Presence Of Divalent Counterions Research Articles

Multivalent counterion induced multilayer adsorption at the air-water interface in dilute Aerosol-OT solutions

The formation of surface multilayer structures, induced by the addition of multivalent counterions in dilute surfactant solutions, has been widely observed in a range of anionic surfactants. The phenomenon is associated with the ability to manipulate surface properties, especially in the promotion of enhanced surface wetting, and in the presence of an extensive near surface reservoir for rapid surface delivery of surfactant and other active components.Hypothesis.In the single alkyl chain anionic surfactants, such as sodium dodecysulfate, SDS, sodium alkylethoxylsulfate, SAES, and alkylestersulfonate, AES, surface multilayer formation is promoted by trivalent counterions such as Al3+, and is generally not observed with divalent counterions, such as Ca2+ or with monovalent counterions. In the di-alkyl chain anionic surfactant, dodecylbenzenesulfonate, LAS, surface multilayer formation now occurs in the presence of divalent counterions. It is attributed to the closer proximity of a bulk lamellar phase, resulting in a greater tendency for surface multilayer formation, and hence should occur in other di-alkyl chain anionic surfactants.Experiments.Aerosol-OT, AOT, is one of the most commonly used di-alkyl chain anionic surfactants, and is extensively used as an emulsifying, wetting and dispersing agent. This paper reports on predominantly neutron reflectivity, NR, measurements which explore the nature of surface multilayer formation of the sodium salt of AOT at the air-solution interface with the separate addition of Ca2+ and Al3+ counterions.Findings.In the AOT concentration range 0.5 to 2.0 mM surface multilayer formation occurs at the air-solution interface with the addition of Ca2+ or Al3+ counterions. Although the evolution in the surface structure with surfactant and counterion concentration is broadly similar to those reported for SDS, SAES and AES, some notable differences occur. In particular the surfactant and counterion concentration thresholds for surface multilayer formation are higher for Ca2+ than for Al3+. The differences encountered reflect the greater affinity of the di-alkyl chain structure for lamellar formation, and how the surface packing is controlled in part by the headgroup structure and the associated counterion binding affinity.

Phase separation of saturated micellar network and its potential applications for nanoemulsification

Phase separation of saturated micellar network, as a result of cross-linking of branched micelles, is established in mixed solutions of the anionic surfactant sodium laurylethersulfate (SLES) and the zwitterionic cocamidopropyl betaine (CAPB) in the presence of divalent counterions: Ca2+, Zn2+ and Mg2+. The saturated network appears in the form of droplets, which are heavier than water and sediment at the bottom of the vessel. In the case of Mg2+, the sedimented drops coalesce and form a separate multiconnected micellar phase – a supergiant surfactant micelle. For this phase, the rheological flow curves show Newtonian and shear-thinning regions. The appearance/disappearance of the Newtonian region marks the onset of formation of saturated network. The addition of small organic molecules (fragrances) to the multiconnected micellar phase leads to an almost spontaneous formation of oil-in-water nanoemulsion. The nanoemulsification capacity of the multiconnected micellar phase decreases with the rise of the volume of the oil molecule. A possible role of the network junctions in the nanoemulsification process can be anticipated. The properties of the multiconnected micellar phases could find applications in extraction and separation processes, in drug/active delivery, and for nanoemulsification at minimal energy input.

Ultrafiltration of saline oil-in-water emulsions stabilized by an anionic surfactant: Effect of surfactant concentration and divalent counterions

The paper presents results of a direct visualization study of membrane fouling by hexadecane-in-water emulsions (0.1% v/v) stabilized by sodium dodecyl sulfate (SDS; 0.1mM or 3mM) in the presence of divalent counterions (Mg2+; 0mM, 6.7mM, or 42.6mM). Direct Observation Through the Membrane (DOTM) tests were performed using an ultrafiltration membrane (Anopore; dpore =0.02µm) and low wettability by hexadecane (contact angle, θ, in 3mM aqueous solution of SDS is ~164°). Three emulsions employed in DOTM tests had different values of interfacial tension and surface charge and exhibited distinctly different behaviors of oil droplets on the membrane surface. In addition to decreasing the solubility of SDS, MgSO4 had two opposing effects on emulsion stability wherein both interfacial tension and ζ-potential of oil droplets decreased; the overall effect of MgSO4 was a more facile droplet coalescence that was further promoted by permeate flux and concentration polarization of oil. The dominant fouling mechanism was cake filtration with multilayer and sub-monolayer coverages observed for different conditions. Because of the relative oleophobicity of the membrane, the attached oil did not form contiguous oil films. Under conditions of extensive coalescence (high MgSO4), oil droplets reached a critical size and were then removed by the crossflow resulting in minimal membrane fouling.

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

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

Adsorption of linear alkyl benzene sulfonates on oil–water interface: Effects of Na+, Mg2+ and Ca2+ ions

Abstract Linear alkyl benzene sulfonates (LAS) are among the most important industrial and house-hold surfactants. Here we study the LAS adsorption properties at oil–water interface in the presence of divalent counter-ions (Mg 2+ and Ca 2+ ). Interfacial tension data are obtained and interpreted using a detailed thermodynamic model for surfactant adsorption, which explicitly accounts for counter-ion binding (Kralchevsky et al. Langmuir 15 (1999) 2351). The obtained results show that the hardness ions (i) reduce very significantly the area-per-molecule in the adsorption layer; (ii) reduce strongly the magnitude of the negative surface potential, neutralizing almost completely the adsorbed LAS molecules; (iii) bind strongly to the adsorption layer via both electrostatic attraction and specific attraction of magnitude around 3.5 kT . The limiting area-per-molecule at oil–water interface is shown to be significantly larger than the respective area at air–water interface. The latter result indicates that the oil molecules are able to intercalate in between the surfactant molecules in the adsorption layer and, thus, to disrupt the molecular packing at the oil–water interface.

Solvation effects on like-charge attraction

We present results of molecular dynamics simulations of the electrostatic interaction between two parallel charged rods in the presence of divalent counterions. Such polyelectrolytes have been considered as a simple model for understanding electrostatic interactions in highly charged biomolecules such as DNA. Since there are correlations between the free charge carriers, the phenomenon of like charge attraction appears for specific parameters. We explore the role of solvation effects and the resulting deviations from Coulomb's law on the nanoscale on this peculiar phenomenon. The behavior of the force between the charged rods in a simulation with atomistic representation of water molecules is completely different from a model in which water is modeled as a continuum dielectric. By calculating counterion-rodion pair correlation functions, we find that the presence of water molecules changes the structure of the counterion cloud and results in both qualitative and quantitative changes of the force between highly charged polyelectrolytes.

Monte Carlo simulations of the electrical double layer forces in the presence of divalent electrolyte solutions: effect of the ion size

In this paper, the effect of ion size on the mean forces between two charged plates in the presence of divalent counterions is analyzed with the help of Monte Carlo simulations in the framework of the primitive model. Inspired by a preliminary work in which a particular and isolated case was presented, we propose a more systematic survey considering both like and oppositely charged plates, different surface charge densities (with magnitudes ranging from 0.01 to 0.2 C m−2) and two very different ionic strengths (0.5 and 500 mM). The effect of ion size is probed comparing systematically results for the most commonly used ionic diameter (0.425 nm) and greater values reported in the scientific literature for hydrated ions. These greater values were previously and successfully employed in other areas (e.g., electrokinetic behaviour in the presence of divalent and trivalent ions). Our simulations show that force–distance profiles strongly depend on the ion size. Consequently, some of the ‘classical’ findings obtained from simulations must be carefully reconsidered. In particular, the widely known and reported attraction between like-charged plates (or macroions) becomes repulsion with increasing ion size.

Electrostatic Bending of Lipid Membranes: How Are Lipid and Electrostatic Properties Interrelated?

Electrostatic modification of lipid headgroups and its effect on membrane curvature are not only relevant in a variety of contexts such as cell shape transformation and membrane tubulation but also are intriguingly implicated in membrane functions. For instance, the gating (open vs closed) properties of mechanosensitive channels can be influenced by membrane curvature and ion valence. However, a full theoretical description of membrane electrostatics is still lacking; in the past, membrane bending has often been considered under a few assumptions about how bending modifies lipid arrangements and surface charges. Here, we present a unified theoretical approach to spontaneous membrane curvature, C(0), in which lipid properties (e.g., packing shape) and electrostatic effects are self-consistently integrated. For the description of electrostatic interactions, especially between a lipid charge and a divalent counterion, we implement the Poisson-Boltzmann (PB) approach by incorporation of finite ionic sizes, so as to capture both lateral and transverse charge correlations on the membrane surface. Our results show that C(0) is sensitive to the way lipid rearrangements and divalent counterions are modeled. Interestingly, it can change its sign in the presence of divalent counterions, thus stabilizing reverse hexagonal (H(II)) phases. Our results show how electrostatic modification of headgroups influences the preferred structure of lipid aggregates (inverted micelles vs bilayers).

Conformational changes of strong polycations in the presence of divalent counterions and their influence upon the flocculation efficiency

The interaction between strong polycations, which possess the ammonium quaternary centers attached to an acrylic macromolecular chain derived from poly(N,N-dimethylaminoethyl methacrylate) (polycations Q x ), and divalent counterions was investigated by viscometry and turbidimetry. Conformational changes of polycations were influenced by the polycation charge density, counterion nature (SO 4 2− or S 2O 8 2−) and concentration. The morphology of the polycation layers deposited onto silicon wafers has been studied by tapping mode atomic force microscopy, a strong influence of the polycation and ammonium persulfate concentration on the surface topography being observed. The optimum flocculation concentration of polycation decreased and the flocculation window increased in the presence of S 2O 8 2−, in the destabilization of kaolin model dispersion. Removal of Congo Red from aqueous solution by the complex system formed between polycations and divalent counterions was also investigated. The behavior of polycations Q x in separation processes was compared with that of one polycation containing 95 mol% N,N-dimethyl-2-hydroxypropyleneammonium chloride units in the backbone (PCA 5).

Main properties and current applications of some polysaccharides as biomaterials

AbstractThis review concerns the applications of some polysaccharides in the domain of biomaterials and bioactive polymers. Natural polysaccharides from different sources have been studied for a long time, and their main properties are summarized in this paper; some of their derivatives obtained by chemical modification are also described. The main polysaccharides currently used in the biomedical and pharmaceutical domains are chitin and its derivative chitosan, hyaluronan and alginates. Alginates are well known for their property of forming a physical gel in the presence of divalent counterions (Ca, Ba, Sr) whereas carrageenans form a thermoreversible gel; these seaweed polysaccharides are mainly used to encapsulate different materials (cells, bacteria, fungi). Other promising systems are the electrostatic complexes formed when an anionic polysaccharide is mixed with a cationic polysaccharide (e.g. alginate/chitosan or hyaluronan/chitosan). An important development of the applications of polysaccharides can be predicted for the next few years in relation to their intrinsic properties such as biocompatibility and biodegradability in the human body for some of them; they are also renewable and have interesting physical properties (film‐forming, gelling and thickening properties). In addition, they are easily processed in different forms such as beads, films, capsules and fibres. Copyright © 2007 Society of Chemical Industry

Electrostatics of phosphatidic acid monolayers: Insights from computer simulations

In this paper, we argue that many of the fascinating electrostatic effects that take place in amphiphilic systems are strongly related to the particular organization of the oxygen atoms within each individual molecule. In particular, we focus on two effects: charge inversion and dielectric overscreening. For that purpose, we present molecular dynamics simulations of phosphatidic acid (DMPA 2−) in the presence of divalent counterions. Our results show that the many oxygens present in DMPA 2− cooperatively create strong binding sites for counterions, which in some cases lead to charge inversion. We also present an analysis of the role of interfacial water and relate our analysis to the phenomenon of dielectric overscreening. Several experimental implications are discussed in the conclusions.

Brownian dynamics simulations of polyelectrolyte solutions with divalent counterions

Brownian dynamics simulations are performed for salt-free polyelectrolyte solutions with divalent counterions. The polymer molecules are modeled as freely jointed charged chains and the counterions are incorporated explicitly. The conformational properties, static structure, and dynamic properties of salt-free polyelectrolyte solutions show interesting behavior that can be attributed to the correlations induced by the counterions. The size of polyelectrolyte chains and the counterion self-diffusion coefficient show a nonmonotonic concentration dependence. There is a sharp peak in the polyion pair correlation functions at short distances and an upturn in the partial static structure factors at low wave vectors. In semidilute solutions, the polyions contract in the presence of divalent counterions, when compared to solutions with monovalent counterions. This contraction is accompanied by the peak in the static structure moving to lower wave vectors. The self-diffusion of polyions is faster with divalent counterions than with monovalent counterions, whereas the collective diffusion at long length scale slows down with divalent counterions compared to the monovalent case. These results are consistent with experiments and suggest that primitive models with strong electrostatic correlations can explain many of the experimental observations.

Effect of divalent counterions on asymmetrically charged lipid bilayers.

We study an asymmetrically charged lipid bilayer in which the inner layer is negatively charged, while the outer one is neutral. In particular, we focus on the interplay between the asymmetrical charge distribution and counterion valency in determining the preferred curvature state of the bilayer. We show that, at low ionic strength, an entropic effect associated with counterion release tends to expand the inner layer-surprisingly, the excess charge of the bilayer opposes this. In the absence of multivalent ions, the entropic effect is dominant and gives rise to a tendency towards negative mean curvature. The presence of divalent counterions, however, counterbalances the entropic effect-they tend to shrink the inner layer, leading to positive mean curvature.

Like-charge colloid–polyelectrolyte complexation

We investigate the complexation of a highly charged sphere with a long flexible polyelectrolyte, both negatively charged in a salt-free environment. Electroneutrality is insured by the presence of divalent counterions. Using molecular dynamics within the framework of the primitive model, we consider different Coulomb coupling regimes. At strong Coulomb coupling we find that the adsorbed chain is always confined to the colloidal surface but forms different conformations that depend on the linear charge density of the chain. A mechanism involving the polyelectrolyte overcharging is proposed to explain these structures. At intermediate Coulomb coupling, the chain conformation starts to become three-dimensional, and we observe multilayering of the highly charged chain while for lower charge density the chain wraps around the colloid. At weak Coulomb coupling, corresponding to an aqueous solvent, we still find like-charge complexation. In this latter case the chain conformation exhibits loops.

Interaction between oppositely charged micelles or globular proteins.

Monte Carlo simulations and the hypernetted chain theory are used to study the interaction between spherical macroions of opposite charge immersed in a solution of monovalent or divalent simple electrolyte. These calculations represent the first step toward studying phase behavior and precipitation kinetics in solutions containing a mixture of macroions with positive and negative net charges. The potential of mean force between colloidal particles is determined as a function of colloid-colloid separation. In addition to having an opposite sign, the calculated potential of mean force is found to be stronger and longer-ranged than observed in the case of equally charged macroparticles. The difference is more pronounced in the presence of divalent counterions and is especially noticeable when we compare distinct Coulombic and hard-core collision contributions to the interaction between equally and oppositely charged colloids. The present observations suggest the dominance of attractive forces between globally neutral but electrostatically heterogeneous macroparticles. While our numerical results cannot be successfully analyzed by existing theories, they provide useful guidance and benchmark data for the development of advanced analytic descriptions.

Macroion−Macroion Correlations in Presence of Divalent Counterions

Monte Carlo results for a primitive model of a micellar solution are presented. The macroions are assumed to carry 20 negative charges and the counterions are monovalent and/or divalent. The ratio of diameters of the ionic species, described as charged spheres, is 3.0 nm:0.4 nm (macroions:small ions). An important conclusion of the study is that by replacement of monovalent counterions with divalent ones it is possible to change the structure of the solution. In solutions with predominantly divalent counterions, the macroions approach each other and share the layer of counterions. This structure is rather stable; in our case the macroion clusters do not “dissolve” upon dilution from 0.02 to 0.01 mole dm-3 and are not significantly affected by moderate addition of simple electrolyte. Thermodynamic considerations indicate an energy−entropy compensation in the process of dilution.

Monte Carlo simulation for the potential of mean force between ionic colloids in solutions of asymmetric salts

A new technique for Monte Carlo sampling of the hard-sphere collision force has been applied to study the interaction between a pair of spherical macroions in primitive-model electrolyte solutions with valences 1:2, 2:1, and 2:2. Macroions of the same charge can attract each other in the presence of divalent counterions, in analogy with earlier observations for planar and cylindrical geometries. The attraction is most significant at intermediate counterion concentrations. In contrast to the entropic depletion force between neutral particles, attraction between macroions is of energetic origin. The entropic contribution to the potential of mean force is generally repulsive at conditions corresponding to aqueous colloids with or without salt. For systems with divalent counterions, the potentials of mean force predicted by mean-field approximations like the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory or the Sogami–Ise (SI) theory are qualitatively different from those observed in the simulations. However, for systems with monovalent counterions, predictions of DLVO theory are in fair agreement with simulation results.

The gellan sol-gel transition

The behavior of gellan during the conformational transition is discussed. This transition corresponds to the formation of a double helix from two separate coils. The influence of the counterion activity on the melting and gelling temperatures of the gels and/or the ordered conformation is also described. The sol-gel transition is studied by conductimetry in the presence of pure monovalent counterions or monovalent divalent mixtures. This method is one of the best ways of characterizing this transition. A two-step mechanism of gelation was demonstrated in the presence of monovalent counterions. In the presence of divalent counterions, at a given ionic strength, the mechanism of gelation and the gel properties depend only on the ratio Y between divalent and monovalent counterions. Three types of gel behavior are described as a function of Y and ionic strength.

Charge reversal in electrical double layers — a balance between energy and entropy

AbstractMonte Carlo simulations and accurate analytical theories predict a charge reversal in an electric double layer under some specific conditions. One typical situation is for very highly charged aggregates in the presence of divalent counterions, where ionic correlations become important. Here, we reexamine the phenomenon in terms of a balance between energy and entropy contributions. High surface charge density, low temperature, low dielectric permittivity or multivalent counterions, all these factors increase the influence of the energetic term and favour a charge reversal. Previous studies have focused on the energy term and attempts to increase that. An alternative and equally valid way is to reduce the entropy term, which can be accomplished by replacing single atomic or molecular counterions by polyelectrolyte chains. The entropy of the charged chains is of course much lower than that of the small ions and the dominance of the energy term will again lead to a charge reversal, but in this case at a much lower and more realistic value of the electrostatic coupling.

Control of the structure of Langmuir-Blodgett films of a discotic liquid crystalline compound via the subphase composition and the adjacent molecular environment

Changes of the ,olecular arrangement that can be induced by means of the LB technique in the multilayers of a disc-shaped multialkynyl amphiphile are monitored by means of small angle X-ray diffraction. Studies of the monolayers at the air-water interface reveal “edge-on” orientation of the discs. Specific effects of the counter-ions (Na +, Cd 2+, Pb 2+ and Ba 2+) and sub-solution pH on the monolayer collapse pressure, transfer efficiency and molecular order in the multilayers are found. A correlation between the monolayer properties and the ability for formation of periodic discotic structures in the presence of divalent counterions is established. The discotic molecules retain their “edge-on” arrangement in the highly compressed transferred films with slight irregular interdigitation of the flexible wings and inclination to the substrate normal. The tilt and the interdigitation are reduced when the discotic monolayers are deposited in alternating LB films with barium arachidate spacer layers.