Flocculation mechanism induced by cationic polymers investigated by light scattering

Three cationic polymers with molecular weights and charge densities of 3.0 x 10(5) g/mol and 10%, 1.1 x 10(5) g/mol and 40%, and 1.2 x 10(5) g/mol and 100% were chosen as flocculants to aggregate silica particles (90 nm), under various conditions, including change in polymer dosage, particle concentration, background electrolyte concentration, and shear rate. The size and structure of flocs produced were determined using the static light scattering technique. On the basis of measurements of polymer adsorption and its effect on the zeta potential and floc properties, it has been found that the polymer charge density plays an important role in determining the flocculation mechanism. Polymers with a 10% charge density facilitate bridging, 40% charged polymers bring about either a combination of charge neutralization and bridging or bridging, depending on the polymer dosage, and polymers with the charge density of 100% induce electrostatic patch flocculation mechanism at the optimum polymer dosage and below but bring about bridging mechanism at the polymer dosage approaching the adsorption plateau value. Bridging aggregation can readily be affected by the particle concentration, and an increase in particle concentration results in the formation of larger but looser aggregates, whereas electrostatic patch aggregation is independent of particle concentration. The addition of a background electrolyte aids in bridging aggregation while it is detrimental to electrostatic patch aggregation. It has also been found that the effect of shear rate on the mass fractal dimension depends on polymer charge density.     

Influence of adsorbed polymers on the removal of mineral particles from a planar surface

 Experimental results on the role of adsorbed polymers on the particle adhesion are presented. Both Brownian (silica particles) and non-Brownian (glass beads) particles were used. The particles were deposited onto the internal surface of a glass parallelepiped cell, and then submitted to increasing laminar flow rates. The pH and the ionic strength of the electrolytes were fixed. The adhesive force was related to the hydrodynamic force required to dislodge 50% of the initially attached beads. We found that high molecular weight PEO had little effect on the adhesion of small silica beads due to the low affinity of the polymer for silica or glass surfaces. On the contrary, PEO greatly enhanced the adhesion of bigger glass beads forced to deposit on the capillary surface because of gravity. The increase was all the more pronounced as the molecular weight of the polymer was high. The effect of high molecular weight cationic copolymers on the adhesion of silica particles was drastic. The maximal force (1500 pN) applied by the device could not enable any particle detachment even using polymers of low cationicity rate (5%), showing the efficiency of electrostatic attractions. When copolymers were adsorbed on both surfaces (particles and plane), the adhesive force exhibited a maximum at intermediate coverage of particles. This optimum was related to the optimum flocculation concentration classically observed in flocculation of suspensions by polymers. Received: 16 February 1996 Accepted: 10 September 1996     

Effect of high salt concentrations on the stabilization of bubbles by silica particles

The stabilization of air bubbles by hydrophobically modified silica particles has been investigated in detail. The silica particles used had a nominal primary particle size of 20 nm and were made hydrophobic by treatment with dichlorodimethylsilane to yield particles with varying percent grafting of alkyl chains ("% SiOR"). Contact-angle (theta) measurements of pure water droplets on flats made from compressed samples of the particles showed a steep increase in theta above ca. 20% SiOR. Other measurements also showed a significant increase in theta when the salt concentration was raised to 1-3 mol dm(-3). Bubbles were formed in a sonicated dispersion of particles by suddenly lowering the pressure. Maximum stability was obtained with 33% SiOR particles and 2-3 mol dm(-3) NaCl. Under these conditions, theta was around 40 degrees. Above a threshold size of around 70 microm, bubbles were extremely stable to disproportionation and coalescence and bubble stability increased significantly with an increase in the NaCl concentration from 0.5 to 3 mol dm(-3). Furthermore, rheological measurements showed that at NaCl concentrations in this range weak particle gels were formed with a finite yield stress. The strength of these gels increased with an increasing NaCl concentration between 0.5 and 3 mol dm(-3) and with an increasing time of aging the dispersions, implicating this as part of the mechanism leading to an increased bubble stability in these systems. Dispersions in the absence of NaCl showed little or no foamability at all. Use of CaCl2 and Al(NO)3 at similar ionic strengths showed that equivalent stability could not be obtained with these salts. Atomic force microscopy (AFM) measurements of the adhesion between a pure (0% SiOR) silica sphere and flat showed a significant increase in the adhesion between 0.5 and 3 mol dm(-3) NaCl, even though in this concentration range no significant change in the electrostatic repulsion might be expected. It is concluded that the increased particle-particle adhesion, effective hydrophobicity, and bubble-stabilization properties of the particles at high NaCl concentrations are probably due to the collapse of protruding polysilicic acid chains on the surface of the silica.     

Investigation of adsorbed polymer layers by flocculation of oppositely charged sols

Measuring the flocculation of oppositely charged sols is a suitable method for investigating the stabilizing effect of thick adsorbed polymer layers since the values of the electrical attractive potential are much higher than those of the van der Waals-London attraction. In this case flocculation occurs at low electrolyte concentrations and thus the precipitation of the polymer that normally occurs at high electrolyte concentration can be prevented. These observations were proved by the mutual flocculation of positively charged Fe₂O₃-sol and negatively AgI-sol, in the presence of large amounts of adsorbed polyvinylalcohol. The total free energy of interaction has been calculated for 1.5 mg/m² adsorbed polymer amount and for two electrolyte concentrations, and the results were in saticfactory agreement with the experimental values.     

Molecular macrocluster formation on silica surfaces in phenol-cyclohexane mixtures

The adsorption of phenol, an aromatic compound with a hydrogen-bonding group, onto a silica surface in cyclohexane was investigated by colloidal probe atomic force microscopy (AFM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and adsorption isotherm measurements. ATR-FTIR measurements on the silica surface indicated the formation of surface macroclusters of phenol through hydrogen bonding. The ATR-FTIR spectra were also measured on the H-terminated silicon surface to observe the effect of the silanol groups on the phenol adsorption. The comparison of the ATR-FTIR spectra for both the silicon oxide and H-terminated silicon surfaces proved that the silanol groups are necessary for the formation of phenol clusters on the surface. The surface force measurement using colloidal probe AFM showed a long-range attraction between the two silica surfaces in phenol-cyclohexane mixtures. This long-range attraction resulted from the contact of the adsorbed phenol layers for the phenol concentrations below 0.6 mol %, at which no significant phenol clusters formed in the bulk solution. The attraction started to decrease at 0.6 mol % phenol due to the exchange of the phenol molecules between the clusters in the bulk phase and on the surface. The surface density of phenol in the adsorbed layer was calculated on the basis of the long-range attraction and found to be much smaller than the liquid phenol density. The plausible structure of the adsorbed phenol layer was drawn by referring to the crystal structure of the bulk phenol and orientation of the phenol molecules on the surface, estimated by the dichroic analysis of ATR-FTIR spectroscopy. The investigation of the phenol adsorption on the silica surface in a nonpolar solvent using this novel approach demonstrated the effect of the aromatic ring on the surface packing density.     

Adsorption characteristics of bottle-brush polymers on silica: effect of side chain and charge density

The adsorption behavior of bottle-brush polymers with different charge/PEO ratio on silica was studied using optical reflectometry and QCM-D. The results obtained under different solution conditions clearly demonstrate the existence of two distinct adsorption mechanisms depending on the ratio of charge/PEO. In the case of low-charge density brush polymers (0-10 mol %), the adsorption occurs predominantly through the PEO side chains. However, the presence of a small amount of charge along the backbone (as low as 2 mol %) increases the adsorption significantly above that of the uncharged bottle-brush polymer in pure water. As the charge density of the brush polymers is increased to 25 mol % or larger the adsorption occurs predominantly through electrostatic interactions. The adsorbed layer structure was studied by measuring the layer dissipation using QCM-D. The adsorbed layer formed by the uncharged brush polymer dissipates only a small amount of energy that indicates that the brush lie along the surface, the scenario in which the maximum number of PEO side chains interact with the surface. The adsorbed layers formed by the low-charge density brush polymers (2-10 mol %) in water are more extended, which results in large energy dissipation, whereas those formed by the high-charge density brush polymers (50-100 mol %) have their backbone relatively flat on the surface and the energy dissipation is again low.     

Hydrogen-bonded macrocluster formation of ethanol on silica surfaces in cyclohexane(1)

Adsorption of ethanol onto silica surfaces from ethanol-cyclohexane binary liquids was investigated by a combination of colloidal probe atomic force microscopy, adsorption excess isotherm measurement, and FTIR spectroscopy using the attenuated total reflection (ATR) mode. An unusually long-range attraction was found between the silica (glass) surfaces in the presence of ethanol in the concentration range of 0.1-1.4 mol % at room temperature. At 0.1 mol % ethanol, the attraction appeared at a distance of 35 +/- 3 nm and turned into a repulsion below 3.5 +/- 1.5 nm upon compression. Half of the attraction range agreed with the adsorption layer thickness estimated from the adsorption excess amount by assuming that the adsorption layer was composed only of ethanol. This indicated that the observed long-range attraction was caused by the contact of opposed adsorption layers of ethanol on the silica surfaces and that the sharp increase of repulsion at shorter distance was caused by the overlap of structured ethanol clusters adjacent to the surface. ATR-FTIR spectra demonstrated that ethanol adsorbed on the silica (silicon oxide) surfaces formed hydrogen-bonded clusters (polymers). Practically no ethanol clusters were formed on the hydrogen-terminated silicon surface. These results indicated that the cluster formation involved hydrogen-bonding interactions between surface silanol groups and ethanol hydroxyl groups in addition to those between ethanol hydroxyl groups. At higher temperatures (30-50 degrees C), the range and the strength of attraction decreased owing to the decrease in the hydrogen-bonded clusters monitored by FTIR spectroscopy, reflecting the nature of hydrogen bonding. The range and the strength of the attraction also changed when the ethanol concentration increased: The long-range attraction started to decrease at 0.6 mol % ethanol at room temperature and disappeared at 1.4 mol % while the adsorption excess amount remained almost constant as did the FTIR peak intensity of the hydrogen-bonded OH group of adsorbed ethanol. In the bulk solution, ethanol clusters appeared at 0.5 mol % ethanol; thus, this change in the attraction could be accounted for in terms of the exchange of ethanol molecules between the surface clusters and bulk clusters. The novel self-assembled structure of alcohol on the surface, found in this study may be called a "surface molecular macrocluster" because the hydrogen-bonded clusters extend to distances of ca. 20 nm longer than the typical sizes of common clusters, 2-4 nm, of alcohol (e.g., ethanol).     

Interactions between Adsorbed Layers of a Low Charge Density Cationic Polyelectrolyte on Mica in the Absence and Presence of Anionic Surfactant

Interactions between two negatively charged mica surfaces across aqueous solutions containing various amounts of a 10% charged cationic polyelectrolyte have been studied. It is found that the mica surface charge is neutralized when the polyelectrolyte is adsorbed from a 10–50 ppm aqueous solution. Consequently no electrostatic double-layer force is observed. Instead an attractive force acts between the surfaces in the distance regime 250–100 Å. We suggest that this attraction is caused by bridging. Additional adsorption takes place when the polyelectrolyte concentration is increased to 100 and 300 ppm, and a long-range repulsion develops. This repulsive force is both of electrostatic and steric origin. The polyelectrolyte layer adsorbed from a 50 ppm solution does not desorb when the polyelectrolyte solution is replaced with an aqueous polyelectrolyte-free solution. Injection of sodium dodecyl sulfate (SDS) into the measuring chamber to a concentration of about 0.01 CMC (8.3 × 10⁻⁵M) does not affect the adsorbed layers or the interaction forces. However, when the SDS concentration is increased to 0.02 CMC (0.166 mM) the adsorbed layer expands dramatically due to adsorption of SDS to the polyelectrolyte chains. The sudden swelling suggests a cooperative adsorption of SDS to the preadsorbed polyelectrolyte layer and that the critical aggregation concentration between the polyelectrolyte and SDS at the surface is about 0.02 CMC. The flocculation behavior of the polyelectrolyte in solution upon addition of SDS was also examined. It was found that 0.16–0.32 mol SDS/mol charged segments on the polyelectrolyte is enough to make the solution slightly turbid.     

Viscosity behavior of silica suspensions flocculated by associating polymers

Associating polymers are hydrophilic long-chain molecules containing a small number of hydrophobic groups, and act as flocculants in aqueous suspensions. The effects of associating and nonassociating polymers on viscosity behavior are studied for silica suspensions. Since flocculation is induced by polymer bridging, the viscosity behavior is converted from Newtonian to shear-thinning profiles. The additions of surfactant cause an increase in viscosity for suspensions prepared with associating polymer, whereas the flow behavior of suspensions with nonassociating polymer is not significantly influenced. In adsorption of associating polymers onto silica particles, the chain may adopt a conformation with a water-soluble backbone attached to the particle surfaces. The hydrophobic groups extending from the chains adsorbed onto different particles can form a micelle by association with surfactant. Therefore, the bridging flocculation is enhanced by surfactant. The cooperative micellar formation between associating polymer and surfactant is responsible for viscosity increase in suspensions.     

Simple model for DNA adsorption onto a mica surface in 1:1 and 2:1 electrolyte solutions

We propose a simple theory of interactions between like-charged polyelectrolyte and a surface based on a mean-field Derjaguin-Landau-Verwey-Overbeek approach. It predicts that the van der Waals attractive interactions are responsible for irreversible physisorption of polyelectrolytes onto charged surfaces. We show that monovalent salts contribute significantly to repulsive interactions, while enhancing the attraction very slightly. The effect of the divalent counterions is reverse. Therefore, to achieve the adsorption, the overall repulsion due to 1:1 electrolyte should be counterbalanced by the stronger van der Waals attraction due to the presence of doubly charged counterions in solution. The theory has been validated experimentally against its ability to predict the minimum polymer/surface interaction energy required for the adsorption using DNA/mica in NaCl, MgCl2, and NiCl2 solutions as a test system. The theory explains the mechanism of linear DNA adsorption to a mica surface for different solvent compositions and can be used as a tool for predicting the optimum conditions for AFM experiments on linear polymer systems. The model can also be used to make general conclusions on the conformation of polymer molecules on a surface. We have shown for the DNA/mica surface system that when the adsorption of DNA is mostly governed by long-range van der Waals forces the molecule adopts an ideal 2D conformation. When the adsorption is mostly due to short-range ion-correlation forces, DNA will appear 3D --> 2D projected in agreement with experimental data.     

Adsorption and flocculation by polymers and polymer mixtures

Polymers of various types are in widespread use as flocculants in several industries. In most cases, polymer adsorption is an essential prerequisite for flocculation and kinetic aspects are very important. The rates of polymer adsorption and of re-conformation (relaxation) of adsorbed chains are key factors that influence the performance of flocculants and their mode of action. Polyelectrolytes often tend to adopt a rather flat adsorbed configuration and in this state their action is mainly through charge effects, including ‘electrostatic patch’ attraction. When the relaxation rate is quite low, particle collisions may occur while the adsorbed chains are still in an extended state and flocculation by polymer bridging may occur. These effects are now well understood and supported by much experimental evidence. In recent years there has been considerable interest in the use of multi-component flocculants, especially dual-polymer systems. In the latter case, there can be significant advantages over the use of single polymers. Despite some complications, there is a broad understanding of the action of dual polymer systems. In many cases the sequence of addition of the polymers is important and the pre-adsorbed polymer can have two important effects: providing adsorption sites for the second polymer or causing a more extended adsorbed conformation as a result of ‘site blocking’.     

Complex formation in mixtures of lysozyme-stabilized emulsions and human saliva

In this paper, we studied the interaction between human unstimulated saliva and lysozyme-stabilized oil-in-water emulsions (10 wt/wt% oil phase, 10 mM NaCl, pH 6.7), to reveal the driving force for flocculation of these emulsions. Confocal scanning laser microscopy (CSLM) showed formation of complexes between salivary proteins and lysozyme adsorbed at the oil-water interface and lysozyme in solution as well. To assess the electrostatic nature of the interaction in emulsion/saliva mixtures, laser-diffraction and rheological measurements were conducted in function of the ionic strength by adding NaCl to the mixture in the range between 0 and 168 mM. Increasing the ionic strength reduced the ability of saliva to induce emulsion flocculation as shown by the decreased floc size and the effect on the viscosity. Turbidity experiments with varying pH (3-7) and ionic strength also showed decreased complex formation in mixtures between saliva and lysozyme in solution upon NaCl addition up to 200 mM. Decreasing the pH increased the turbidity, in line with the increase of the positive net charge on the lysozyme molecule. We conclude that electrostatic attraction is the main driving force for complex formation between saliva components and lysozyme adsorbed at the oil droplets and in solution.     

An improved collision efficiency model for particle aggregation

A generalized geometric model is presented which describes the collision efficiency factor of aggregation (the probability of a binary particle or aggregate collision resulting in adhesion) for systems comprised of two oppositely charged species. Application of the general model to specific systems requires calculation of the area of each species available for collision with a second species. This is in contrast to previous models developed for polymer-particle flocculation that are based on the fractional surface coverage of adsorbed polymer. The difference between these approaches is suggested as an explanation for previously observed discrepancies between theory and observation. In the current work the specific case of oppositely charged nondeformable spherical particles (heteroaggregation) is quantitatively addressed. The optimum concentration of oppositely charged particles for rapid aggregation (maximum collision efficiency) as a function of relative particle size is calculated and an excellent correlation is found with data taken from literature.     

Mathematical modeling of polymer-induced flocculation by charge neutralization

A detailed mathematical model for flocculation of colloidal suspensions in presence of salts and polymers is described and validated. In former case, the classical DLVO theory, which accounts for relevant variables such as pH and salt concentration, is incorporated into a geometrically sectioned discrete population balance model. For processes involving polymers, flocculation via simple charge neutralization is modeled using a modified DLVO theory in which the effect of adsorbed polymer layers on van der Waals attraction is included. The fractal dimension of aggregates is obtained by dynamic scaling of experimental data for time evolution of mean aggregate size. The particle surface potential is assumed to be approximately equal to the zeta potential. The model predictions are in close agreement with experimental results for flocculation of colloidal hematite suspensions in the presence of KCl and polyacrylic acid at different concentrations. In particular, given values of model parameters, e.g., Hamaker constant, fractal dimension, surface potential, and thickness of adsorbed polymer layer, the model can realistically describe the kinetics of flocculation by a simple charge neutralization mechanism and track the evolution of floc size distribution. Representative examples of sensitivity of the flocculation model to perturbations in surface potential and fractal dimension and to modification in the DLVO theory for polymer-coated particles are included.     

Rheological behavior of silica suspensions in aqueous solutions of associating polymer

Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups. The aqueous solutions show viscoelastic responses above some critical concentrations because a three-dimensional structure is formed by association of hydrophobic groups. When the associating polymers are added to silica suspensions at low concentrations, the flocculation is induced by bridging mechanisms, and the flow of suspensions become shear-thinning. For suspensions prepared with polymer solutions in which the associating network is developed, the viscosity decreases, shows a minimum, and then increases with increasing particle concentration. The viscosity decrease may arise from the breakdown of associating network due to adsorption of polymer chains onto the silica surfaces. As the particle concentration is increased, the polymer concentration in solution is decreased, and finally, all polymer chains are adsorbed on the surfaces. Beyond this point, the partial coverage of particle surfaces takes place and strong interactions are generated between particles by polymer bridging. Since the stable suspensions are converted to highly flocculated systems, the viscosity is increased and the flow becomes shear-thinning. The concentration effect of silica particles on the viscosity behavior of suspensions can be explained by a combination of viscosity decrease in solution due to polymer adsorption and viscosity increase due to flocculation.     

Coupling between polymer adsorption and colloidal particle aggregation

Studies of the adsorption of high molecular weight polymers on colloidal latex and silica particles and their subsequent flocculation were carried out. Neutral polyethylene oxide samples with both a narrow and a broad molecular weight distribution were used together with low charged cationic copolymers. The influence of the particle concentration and polymer dose on the flocculation were systematically investigated under quiescent conditions.Equilibrium bridging only occurred with polyelectrolyte, even in very dilute suspensions, at high particle coverage. In contrast to this, non-equilibrium bridging occurred with both neutral polymer and polyelectrolytes but only for more concentrated suspensions and small amounts of adsorbed polymer. Polymer adsorption in dilute suspensions, which did not show particle aggregation was measured an electrophoretic technique. In more concentrated suspensions, where flocculation takes place, we found that aggregation prevents further polymer adsorption and induces both an excluded volume and a surface effect. The consequences on the shape of the isotherms differ according to the aggregation mechanism.A significant decrease of the amount, , of adsorbed polymer is observed with non-equilibrium bridging. When both mechanisms simultaneously contribute to the aggregation, the value of depends on their relative importance. In the intermediate range of copolymer dose their respective contributions are critically sensitive to the details of the mixing step and stirring, leading to non reproducible experimental results.     

Temperature influence of nonionic polyethylene oxide and anionic polyacrylamide on flocculation and dewatering behavior of kaolinite dispersions

Nonionic polyethylene oxide (PEO) and anionic polyacrylamide (PAM) flocculation of kaolinite dispersions has been investigated at pH 7.5 in the temperature range 20-60 degrees C. The surface chemistry (zeta potential), particle interactions (shear yield stress), and dewatering behavior were also examined. An increase in the magnitude of zeta potential of kaolinite particles, in the absence of flocculant and at a fixed PEO and PAM concentration, with increasing temperature was observed. The zeta potential behavior of the flocculated particles indicated a decrease in the adsorbed polymer layer thickness, while at the same time, however, the adsorbed polymer density showed a significant increase with increasing temperature. These results suggest that polymer adsorption was accompanied by temperature-influenced conformation changes. The hydrodynamic diameter and supernatant solution viscosity of both polymers decreased with increasing temperature, consistent with a change in polymer-solvent interactions and conformation, prior to adsorption. The analysis of the free energy (DeltaG(ads)) of adsorption showed a strong temperature dependence and the adsorption process to be more entropically than enthalpically driven. The polymer conformation change and increased negative charge at the kaolinite particle surface with increasing temperature resulted in decreased polymer bridging and flocculation performance. Consequently, the shear yield stress and the rate and the extent of dewatering (consolidation) of the pulp decreased significantly at higher temperatures (>40 degrees C). The temperature effect was more pronounced in the presence of PEO than PAM, with 40 and 20 degrees C indicated as the optima for enhanced performance of the latter and former flocculants, respectively. The results demonstrate that a temperature-induced conformation change, together with polymer structure type, plays an important role in flocculation and dewatering behavior of kaolinite dispersions.     

Weak flocculation of aqueous kaolin suspensions initiating by NaCMC with different molecular weights

The present work is an investigation of the effect of NaCMC with different viscosities (molecular weights) on the stability of aqueous kaolin suspensions at pH 5-6. The stabilizing effect of polymers was characterized by measuring the sedimentation volumes (for 2.5% kaolin suspensions) and some important rheological parameters (for 40% and 50% kaolin suspensions). In certain cases the stability of suspensions was also studied in the presence of 0.5-1.0% NaCl. The additives were incorporated into the suspensions separately and simultaneously, as well. In certain cases the effect of mixing order of NaCMCs was also studied. The lower viscosity NaCMC was found to be a better stabilizing agent than its medium viscosity counterpart at the studied polymer concentrations (0.005-1.0%). This was manifested in smaller sedimentation volumes and lower rheological parameters (viscosity, yield stress, degree of thixotropy and elasticity). The lower and the medium viscosity polymer were simultaneously and consecutively added in a mass ratio of 50:1 and 10:1. The resulted observation of low viscosity and yield stress, and more importantly thixotropy and elasticity, can be interpreted in terms of a “site-blocking” type flocculation.     

Phase behavior and 3D structure of strongly attractive microsphere-nanoparticle mixtures

We investigate the phase behavior and 3D structure of strongly attractive mixtures of silica microspheres and polystyrene nanoparticles. These binary mixtures are electrostatically tuned to promote a repulsion between like-charged (microsphere-microsphere and nanoparticle-nanoparticle) species and a strong attraction between oppositely charged (microsphere-nanoparticle) species. Using confocal fluorescence scanning microscopy, we directly observe the 3D structure of colloidal phases assembled from these mixtures as a function of varying composition. In the absence of nanoparticle additions, the charged-stabilized microspheres assemble into a polycrystalline array upon sedimentation. With increasing nanoparticle volume fraction, nanoparticle bridges form between microspheres, inducing their flocculation. At even higher nanoparticle volume fractions, the microspheres become well coated with nanoparticles, leading to their charge reversal and subsequent restabilization. We demonstrate how this fluid-gel-fluid transition can be utilized to control the morphology of the colloidal phases formed under gravity-driven sedimentation.     

Ion-Exchange Chromatographic Supports Obtained by Formation of Polyelectrolyte Multi-Layers for the Separation of Proteins

Summary High performance liquid chromatography (HPLC) was used to study the mechanism of formation of polyelectrolyte multilayers on porous silicas. The coatings were produced by alternating the adsorption of positively and negatively charged polymers. The stationary phases formed by adsorbing a single layer, double layers and triple layers were tested by studying the elution behavior of model proteins. The double polymer coating was achieved by adsorbing first a polycation such as hexadimethrine bromide (HB) on the HPLC silica support and then a polyanion such as dextran sulfate (DS) on the cationic layer formed. The retention properties of this support are mainly those of a cation exchanger as the negatively charged proteins were strongly retained while positively charged ones were weakly adsorbed. This work demonstrated the importance of the first underlying layer as the retention behavior of proteins was greatly affected by the properties of this coating. The triple polymer coating was achieved by adsorbing the polycation (HB) on the double layer coating (HB-DS). Its retention behavior was that of an anion exchange support. The HB-DS stationary phase displayed good chromatographic performances, with an adsorbed layer relatively stable. The polyelectrolyte multilayer coating procedure was useful to easily synthesize cation-exchange supports for the separation of basic proteins.