Polyelectrolyte adsorption and stabilization of silica-suspensions

The adsorption of cationic polyelectrolytes on colloidal silica-particles is investigated. The polyelectrolytes poly(diallyl-dimethyl-ammoniumchloride) PDADMAC of different molar mass have been used. The adsorbed amount is influenced by the ionic strength and pH of the suspension and the molar mass of the macromolecule. The adsorption determines the zetapotential of the covered particles. The electrostatic interaction between the particles as well as the structure of the adsorbed polyelectrolytes play an important role in the stabilization and flocculation behaviour of the polyelectrolyte covered suspensions.     

Effect of microfibrillated cellulose and fines on the drainage of kraft pulp suspension and paper strength

Different types of microfibrillated cellulose (MFC) and fines suspensions were produced, characterized, and then added to a papermaking pulp suspension. High and medium molar mass cationic polyelectrolytes were used as fixatives. The drainage behavior of the pulp suspensions with additives were evaluated against the strength properties of hand sheets made thereof. The effects of salt concentration, pH, fixative type, dosage and type of fibrillar material on drainage were examined. All the MFC and fines samples produced had clearly different properties due to their dissimilar production methods, and they also introduced specific responses on the measured drainage and paper strength. Generally, the addition of MFC decreased the drainage rate of pulp suspension and increased the strength of paper. However, it was shown that by optimum selection of materials and process conditions an enhancement of the strength properties could be achieved without simultaneously deteriorating the drainage.     

Properties of spruce sulfite pulp and birch kraft pulp after sorption of cationic birch xylan

In this study, correlations between the charge density of adsorbed cationic xylans and the mechanical properties of selected pulps are discussed. Hand-sheet experiments were carried out using birch sulfate pulp and spruce sulfite pulp after the adsorption of 2-hydroxypropyl-trimethylammonium-4-O-methylglucuronoxylans (HPMAGXs) with different molar degrees of substitution (MS) in the range of 0.06–0.19. The charge density of the HPMAGX in water was determined by polyelectrolyte titration. Properties such as the tensile and burst index increased after HPMAGX addition and showed an optimum depending on the MS, which was 0.1 for both pulps. Other properties like the tear-index or the specific volume changed depending on the pulp. Beating experiments were also performed and showed an increasing tensile index of birch kraft pulp in the range of 53.7 to 85.7 N m g⁻¹ for a beating time of 10 min. The adsorption of HPMAGXs with different MS onto thiol-based self-assembled monolayers (SAMs) on gold and regenerated cellulose surfaces were studied using surface plasmon resonance (SPR). Electrostatic interactions were found to be the most important factors affecting HPMAGX adsorption, and a strong correlation between HPMAGX adsorption onto carboxyl-terminated SAMs (SAM-COOH) and paper strength when HPMAGX was used as a papermaking additive was observed.     

Adsorption of polyelectrolytes on colloidal latex particles, electrostatic interactions and stability behaviour

The stabilization and flocculation behaviour of colloidal latex particles covered with cationic polyelectrolytes (PE) is studied with photon correlation spectroscopy and zetapotential measurements. Diffusion coefficients, flocculation rate constants and zetapotentials have been determined as a function of adsorbed amount of cationic poly-(diallyl-dimethyl-ammoniumchloride) (PDADMAC) of different molar masses and of statistic copolymers of DADMAC and N-methyl-N-vinyl-acetamide (NMVA) of various compositions in water and at high ionic strength. Flocculation by van der Waals attraction can be observed if the zetapotential is low. This occurs, if the surface charge is screened by the oppositely charged cations. Furthermore, in the case of adsorption of high molecular polycations mosaic flocculation occurs if the adsorbed amount is low. At high ionic strength, flocculation takes place if the adsorbed amount is below the adsorption plateau. If the adsorption plateau is reached the suspensions become stabilized. In water the charge reversal at full coverage leads to electrosteric stabilization both with low and high molar mass polycations. At high ionic strength only polycations with high molar mass are able to stabilize the suspension. If a certain molar mass of the polycation is exceeded, steric stabilization of the suspension occurs due to the formation of long adsorbed PE tails and their osmotic repulsion. The layer thicknesses are determined as a function of the molar mass. Received: 4 July 2000/Accepted: 18 August 2000     

Determination of zeta potential and cationic demand in ECF and TCF bleached pulp from eucalyptus and flax. Influence of measuring conditions

The electrical nature of cellulose fibres is known to govern flocculation, retention and drainage mechanisms during the papermaking process. Zeta (ζ) potential provides useful information towards better control of wet-end chemistry in respects such as the dosaging of chemical aids. The purpose of this work was to study two electrokinetic properties (ζ potential and cationic demand) in ECF (elementary chlorine free) and TCF (totally chlorine free) bleached pulps from eucalyptus and flax, and examine the influence of pH and conductivity on measurements of such properties made with various methods based on the streaming potential, electrophoresis, polyelectrolyte titration and colloidal titration. Measurements of the electrokinetic properties made at high conductivities (C > 0.1 mS/cm) afforded no discrimination between pulp types in terms of electric charge. In fact, the conductivity used had a strong influence on measurements and shifted ζ potential to less negative values at high levels and to more negative values at low levels. The streaming potential technique proved to be more sensitive to changes in the properties (pH and conductivity) of the fibre suspension than did electrophoresis. Conductivity also influenced polyelectrolyte adsorption in the determination of cationic demand. The study also involved assessing the effect of low conductivities (0.01 < C < 0.1 mS/cm), which allowed fibre types and bleaching processes to be easily distinguished. Based on the results, accurately characterizing and identifying not only pulp types, but also the effects of mechanical, chemical and biochemical treatments on fibres, requires measuring the electrokinetic properties at a fixed pH and low conductivity.     

Flocculation of kaolin particles by two typical polyelectrolytes: A comparative study on the kinetics and floc structures

The flocculation kinetics of kaolin particles induced by two polyelectrolytes is studied by using small-angle laser light scattering (SALLS). Two different methods, image analysis and SALLS, are used to calculated the fractal dimensions of flocs formed under different flocculation mechanisms. For a high charge density of polydiallyldimethylammonium chloride (PDADMAC), the initially flocculation rates are slow due to the quite low molecular weight. Smaller and more compact flocs are in the particle–particle connections, and restructuring of the flocs occurs in the flocculation process. With cationic polyacrylamide C498 of very high molecular weight and low charge density, however, the initially flocculation rates are much higher due to its rapid adsorption on kaolin particles, but it will take the adsorbed polymer a much longer time to reach equilibrium due to re-conformation. High potentialities of adsorption prevent the particles from entering the interior of the floc structure or rearrangement, which results in a more open floc structure. Different underlying flocculation mechanisms are evident for these two kinds of polyelectrolytes, in which charge neutralization is mainly involved for the low molecular weight and high charge density polymer of PDADMAC while polymer bridging is suggested to be the dominant mechanism for the high molecular weight polyelectrolyte of C498.     

Adsorption of polyethylene oxide on surface modified silica – stability of bare and covered particles in suspension

 The adsorption of poly-ethylene oxide (PEO) on modified colloidal silica and the stability of the aqueous suspension was investigated. With octanol some silanol groups at the silica surface were replaced by octylgroups. The size of the modified silica particles and the charge and chemical groups on the surface were charaterized by ultracentrifugation, photon correlation spectrometry, polyelectrolyte titration and IR spectrometry. The adsorbed amounts of polyethylene oxides of different molar mass were determined on the modified silica in water. With photon correlation spectrometry (PCS) the hydrodynamic layer thickness of the PEO layers on the particles were measured. The dependences of the layer thicknesses on molar mass of the PEO, polymer concentration and adsorption time were determined. The aggregation of the suspended PEO coated and uncoated modified silica particles was examined with PCS by the time dependence of the diffusion coefficient at different salt concentrations. The influence of molar mass and concentration of PEO as well as of the age of the dispersion was explored. The measured dependences are discussed and compared with the behavior of unmodified silica- and latex-particles. Received: 6 April 1998 Accepted: 27 May 1998     

Effects of surfactants and electrolytes on adsorbed layers and particle stability

Adsorbed polymer and polyelectrolyte layers on colloidal silica nanoparticles have been studied in the presence of various salts and surfactants using photon correlation spectroscopy and solvent relaxation NMR. Poly(ethylene oxide) (PEO; molar mass 103.6 kg mol (-1)) adsorbed with a relatively high affinity and gave a layer thickness of 4.2 +/- 0.2 nm. While the nonionic surfactant used only increased this thickness slightly, anionic surfactants had a much greater effect, mainly due to repulsions between adsorbed aggregates, leading to expansion of the layer. A nonionic/anionic surfactant mixture was also tested and resulted in a larger increase in layer thickness than any of the individual surfactants. The dominant factor on addition of salt was generally the reduced solvency of PEO, which resulted in a further increase in the layer thickness but in some cases caused flocculation. This was not the case when the surfactant was sodium dodecylbenzenesulfonate; instead screening of the intermicellar repulsions possibly combined with surfactant-cation binding resulted in a reduction in the layer thickness. In comparison the affinity between silica and sodium polystyrenesulfonate was very weak. Anionic surfactants and salts did not noticeably increase the strength of adsorption, but instead encouraged flocculation. The situation was different with a nonionic surfactant, which was able to adsorb to silica itself and apparently facilitated a degree of polyelectrolyte adsorption as well.     

甲壳素-壳聚糖对皂土悬浮液的絮凝

研究了三种摩尔质量大体相同而脱乙酰度分别为93%、78%和62%的甲壳素-壳聚糖在pH=3～6范围内对皂土悬浮液的絮凝性质, 并与非离子型絮凝剂聚丙烯酰胺作了比较, 证明含自由氨基的甲壳素-壳聚糖在酸性溶液中形成阳离子聚电解质后. 对带负电的皂土悬浮颗粒有很好的絮凝能力. 通过对上述三种聚合物的絮凝能力、在皂土颗粒表面的吸附量等测定, 认为絮凝中桥连机制起了主导作用, 但聚合物所带正电荷则有利于被皂土颗粒所吸附, 电中和还降低了颗粒间的静电排斥, 这都对桥连絮凝起了促进作用. 聚丙烯酰胺分子在水中不带电, 故当介质pH升高使皂土颗粒双电层变厚时, 絮凝能力迅速下降.     

Adsorption of low charge density polyelectrolytes to an oppositely charged porous substrate

The adsorption behavior of a low charge density cationic polyelectrolyte to cellulosic fibers has been studied. Cationic dextran served as a model polyelectrolyte, as it can be prepared over a range in molecular mass and charge density. The adsorption behavior of the cationic dextran was measured in electrolyte-free conditions using polyelectrolyte titration techniques. By fluorescent labeling the cationic dextran, the extent to which adsorption occurs inside the porous structure was further determined by fluorescent confocal laser scanning microscopy. Cationic dextran having a sufficiently low charge density adsorbed into the pores, although the extent the cationic dextran adsorbed was governed by the molecular mass. The adsorption behavior of the cationic dextran was also studied in various electrolyte concentrations. The adsorbed mass monotonically decreased with increasing electrolyte, as the electrostatic interaction with the substrate was more effectively screened. This behavior also suggests that the interactions between adsorbed polyelectrolyte chains, i.e. lateral correlation effects, are negligible for low charge density polyelectrolytes. Finally, the effect of having a preadsorbed layer of cationic dextran on the adsorption behavior was determined in electrolyte-free conditions using fluorescent double staining techniques. The preadsorbed cationic dextran had almost no effect on the adsorption of low molecular mass fractions. Low molecular mass fractions directly adsorbed into the pore structure, as opposed to adsorbing to a free surface and diffusing into the pores. It was also shown that cationic dextran can be selectively adsorbed to different locations, such that the surface of a porous substrate can be treated uniquely from the bulk.     

pePC-SAFT: Modeling of polyelectrolyte systems 2. Aqueous two-phase systems

This work considers aqueous two-phase systems (ATPS) containing one polymer-polyelectrolyte as well as one salt. To model the liquid-liquid equilibria (LLE) of these systems, the recently presented model pePC-SAFT has been employed. ATPS containing poly(acrylic acid) of different degrees of neutralization or poly(vinyl pyrrolidone), respectively, were considered. The binary interaction parameters used between water-poly(acrylic acid) and water-poly(vinyl pyrrolidone) were adjusted to vapor-liquid equilibrium (VLE) data of these systems. ATPS consisting of poly(vinyl pyrrolidone)-water-sodium sulfate were predicted as function of temperature as well as of molar mass of the polymer. For poly(acrylic acid) systems, ATPS were predicted as function of charge density (degree of neutralization) for different types of salt. For these calculations, the polyelectrolyte model parameters were determined from the non-charged polymer whereas the effect of increasing charge density has been purely predicted by the model. Using this approach, it is possible to predict the shrinking of the liquid-liquid equilibrium region with increasing charging of the polyelectrolyte.     

The effect of pretreatment of calcite dispersions with anionic sodium polyacrylate on their flocculation behavior induced by cationic starch

The flocculation performance of cationic starches on calcite pretreated with anionic sodium polyacrylate (NaPA) was investigated by measuring the mean particle size and the dynamic mobility of the calcite dispersions. Cationic starches of different molecular weight and degree of substitution were used. By varying the amount of anionic sodium polyacrylate, which has a strong affinity to the calcium carbonate surface, one is able to anionically modify the particles and reverse the charge character of the originally cationic calcium carbonate. By such modification of the charge character of the calcium carbonate dispersion, it is possible to approach the mechanisms of flocculation caused by cationic macromolecules like starch. The importance of different mechanisms of flocculation, such as bridging, charge neutralization, and flocculation induced by polyelectrolyte complexes (PEC), was further investigated in this work. It was found that when the NaPA is completely absorbed at the calcite surfaces the mechanism of the flocculation induced by the starch is mainly bridging flocculation. Excess NaPA in the calcium carbonate dispersion will result in polyelectrolyte complexes formed between the non-absorbed NaPA and the oppositely charged starch polymers. These complexes will in most cases strongly enhance the flocculation due to mainly charge neutralization. Depending on the ratio of non-absorbed NaPA and the starch in the aqueous phase, the calcite dispersion is either re-stabilized or more strongly flocculated due to the formed polyelectrolyte complexes. Both the mobility and the particle size measurements support the mechanisms described. It was further demonstrated that the molecular weight and degree of substitution of the starches might be adjusted to control the flocculation behavior.     

Flocculation and stabilization of colloidal silica by the adsorption of poly-diallyl-dimethyl-ammoniumchloride (PDADMAC) and of copolymers of DADMAC with N-methyl-N-vinyl- acetamide (NMVA)

 The stabilization and flocculation behavior of colloidal silica-particles with cationic polyelectrolytes (PE) is investigated. The zetapotentials, diffusion coefficients and flocculation rate constants of silica particles have been measured as a function of the adsorbed amount of cationic polyelectrolytes poly(diallyl-dimethyl-ammoniumchloride) (PDADMAC) of different molar masses and of statistic copolymers of DADMAC and N-methyl-N-vinyl-acetamide (NMVA) of various compositions at different salt concentrations and pH-values. Very fast flocculation due to van der Waals attraction occurs if the zetapotential is small. At low ionic strength this condition occurs just below the plateau of the adsorption isotherms where the surface charges are screened by adsorbed polycations. Additionally with high molecular polycations slow mosaic flocculation is observed at lower PE concentrations. At high ionic strength fast flocculation takes place at low macroion concentration due to the screening of the surface charges by adsorbed polycations and salt ions. At medium concentrations of polycations below plateau adorption slow bridging flocculation is observed. At plateau adsorption the suspensions become stabilized up to high ionic strength. At low salt concentration charge reversal at full coverage with polycations results in electrostatic repulsion. At high ionic strength the particles are stabilized sterically due to the osmotic repulsion of the long adsorbed PE tails. Therefore macroions of high molar mass are necessary to stabilize the suspension at high ionic strength. Received: 27 January 1998 Accepted: 23 March 1988     

Titrimetric methods for the determination of surface and total charge of functionalized nanofibrillated/microfibrillated cellulose (NFC/MFC)

Total and surface charge of three different carboxymethylated nanofibrillated/microfibrillated cellulose (NFC/MFC) samples were investigated by using titrimetric methods (conductometric and polyelectrolyte (PE) titrations). Conductometric titration was found to be suitable method for the NFC total charge measurements when the back titration with HCl was applied. Surface charge measurements of NFC/MFC were conducted by using both indirect and direct PE titrations. The direct PE titration was found to be a more suitable method for the surface charge determination of NFC/MFC whereas the indirect PE titration produced too high surface charge values. This is presumably due to kinetically locked polyelectrolyte conformations on the NFC/MFC surfaces or entrapment of residual polymer after adsorption onto the NFC/MFC gel network. Finally, NFC was propargyl-functionalized and the changes in surface and total charge were successfully monitored and compared to those of propargyl-functionalized pulp. A good correlation between the titrimetric methods and elemental analysis was observed.     

Swelling of mechanical pulp fines

The objective of the study was to determine the swelling of different types of mechanical pulp fines. The physical and chemical characteristics of the fines were also examined. It was found that the degree of swelling correlates with the proportion of fibrillar material, that is fibrillar content of the fines. The fines with the lowest fibrillar content had a swelling comparable to mechanical pulp fibres (0.69g/g), whereas the fines with a high fibrillar content had a swelling comparable to neverdried kraft pulp fibres (1.41g/g). Hemicellulose content and charge could not explain the differences in swelling of different types of the mechanical pulp fines. While the lignin content appears to be an important factor in the degree of swelling of mechanical pulp fines, the results suggest that structural differences between the particles are also important. The bulk elastic modulus was determined by measuring the change in swelling for a known change in osmotic pressure. All the mechanical pulp fines had a high bulk elastic modulus compared to kraft fines. However, fibrillar fines had a lower bulk elastic modulus than flakelike fines. Mechanical pulp fines, both fibrillar and flakelike varieties, did not hornify appreciably. The swelling of both the fines and the fibre fractions increased slightly with the specific energy consumption in the refining.     

Comparison of polyelectrolyte multilayers built up with polydiallyldimethylammonium chloride and poly(ethyleneimine) from salt-free solutions by in-situ surface plasmon resonance measurements

Polyelectrolytes offer a widespread potential for the defined modification of planar inorganic or polymer surfaces. Essential parameters for the regular adsorption of subsequent polymer layers by electrostatic interactions are the charge of polyelectrolyte and of the outermost surface region, the surface of the substrate, and the molar mass of the polyelectrolyte. To study such effects in mono- and multilayers we used poly(diallyldimethylammonium chloride (PD) with a molar mass from 5000 to 400000 g/mol as a strong polycation and poly(ethyleneimine) (PEI) with 75000 g/mol as a weak polycation and poly(sodium styrenesulfonate) (PSS) from 70000 to 1Mio g/mol in the diluted and semi-diluted region. The characterization of the layers was performed by streaming potential, in-situ SPR and UV-Vis spectroscopy. Thereby the layer built up at the solid/liquid-interface could be followed and quantified at the molecular level. SPR revealed that the thicknesses of the multilayer depends strongly on pK values of the polyelectrolyte (strong or weak) and the molar masses. We observed a linear growth if both polyelectrolytes are strong and an exponential growth if one polyelectrolyte is weak. The thickness increased with higher molar masses of the polyelectrolytes. The process was followed in-situ in short time steps.     

Adsorption of highly charged polyelectrolytes onto an oppositely charged porous substrate

The adsorption behavior of highly charged cationic polyelectrolytes onto porous substrates is electrostatic in nature and has been shown to be highly dependent on the polyelectrolyte properties. Copolymers of acrylamide (AM) and diallyldimethylammonium chloride (DADMAC) were synthesized to have a range of macromolecular properties (i.e., charge density and molecular mass). Traditional titration methods have been complemented by fluorescence labeling techniques that were developed to directly observe the extent that fluorescently labeled poly(AM- co-DADMAC) adsorbs into the pore structure of a cellulosic substrate. Although contributing to the electrostatic driving force, the charge density acts to limit adsorption to the outermost surface under electrolyte-free conditions. However, adsorption into the pores can occur if both the molecular mass and charge density of poly(AM- co-DADMAC) are sufficiently low. Adsorption initially increases as the electrolyte concentration is increased. However, the electrostatic persistence length of poly(AM- co-DADMAC) restricts the polyelectrolyte from entering the pores. Therefore, changes in the adsorption behavior at moderate electrolyte concentrations have been attributed to swelling of the polyelectrolyte layer at the fiber exterior. The adsorption behavior changes again at high electrolyte concentrations such that poly(AM- co-DADMAC) could adsorb into the pore structure. This occurred when the electrolyte concentration was sufficient to screen the electrostatic persistence length of poly(AM- co-DADMAC), provided that the entropic driving force for adsorption still existed. It is suggested that adsorption into the pore structure is a kinetic process that is governed by localized electrostatic interactions between poly(AM- co-DADMAC) and the charges located within the pores.     

On the indirect polyelectrolyte titration of cellulosic fibers. Conditions for charge stoichiometry and comparison with ESCA

The effect of electrolyte (NaHCO3) concentration on the adsorption of poly-DADMAC (poly-diallyldimethylammonium chloride) onto cellulosic fibers with different charge profiles was investigated. Surface carboxymethylated fibers were obtained by grafting carboxymethyl cellulose (CMC) onto the fiber surface and bulk carboxymethylated fibers were obtained by reacting the fibers with monochloroacetic acid. It was shown that nonionic interactions do not exist between cellulose and poly-DADMAC, rather electrostatic interactions govern the adsorption. Charge stoichiometry prevails under electrolyte-free conditions, whereas surface charge overcompensation occurs at higher electrolyte concentrations. It was shown that charge stoichiometry prevails if the thickness of the electric double layer kappa(-1) was larger than the mean distance between the charges on the fiber surface, as predicted by polyelectrolyte adsorption theories, taking lateral correlation effects into account. In a second set of experiments the ESCA technique served to independently calibrate the polyelectrolyte titrations for determining the surface charge of cellulosic fibers. Various molecular masses of poly-DADMAC were adsorbed to carboxymethylated fibers having different charge profiles. The adsorption of low M(w) poly-DADMAC (7.0 x 10(3)), analyzed by polyelectrolyte titration, was about 10 times higher than that of the high M(w) poly-DADMAC (9.2 x 10(5)). Despite the difference in accessibility of these two polyelectrolytes to the fiber cell wall, ESCA surface analysis showed, as expected, only slight differences between the two polyelectrolytes. This gives strong credibility to the idea that surface charge content of cellulosic fibers can be analyzed by means of adsorption of a high-molecular-mass cationic polymer, i.e., by polyelectrolyte titration.     

Lignin adsorption on cellulose fibre surfaces: Effect on surface chemistry, surface morphology and paper strength

The adsorption of lignin on cellulose fibres at neutral pH and the effects of calcium ions and a cationic polyelectrolyte (PDADMAC) on the adsorption have been studied. The surface coverage by lignin was determined by electron spectroscopy for chemical analysis (ESCA). The morphology of the lignin layer was studied by atomic force microscopy (AFM). The effect of adsorbed polyelectrolyte and lignin on the strength properties of the paper was also studied. The adsorbed amount of lignin increased monotonically with lignin concentration. Addition of calcium ions resulted in a very high surface coverage by lignin. PDADMAC did not enhance the adsorption of lignin, but without addition of polyelectrolyte the lignin was very weakly attached to the fibre surface. PDADMAC formed complexes with lignin in solution. At high polymer/lignin concentration ratios the charge of the complex was positive and it adsorbed irreversibly as large blobs. At low ratios the complex was easily washed away from the fibre surface. When PDADMAC was pre-adsorbed on the fibre surface the lignin adsorbed as small granules at all lignin concentrations. Neither PDADMAC nor lignin alone increased the strength of pulp sheets significantly. However, together they increased the bonding between fibres.     

Characterization and Aggregation Behaviors of Mixed DDAB/SDS Solution With and Without Poly(4-styrenesulfonic Acid-Co-Maleic Acid) Sodium

Synthetic vesicles are formed by cationic and anionic surfactants, didodecyldimethylammonium bromide (DDAB), and sodium dodecylsulfate (SDS). The morphology, size, and aqueous properties of cationic/anionic mixtures are investigated at various molar ratios between cationic and anionic surfactants. The charged vesicular dispersions made of DDAB/SDS are contacted or mixed with negatively charged polyelectrolyte, poly(4-styrenesulfonic acid-co-maleic acid) sodium (PSSAMA), to form complexes. Depending on DDAB/SDS molar ratio or PSSAMA/vesicle charge ratio, complexes flocculation or precipitation occur. Characterization of the cationic/anionic vesicles or complexes formed by the catanionic vesicles and polyelectrolytes is performed by transmission electron microscope (TEM), dynamic light scattering (DLS), conductivity, turbidity, and zeta potential measurements. The size, stability, and the surface charge on the mixed cationic/anionic vesicles or complexes are determined.