Ionic-like behavior of oppositely charged nanoparticles

Mixtures of oppositely charged nanoparticles of various sizes and charge ratios precipitate only at the point of electroneutrality. This phenomenon-specific to the nanoscale and reminiscent of threshold precipitation of ions-is a consequence of the formation of core-and-shell nanoparticle aggregates, in which the shells are composed of like-charged particles and are stabilized by efficient electrostatic screening.     

Phase behaviour of oppositely charged protein and surfactant mixtures: DOTAC-BLG-Water

The interactions between the negatively charged protein, β-lactoglobulin (BLG) and the cationic surfactant dodecyltrimethylammonium chloride (DOTAC) in water have been investigated by determining the phase equilibria of the ternary system within the concentration range of 20 wt.% of both protein and surfactant. Three main regions are formed—an isotropic solution phase, a white precipitation region and a blueish, isotropic, highly viscous gel phase. The protein solution can solubilize 1 mole surfactant, [DOTAC] per mole protein, [BLG] prior to precipitation. The protein-surfactant precipitate complex is neutral and consists of 8 [DOTAC]/[BLG]. The net charge of the protein in water at aqueous pH is −7 and this agrees with the determined composition. The pH is, however, decreased on addition of DOTAC, but this does not seem to affect the composition of the precipitate significantly. The amount of precipitate reaches a maximum at about 8 [DOTAC]/[BLG] and thereafter a plateau region occurs where no more precipitate seems to be formed. On further increasing the surfactant concentration the precipitate redissolves either into a solution phase directly or into a solution phase via a gel phase depending on the protein concentration. On decreasing salt concentration the ternary system shows similar phase behaviour, but the stability of the regions are different. It is also observed that oppositely charged protein-surfactant systems show similar phase behaviour irrespective of nature of the net charge on the protein.     

Phase diagrams of binary mixtures of oppositely charged colloids

Phase diagrams of binary mixtures of oppositely charged colloids are calculated theoretically. The proposed mean-field-like formalism interpolates between the limits of a hard-sphere system at high temperatures and the colloidal crystals which minimize Madelung-like energy sums at low temperatures. Comparison with computer simulations of an equimolar mixture of oppositely charged, equally sized spheres indicate semiquantitative accuracy of the proposed formalism. We calculate global phase diagrams of binary mixtures of equally sized spheres with opposite charges and equal charge magnitude in terms of temperature, pressure, and composition. The influence of the screening of the Coulomb interaction upon the topology of the phase diagram is discussed. Insight into the topology of the global phase diagram as a function of the system parameters leads to predictions on the preparation conditions for specific binary colloidal crystals.     

A modified Poisson-Boltzmann model including charge regulation for the adsorption of ionizable polyelectrolytes to charged interfaces, applied to lysozyme adsorption on silica

The equilibrium adsorption of polyelectrolytes with multiple types of ionizable groups is described using a modified Poisson-Boltzmann equation including charge regulation of both the polymer and the interface. A one-dimensional mean-field model is used in which the electrostatic potential is assumed constant in the lateral direction parallel to the surface. The electrostatic potential and ionization degrees of the different ionizable groups are calculated as function of the distance from the surface after which the electric and chemical contributions to the free energy are obtained. The various interactions between small ions, surface and polyelectrolyte are self-consistently considered in the model, such as the increase in charge of polyelectrolyte and surface upon adsorption as well as the displacement of small ions and the decrease of permittivity. These interactions may lead to complex dependencies of the adsorbed amount of polyelectrolyte on pH, ionic strength, and properties of the polymer (volume, permittivity, number, and type of ionizable groups) and of the surface (number of ionizable groups, pK, Stern capacity). For the adsorption of lysozyme on silica, the model qualitatively describes the gradual increase of adsorbed amount with pH up to a maximum value at pHc, which is below the iso-electric point, as well as the sharp decrease of adsorbed amount beyond pHc. With increasing ionic strength the adsorbed amount decreases (for pH > pHc), and pHc shifts to lower values.     

Synergistic effects in mixtures of oppositely charged surfactants as calculated from the Poisson-Boltzmann theory: a comparison between theoretical predictions and experiments

Critical micelle concentrations in mixtures of an anionic surfactant and a cationic amphiphilic drug have been investigated using a model-independent procedure to quantify observed synergistic effects. Experimental results were compared with a theory based on the Poisson-Boltzmann mean field approximation of a charged interface with a diffuse layer of counterions. Explicit expressions for the activity coefficients from which the critical micelle concentration can be calculated and quantitatively predicted have been derived and excellent agreement between experimental data and theory was obtained. As a result, we demonstrate that it is possible to rationalize and predict the magnitude of synergism in mixtures of oppositely charged surfactants in the presence of added salt.     

Heteroaggregation, repeptization and stability in mixtures of oppositely charged colloids

We report a study of mixtures of initially oppositely charged particles with similar size. Dispersions of silica spheres (negatively charged) and alumina-coated silica spheres (positively charged) at low ionic strength, mixed at various volume ratios, exhibited a surprising stability up to compositions of 50% negative colloids as well as spontaneous repeptization of particles from the early-stage formed aggregates. The other mixtures were found to contain large heteroaggregates, which were imaged using cryogenic electron microscopy. Electrophoretic mobility, electrical conductivity, static and dynamic light scattering and sedimentation were studied as a function of volume fraction of the mixed dispersions to investigate particle interactions and elucidate the repeptization phenomenon.     

Anomalous phase behavior of water-soluble polyelectrolyte and oppositely charged surfactants

Aqueous mixtures of anionic surfactants with cationically substituted quaternary ammonium derivatives of hydroxyethylcellulose, JR and LR series, were investigated by several techniques. On adding sodium dodecyl sulfate (SDS) to a polyelectrolyte solution, phase separation with precipitation occurs in a co-operative way, and redissolution of precipitation is observed at the critical micelle concentration (CMC) of SDS. This is due to admicelle formation on the polyelectrolyte. The phase separation for the two-headed anionic surfactant systems is also seen, while the concentration where this takes place is near the CMC of the surfactant. This is remarkable in the case of the triethanolamine cocoyl glutamate (TCG)–JR 400 system, in which TCG has a CMC over 1 order of magnitude smaller CMC than that of SDS. Surface tension and the dynamic light scattering measurements show the existence of not only electrostatic interaction between the cationic polyelectrolyte and the two-headed anionic surfactant but also intraction between the adsorbed polymers. The scaling analysis of the precipitation line of the surfactant with polyelectrolyte concentration elucidates that one molecule of TCG can neutralize approximately two charges on JR 400. Received: 9 February 1999 Accepted in revised form: 23 June 1999     

Phase behavior of polyampholytes from charged hard-sphere chain model

A molecular thermodynamic theory is developed for polyampholytes from the coarse-grained charged hard-sphere chain model. The phase behavior of polyampholytes with variations in sequence and chain length is satisfactorily predicted by the theory, consistent with simulation results and experimental observations. At a fixed chain length, the phase envelope expands as the sequence of charge distribution becomes less random. With increasing chain length, the phase envelope expands for diblock and random polyampholytes, but shrinks for zwitterionic polyampholytes. The predicted critical temperature, density, and pressure exhibit scaling relations with chain length for all the three (diblock, random, and zwitterionic) polyampholytes.     

Mixtures of mucin and oppositely charged surfactant aggregates with varying charge density. Phase behavior, association, and dynamics

The nonionic surfactant Tween80 is a commonly used excipient in drug formulations containing an active substance with low aqueous solubility. Model drug vehicles with varying charge density were obtained by mixing Tween80 (PS-80) with the cationic surfactant Tetradecyltrimethylammonium chloride (TTAC), thus forming mixed micelles. The micelles were mixed with the negatively charged polyelectrolyte mucin, which is a component in the protective mucus layer covering epithelial cell linings. Depending on the composition of the mixture, complex-formation could be followed by precipitation. Using X-ray diffraction, it was found that the precipitate contained a lamellar phase with properties sensitive to the proportion of PS-80. Higher amounts of PS-80 were found to oppose phase separation. Further analysis in the one-phase region, or alternatively of the supernatant of two-phase samples, by (1)H NMR, HPLC, and diffusion measurements with PGSE-NMR led to the conclusions that at low proportion of PS-80 aggregates composed of mixed (PS-80 and TTAC) micelles and mucin were formed, whereas increased concentrations of PS-80 favored the dissolution of the precipitate and limited the interactions between mixed micelles and the polymer.     

Multilayers of oppositely charged SiO2 nanoparticles: effect of surface charge on multilayer assembly

The growth behavior of all-silica nanoparticle multilayer thin films assembled via layer-by-layer deposition of oppositely charged SiO2 nanoparticles was studied as a function of assembly conditions. Amine-functionalized SiO2 nanoparticles were assembled into multilayers through the use of three different sizes of negatively charged SiO2 nanoparticles. The assembly pH of the nanoparticle suspensions needed to achieve maximum growth for each system was found to be different. However, the surface charge /z/ of the negatively charged silica nanoparticles at the optimal assembly pH was approximately the same, indicating the importance of this parameter in determining the growth behavior of all-nanoparticle multilayers. When /z/ of the negatively charged nanoparticles lies between 0.6z(0) and 1.2z(0) (where z(0) is the pH-independent value of the zeta-potential of the positively charged nanoparticles used in this study), the multilayers show maximum growth for each system. The effect of particle size on the film structure was also investigated. Although nanoparticle size significantly influenced the average bilayer thickness of the multilayers, the porosity and refractive index of multilayers made from nanoparticles of different sizes varied by a small amount. For example, the porosity of the different multilayer systems ranged from 42 to 49%. This study further demonstrates that one-component all-nanoparticle multilayers can be assembled successfully by depositing nanoparticles of the same material but with opposite surface charge.     

Chiral anion exchangers applied to capillary electrochromatography enantioseparation of oppositely charged chiral analytes: investigation of stationary and mobile phase parameters

Weak anion-exchange (WAX) type chiral stationary phases (CSPs) based on tert.-butyl carbamoyl quinine as chiral selector (SO) and different types of silica particles (porous and non-porous) as chromatographic support are evaluated in packed capillary electrochromatography (CEC). Their ability to resolve the enantiomers of negatively charged chiral analytes, e.g., N-derivatized amino acids, in the anion-exchange mode and their electrochromatographic characteristics are described in dependence of several mobile phase parameters (pH, buffer type and concentration, organic modifier type and concentration) and other experimental variables (electric field strength, capillary temperature). The inherent "zwitterionic" surface character of such silica-based WAX type CSPs (positively charged SO and negatively charged residual silanols) allows the reversal of the electroosmotic flow (EOF) towards the anode at pH values below the isoelectric point (pI) of the modified surface, whereas a cathodic EOF results at pH values above the pI. Since for negatively charged analytes also an electrophoretic transport increment has to be considered, which can be either in or against the EOF direction, several distinct modes of elution have been observed under different stationary phase and mobile phase conditions: (i) co-electrophoretic elution of the negatively charged solutes with the anodic EOF in the negative polarity mode, (ii) counter-electrophoretic elution with the cathodic EOF in the positive polarity mode, and (iii) electrophoretically dominated elution in the negative polarity mode with a cathodic EOF directed to the injection end of the capillary. Useful enantioseparations of chiral acids have been obtained with all three modes. Enantioselectivity values as high as under pressure-driven conditions and theoretical plate numbers up to 120000 per meter could be achieved under electrically driven conditions. A repeatability study yielded RSD values below 2% for retention times and RSD values in the range of 5-10% for theoretical plate numbers and resolution, thus clearly establishing the reliability of the investigated anion-exchange type CEC enantioseparation methods.     

Salt effects on macrophase separations in non‐stoichiometric mixtures of oppositely charged macromolecules: Theory and experiment

In the field of biological applications, polyelectrolyte complexes are proposed to encapsulate bioactive compounds, to deliver drugs, and also to transfect genes into cells under the name of polyplexes. Complex formation is obtained by addition of a polycation solution into a polyanion solution or vice‐versa. This work proposes a theoretical approach to describe complex formation in the case of non‐stoichiometric mixtures of oppositely charged macroions having different degrees of ionization and different degrees of polymerization under different salt conditions. In a second part, comparison was made with experimental data collected when a weak polybase, namely poly(l ‐lysine) under its bromide form was added stepwise to solutions of various polyanions under their sodium salt form, namely poly(l ‐lysine citramide imide), poly(l ‐lysine citramide), and poly(β‐malic acid), the latter lacking hydroxyl groups attached to the main chain. The stability of stroichiometric complexes made of poly(l ‐lysine) and poly(l ‐lysine citramide) having different molecular masses is discussed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1717–1730     

Phase behavior and molecular thermodynamics of coacervation in oppositely charged polyelectrolyte/surfactant systems: a cationic polymer JR 400 and anionic surfactant SDS mixture

Coacervation in mixtures of polyelectrolytes and surfactants with opposite charge is common in nature and is also technologically important to consumer health care products. To understand the complexation behavior of these systems better, we combine multiple experimental techniques to systematically study the polymer/surfactant binding interactions and the phase behavior of anionic sodium dodecyl sulfate (SDS) surfactant in cationic JR 400 polymer aqueous solutions. The phase-behavior study resolves a discrepancy in the literature by identifying a metastable phase between the differing redissolution phase boundaries reported in the literature for the surfactant-rich regime. Isothermal titration calorimetry analyzed within the framework of the simple Satake-Yang model identifies binding parameters for the surfactant-lean phase, whereas a calculation for polymer-bound micelles coexisting with free micelles is analyzed in the surfactant-rich redissolution regime. This analysis provides a preliminary understanding of the interactions governing the observed phase behavior. The resulting thermodynamic properties, including binding constants and the molar Gibbs free energies, enthalpies, and entropies, identify the relative importance of both hydrophobic and electrostatic interactions and provide a first approximation for the corresponding microstructures in the different phases. Our study also addresses the stability and metastability of oppositely charged polyelectrolytes and surfactant mixtures.     

Thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution

Microcalorimetric techniques, combined with turbidity measurements, were used to study the thermodynamics of self-assembling of hydrophobically modified cationic polysaccharides and their mixtures with oppositely charged surfactants in aqueous solution. The studied polyelectrolytes were a series of polymers based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-alkylammonium chloride groups randomly distributed along the polymer backbone. The parameters for their micellization process are evaluated from the results of the observed dilution enthalpy curves and compared with those of the related cationic surfactants (DTAC and CTAC). The microcalorimetric results for the mixed systems (polyelectrolytes with oppositely charged surfactants) are used along with turbidity measurements to characterize systematically the thermodynamics of their interaction. The phase behavior is described and the interaction enthalpies are derived from the differences between the observed enthalpy curves with and without polyelectrolyte. Therefore, we discuss in detail the effect of changing the alkyl chain length of polyelectrolyte pendant groups, the molecular weight of the dextran backbone, and the temperature of the measurements on the interactions between polyelectrolyte and surfactant.     

Characterization of complexation and phase behavior of mixed systems of unmodified and hydrophobically modified oppositely charged polyelectrolytes

This paper reports turbidity, rheology, zeta potential, and rheo-small angle light scattering measurements on aqueous mixtures of oppositely charged and hydrophobically modified hydroxyethylcellulose derivatives (HM-HEC(?) and HM-HEC(+)) and mixtures of oppositely charged hydroxyethylcellulose (HEC(?) and HEC(+)). The experiments were restricted to the one-phase region, i.e., at mixing ratios before and after the two-phase area. The associative phase separation behavior usually observed when mixing oppositely charged polyelectrolytes was undetectable in the mixtures of the polyelectrolytes without attached hydrophobic groups. Upon modification of HEC by incorporation of pendant hydrophobic groups and by introducing charges of negative or positive sign (HM-HEC(?) and HM-HEC(+)), the mixtures showed phase separation over a certain mixing interval, revealing the existence of large polyelectrolyte complexes. The zero shear viscosity was strongly dependent on both the hydrophobicity of the polymers and the mixing ratio, increasing significantly with hydrophobic modification of polyelectrolytes. The strong enhancement of the turbidity and the viscosity drop as the two-phase area is approached suggest the formation of fragmented non-connected complexes. This work demonstrates that if the oppositely charged polyions have a hydrophilic character, it is not necessary that the attractive Coulombic forces induce insoluble polyelectrolyte complexes.     

Fluorescence and ESR studies of the conformational behavior of oppositely charged polyelectrolytes at solid/liquid interfaces

The conformational behavior of oppositely charged polyelectrolytes on alumina in solutions was investigated by means of excimer fluorescence and electron spin resonance spectroscopy using maleic acid-propene copolymer labeled with pyrene or TEMPO. It was found that the ability of the polyanion at the surface for conformational rearrangements is strongly influenced by the constraints of the adsorbed state that restrict its complexation. Polyelectrolyte complexes (PEC) formed by mixing of the oppositely charged polyelectrolytes exhibited extreme coiling due to the screening of the charged groups. The polyelectrolytes undergo spreading during the adsorption process due to the electrostatic attraction. Surface binding can irreversibly limit the flexibility for the reconformation process to a great extent. It is also shown here that a flatter adsorbed state could be reached by sequential adsorption of polyanion and polycation than could be reached by the direct adsorption of the polyelectrolyte complex itself.     

Self-aggregation of mixtures of oppositely charged polyelectrolytes and surfactants studied by rheology, dynamic light scattering and small-angle neutron scattering

In this study, the phase behavior, structure and properties of systems composed of the cationic, cellulose-based polycation JR 400 and the anionic surfactants sodium dodecylbenzenesulfonate (SDBS) or sodium dodecylethoxysulfate (SDES), mainly in the semidilute regime, were examined. This system shows the interesting feature of a very large viscosity increase by nearly 4 orders of magnitude as compared to the pure polymer solution already at very low concentrations of 1 wt%. By using rheology, dynamic light scattering (DLS), and small-angle neutron scattering (SANS), we are able to deduce systematic correlations between the molecular composition of the systems (characterized by the charge ratio Z=[+(polymer)]/[?(surfactant)]), their structural organization and the resulting macroscopic flow behavior. Mixtures in the semidilute regime with an excess of polycation charge form highly viscous network structures containing rodlike aggregates composed of surfactant and polyelectrolyte that are interconnected by the long JR 400 chains. Viscosity and storage modulus follow scaling laws as a function of surfactant concentration (η~c(s)(4); G(0)~c(s)(1.5)) and the very pronounced viscosity increase mainly arises from the strongly enhanced structural relaxation time of the systems. In contrast, mixtures with excess surfactant charges form solutions with viscosities even below those of the pure polymer solution. The combination of SANS, DLS, and rheology shows that the structural, dynamical, and rheological properties of these oppositely charged polyelectrolyte/surfactant systems can be controlled in a systematic fashion by appropriately choosing the systems composition.     

Soluble complexes in aqueous mixtures of low charge density comb polyelectrolyte and oppositely charged surfactant probed by scattering and NMR

A low charge density polyelectrolyte with a high graft density of 45 units long poly(ethylene oxide) side-chains has been synthesized. In this comb polymer, denoted PEO(45)MEMA:METAC-2, 2 mol% of the repeating methacrylate units in the polymer backbone carry a permanent positive charge and the remaining 98 mol% a 45 unit long PEO side-chain. Here we describe the solution conformation of this polymer and its association with an anionic surfactant, sodium dodecylsulfate, SDS. It will be shown that the polymer can be viewed as a stiff rod with a cross-section radius of gyration of 29 A. The cross section of the rod contracts with increasing temperature due to decreased solvency of the PEO side-chains. The anionic surfactant associates to a significant degree with PEO(45)MEMA:METAC-2 to form soluble complexes at all stoichiometries. A cooperative association is observed as the free SDS concentration approaches 7 mM. At saturation the number of SDS molecules associated with the polymer amounts to 10 for each PEO side-chain. Two distinct populations of associated surfactants are observed, one is suggested to be molecularly distributed over the comb polymer and the other constitutes small micellar-like structures at the periphery of the aggregate. These conclusions are reached based on results from small-angle neutron scattering, static light scattering, NMR, and surface tension measurements.     

Phase behavior and rheological properties of DNA-cationic polysaccharide mixtures

Associative aqueous mixtures over a range of concentrations of double- (ds) or single- (ss) stranded DNA with dilute or semidilute solutions of two cationic derivatives of hydroxyethyl cellulose (cat-HEC and cat-HMHEC,(1) the latter carrying grafted hydrophobic groups), were studied. The phase behavior showed an interesting asymmetry: Phase separation occurred immediately when small (sub-stoichiometric) amounts of cationic polyelectrolyte were added to the DNA solution, but redissolution into a single cat-(HM)HEC/DNA/H(2)O phase occurred already with a modest charge excess of the cationic polyelectrolyte, at a charge ratio approximately independent of the overall polyelectrolyte concentration. Cat-HEC/dsDNA/H(2)O and cat-HEC/ssDNA/H(2)O systems presented a considerable difference in the extension of the phase separation region. The one-phase samples with excess cationic polyelectrolyte were studied by rheology. The presence of DNA strengthened the viscoelastic behavior of the solutions of the cationic polyelectrolytes, reflected in an increase in storage modulus and viscosity. Differences in phase behavior and rheology were observed, particularly between systems containing cat-HEC or cat-HMHEC, but also between dsDNA and ssDNA. Thus, these systems allow for the preparation of DNA formulations with widely variable rheology and water uptake.     

Enhancement of selectivity and resolution in the enantioseparation of uncharged compounds using mixtures of oppositely charged cyclodextrins in capillary electrophoresis

The enantiomeric separation of some nonsteroidal anti-inflammatory drugs (NSAIDs) was investigated in capillary electrophoresis (CE) using dual systems with mixtures of charged cyclodextrin (CD) derivatives. A significant enhancement of selectivity and resolution could be achieved in the enantioseparation of these analytes in their uncharged form by the simultaneous addition of two oppositely charged CD derivatives to the background electrolyte. The combination of the single-isomer cationic CD, permethyl-6-monoamino-6-monodeoxy-beta-CD (PMMAbetaCD) and the single-isomer polyanionic CD, heptakis-6-sulfato-beta-cyclodextrin (HSbetaCD) in a pH 2.5 phosphoric acid-triethanolamine buffer, was designed and employed for the enantioseparation of profens. The improvement in selectivity and resolution can be attributed to the fact that the two CDs, which lead to independent and enantioselective complexation with the analyte enantiomers, have not only opposite effects on the electrophoretic mobility of these compounds but also opposite affinity patterns towards the enantiomers of these compounds. Binding constants for these enantiomers with each CD were determined using linear regression approach, in order to be able to predict the effect of the concentrations of the two CDs on enantiomeric selectivity and resolution in such dual systems.