exiting linker DNA to screen electrotatic repulsion among the linker DNAs. This tendency of the H3 tails to attach to linker DNA, however, decreases significantly upon the addition of linker histones due to competition effects. The H2A and H2B tails do not mediate significant internucleosomal interactions but are important for mediating fiber/fiber intractions, especially in relatively unfolded chromatin in monovalent salt environments.

Formation of Block Liposomes is a General Phenomenon of Charged Membranes

We have recently reported on the discovery of block liposomes (BLs), a new class of chain-melted (liquid) vesicles, formed in mixtures of curvature-stabilizing hexadecavalent cationic lipid MVLBG2, DOPC and water (Zidovska et al., Langmuir, 2008). BLs consist of distinctly shaped, yet connected spheres, pears, tubes, or rods. Unlike typical liposome systems, where spherical vesicles, tubular vesicles, and cylindrical micelles are separated on the macroscopic scale, within a BL, shapes are separated on the nanometer scale. We carried out structural studies of BLs with differential-contrast-microscopy (DIC) and cryo-transmission-electron-microscopy (cryo-TEM) and identified membrane charge and spontaneous membrane curvature as key parameters controlling the BL-formation. BL-formation was believed to be a special capacity of MVLBG2, a newly synthesized highly charged (16+) lipid (Ewert et al., JACS, 2006) with giant dendrimer-like headgroup leading to a conical molecular shape resulting into high spontaneous membrane curvature, when incorporated into lipid bilayer. In this work we report formation of BLs for other cationic lipids, demonstrating that BL-formation is a general phenomenon of all charged membranes. We carried out systematic study of binary lipid mixtures comprised of DOPC and a cationic lipid, varying the headgroup size and charge of cationic lipid from 1+, 3+ to 5+. We find that all cationic lipids form BLs on the micrometer and nanometer scale. We have also found that pure DOPC forms BLs in presence of monovalent salt, which is known to cause zwitterionic DOPC to become negatively charged. This latter finding confirms that BL-formation is a general capacity of charged membranes independent of the charge nature. Block liposomes may find a range of applications in chemical and nucleic acid delivery and as building blocks in the design of templates for hierarchical structures. Funding by DOE DE-FG-02-06ER46314, NIH GM-59288, NSF DMR-0503347.

Effect of monovalent salt on the conformation of polyelectrolyte-surfactant complexes

We study the conformation of polyelectrolyte-surfactant complexes in the presence of monovalent salt. A simple model for the formation of these structures is presented in the framework of the Debye-Hückel-Bjerrum-Manning and Flory theories, with the hydrophobic interactions between the hydrocarbon tails of surfactant molecules treated in the spirit of van der Waals theory as an effective attraction. The extension of the polyelectrolyte-surfactant complexes is analyzed as a function of the salt concentration and a discrete conformational transition between a compact globule and an elongated coil is found, in agreement with experimental results for the unfolding transition of a DNA-cationic surfactant complex.

Laser light scattering and isothermal titration calorimetric studies of poly(ethylene oxide) aqueous solution in presence of sodium dodecyl sulfate

The aqueous solution of poly(ethylene oxide) (PEO) in the presence of different concentrations of sodium dodecyl sulfate (SDS) was examined by laser light scattering and isothermal titration calorimetric techniques. A small fraction of PEO aggregates were found to coexist with unimeric PEO chains in dilute solution. The presence of monovalent salt does not alter the hydrodynamic properties of PEO in aqueous solution. Addition of a monovalent anionic surfactant, such as SDS, induces cooperative binding of surfactant monomers to PEO backbones at SDS concentrations ranging from 4.0 mM (critical aggregation concentration) to 16.5 mM (saturation concentration). The hydrodynamic radius of PEO unimers decreases initially and then increases with SDS concentration, resulting from the structural reorganization of the PEO/SDS complex. Beyond the saturation concentration, the hydrodynamic radii of PEO/SDS complex are independent of SDS concentration.

Flexible polyelectrolytes with monovalent salt

We present a model for describing flexible polyelectrolytes in a good solvent and in the presence of monovalent salt. The molecule composed by N monomers is characterized by the end-to-end distance R e = b ( Z - 1 ) γ and the number of associated counterions n. At high temperatures the polyelectrolyte behaves as a neutral polymer ( γ = 0.588 ). Decreasing the temperature, the macromolecule changes from this extended configuration ( γ = 0.588 ) to a stretched form ( γ ≈ 1 ). At even lower temperatures, above the Manning condensation threshold, the polyelectrolyte collapses ( γ ≈ 0.3 ). Our results show good agreement with simulations.

Calculation of the binding free energy for magnesium–RNA interactions

The nature of the interaction between nucleic acids and divalent ions in solution is complex. It includes long-range electrostatic and short-range nonelectrostatic forces. Water molecules can be in an inner coordination shell that intervenes between the ion and its binding site. This work describes a method for calculating the binding free energy and applies it to a specific Mg-RNA system in the presence of monovalent salt. The approach combines high-level ab initio theory with Poisson-Boltzmann calculations and provides an accurate description of all terms of the binding free energy for magnesium ions located at the RNA surface (including nonelectrostatic interactions). Some alternative macroscopic approaches are also discussed.

Spherical polyelectrolyte block copolymer micelles: Structural change in presence of monovalent salt

Spherical polyelectrolyte block copolymer micelles were investigated as a function of added NaCl salt concentration using Small-Angle Neutron Scattering (SANS) and Light Scattering (LS). The micelles are formed by the self-association of charged-neutral copolymers made of a long deuterated polyelectrolyte moiety (NaPSS(d))251 and a short hydrophobic moiety (PEP)52. In presence of salt, the core shape and the aggregation number of the micelles are not affected. The hydrodynamic radius of the micelle is found to be identical to the radius of the whole micelle deduced from neutron scattering and thus the hydrodynamic radius is a valid measure of the corona thickness. At the lowest salt concentrations investigated the thickness of the corona, R(s), remains essentially constant and a contraction is observed above an added-salt concentration c(s) of 2 x 10(-2) M where this crossover concentration corresponds to the average ionic strength of the free counterions in the corona. The contraction takes place while maintaining a rod-like behavior of the chains at short scale and obeys to: R(s) approximately c(s)(-0.18). The exponent 0.18 suggests an electrostatic persistence length proportional to the Debye screening length.

Phase diagram of aggregation of oppositely charged colloids in salty water.

Aggregation of two oppositely charged colloids in salty water is studied. We focus on the role of Coulomb interaction in strongly asymmetric systems in which the charge and size of one colloid is much larger than the other one. In the solution, each large colloid (macroion) attracts a certain number of oppositely charged small colloids (Z-ion) to form a complex. If the concentration ratio of the two colloids is such that complexes are not strongly charged, they condense in a macroscopic aggregate. As a result, the phase diagram in a plane of concentrations of two colloids consists of an aggregation domain sandwiched between two domains of stable solutions of complexes. The aggregation domain has a central part of total aggregation and two wings corresponding to partial aggregation. A quantitative theory of the phase diagram in the presence of monovalent salt is developed. It is shown that as the Debye-Hückel screening radius r(s) decreases, the aggregation domain grows, but the relative size of the partial aggregation domains becomes much smaller. As an important application of the theory, we consider solutions of long double-helix DNA with strongly charged positive spheres (artificial chromatin). We also consider implications of our theory for in vitro experiments with the natural chromatin. Finally, the effect of different shapes of macroions on the phase diagram is discussed.