K. Lunkwitz

Higher efficiency in the flocculation of clay suspensions by using combinations of oppositely charged polyelectrolytes

Abstract Dual systems—the highly charged polycation poly(diallyll-dimethyl-ammoniumchloride) (PDADMAC) in combination with different high molecular weight polyanions of the polyacrylamide type—were tested as flocculants for clay suspensions and natural waste waters from gravel pits. The flocculant performance was characterised by measuring the sedimentation behaviour as well as the residual turbidities and the extent of polymer adsorption. Whereas the molecular weight of the high molecular weight polycation is of lower influence, the removel of suspended particles is greatly affected by the molecular weight of the polyanion. Our experiments showed that degradation of the very high molecular weight polyanions leads to higher turbidity of the supernatant. The order of addition of the two polyelectrolytes influences the flocculation mechanism. Most effective is the addition of polycation followed by the polyanion. So a combination of patching and bridging is obtained.

Polyelectrolyte adsorption on charged surfaces: study by electrokinetic measurements

Abstract In order to describe the influence of cationic polyelectrolytes on flocculation of disperse systems the adsorption of poly (diallyldimethylammonium chloride) (PDADMAC) onto silica, mica and acidic polymer latex was investigated. The plateau value of the adsorption isotherms grows with increasing surface charge density of the substrates and electrolyte concentration. The adsorbed layer of the polycation was characterized by zeta potential measurements with KCl solutions of constant ionic strength and varied pH. The zero point of the charge as well as the shape of the zeta potential–pH plot depends on the coverage of the surface with polycations. For fully covered substrates the zero point of the charge as well as the pKA and pKB values calculated by a stochastic search programme are independent of the substrate. Maximum flocculation was observed at about 30% of the plateau value of the adsorption isotherms.

Effect of a low-molecular-weight salt on colloidal dispersions of interpolyelectrolyte complexes

Abstract Colloidal dispersions of an interpolyelectrolyte complex were prepared by mixing dilute aqueous solutions of poly(dimethyldiallylammonium chloride) and the sodium salt of the alternating copolymer of maleic acid propene in amounts providing about a threefold excess of the charged groups of the cationic polyelectrolyte over those of the anionic polyelectrolyte. These dispersions were examined by means of analytical sedimentation, quasielastic light scattering, and laser Doppler microelectrophoresis. The experimental results obtained suggest that the particles of the interpolyelectrolyte complex are multicomplex aggregates bearing cationic charge. Such aggregates were assumed to consist of a hydrophobic core formed by coupled oppositely charged macromolecules and a hydrophilic shell formed by cationic macromolecules. Hydrodynamic and electrophoretic properties of these aggregates were found to be rather sensitive to variations in the ionic strength of the surrounding medium: with rising salt concentration, their sedimentation coefficient and hydrodynamic size increase, these increases becoming more strongly pronounced at higher salt concentrations, whereas their electrophoretic mobility gradually decreases. The salt effects revealed suggest that the aggregation level of the particles of the interpolyelectrolyte complex rises in response to an increase in the ionic strength of the surrounding medium. This phenomenon was associated with the salt-induced decrease of the stabilizing effect of the hydrophilic shells that protect such particles from progressive aggregation.

The use of oppositely charged polyelectrolytes as flocculants and retention aids

Abstract New results for flocculation of cellulose-clay mixtures with a so-called dual system, consisting of the polycation poly(dimethyldiallylammonium chloride), PDMDAAC, and different polyanions are presented. The flocculation behaviour was investigated by different procedures: polyelectrolyte titration, dewatering test, and measuring the residual turbidity. It should be pointed out that the flocculation mechanism depends on the molecular weight of the polyanion used. In the first case, when the molecular weight of the polyanionic compound is relatively low, the complexation between the polycation and the polyanion is said to be the mechanism and the ratio of anionic to cationic charges (n−:n+) is of crucial importance. Using a very high molecular weight polyanion (⪢ 1 million) as one component of the dual system, the mechanism of reaction consists of bridging and patch effects.

The interaction between polyelectrolyte complexes made from poly(dimethyldiallylammonium chloride) (PDMDAAC) and poly(maleic acid-co-α-methylstyrene) (P(MS-α-MeSty)) and cellulosic materials

Abstract The interaction between the oppositely charged polyelectrolytes poly(dimethyldiallylammonium chloride) (PDMDAAC) and poly(maleic acid-co-α-methylstyrene) (P(MS-α-MeSty)) in the presence of cellulose or a mixture of cellulose and clay, respectively, has been investigated. The application of different methods allowed the characterization of flocculation and surface modification. It has been found for step-by-step addition that the flocculation behaviour of this two-component system (dual system) depends on the molar ratio of the two polymers used and also on the concentration of polycation. Only at a certain concentration of PDMDAAC does the two component system lead to a considerable improvement in the flocculation behaviour and in surface modification, as compared to a pure polycation. The addition of a premixed polyelectrolyte complex has also been found to be a successful method. However, the mechanism of flocculation is supposed to be different for complexes with different molar ratios.

Surface modification by polyelectrolyte complexes: influence of different polyelectrolyte components and substrates

Abstract The complex formation between oppositely charged polyelectrolytes (PELs) and their interaction with inorganic particles have been investigated. The PELs studied were poly(ethyleneimine)s differing in their molar masses, poly(diallyldimethylammonium chloride) and copolymers of maleic acid and propylene or styrene. To obtain comprehensive information, the results of PEL titration, electrophoretic mobility investigations and scanning force microscopy measurements are compared. Polyelectrolyte complexes (PECs) are formed by combination of aqueous PEL solutions containing definite amounts of charges (n−, polyanion; n+, polycation). The point of zero charge of the PEC depended on the selection of different PELs. Changing the molar ratio n − n + permitted the preparation of non-stoichiometric polyelectrolyte complexes (nPECs) showing differences in the residual amount of charge and the zeta potential. The nPEC carrying cationic excess charges was used to modify the surface of silicate powders. In contradiction to the adsorption of polycations, nPECs were able to generate a strongly cationic surface charge in most of the cases. It is concluded that the preformed PEC adsorbs roughly with the same stoichiometry as in solution on the surface, but the efficiency of surface modification is quite different for nPECs made from altered PELs. We believe that steric factors of the PELs are responsible for the conformation of the PECs and also for the flexibility with which the complexes rearrange on the surface. The influence of the surface charge density of the bare substrates on the extent of surface modification by PEC is also described.

INVESTIGATION OF THE STABILITY OF SURFACE MODIFICATION BY POLYELECTROLYTE COMPLEXES: INFLUENCE OF POLYELECTROLYTE COMPLEX COMPONENTS AND OF SUBSTRATES AND MEDIA

Abstract Silicate surfaces were modified by adsorption of polyelectrolyte complexes (PEC) and parameters that influence the permanence of this kind of surface modification were investigated; for example, different polyelectrolyte (PEL) components, substrate surface charge density and interacting media (pH, salt). The PEL tested were poly(ethyleneimine)s differing in the molar mass, poly(diallyldimethylammonium chloride) and a copolymer of maleic acid and propene. For sensitive detection of the polyanion concentration the poly(maleic acid-co-propene) was labelled with a fluorescent dye. To obtain more comprehensive information about the performance of the surface charge modification the results of PEL titration, electrophoretic mobility investigations, scanning force microscopy (SFM) and fluorescence measurements have been compared. Non-stoichiometric PECs were able to recharge the negatively charged powders and to generate cationic surface charges. However, the stability of the modification layer has been influenced by rinsing the PEC-modified substrate (MS-P/Polymin P) with 10−2 mole/l NaOH that led to an increase in the surface charge density and the zeta potential due to desorption of the polyanion from the surface of the polymer layer. The adsorbed amounts were found to be dependent on the NaOH concentration. The addition of 10−2 mole/l HCl partly causes the release of the PEC and a negative surface charge density has been observed, whereas the rinsing with NaOH has always been more effective than rinsing with HCl. Substituting the weak Polymin P by PDADMAC leads to decreasing desorbed amounts. The decrease of the substrates surface charge density and the addition of low molecular weight salts strongly reduce the stability of the PEC surface layer.

Interaction of colloidal dispersions of non-stoichiometric polyelectrolyte complexes and silica particles

Abstract Stable non-stoichiometric polyelectrolyte complex dispersions (PEC) with cationic excess charges were used to study the efficiency of charge modification of silica particles. The complex dispersions formed by the interactions of poly(diallyldimethyl-ammonium chloride) (PDADMAC) or polyethylenimine (PEI) with poly(maleic acid-co-α-methylstyrene) are stable but differ in their cationic net charge. It can be seen from the results that the amount of preformed complex dispersion necessary for complete adsorption and for discharging the negative surface of the substrates tested is quite different for the PDADMAC complex and the PEI complex. Depending on the molecular weight and the type of functional groups of PDADMAC and PEI, polyelectrolyte complex particles that differ in their conformation, diameter and charge accessibility are formed. In the case of glass pearls, a substrate with a very high surface charge density, we can demonstrate that the effectiveness of charge modification with the PDADMAC complex is higher than with the PEI complex. Accordingly, the results suggest that the steric differences between the polyelectrolyte complexes tested is less significant for silicic acid, a substrate with a large specific surface.

The interaction between oppositely charged polyelectrolytes in the presence of solid surfaces

Abstract The interaction between the oppositely charged polyelectrolytes poly(dimethyldiallylammonium chloride) (PDMDAAC) and poly(maleic acid-co-α-methylstyrene) (P(MS-α-MeSty)) in the presence of inorganic fine particles such as clay, silicic acid and glass pearls has been investigated. In experiments involving only adsorption of PDMDAAC the driving forces are electrostatic attractions between the anionic particles and the polycation. The amount of adsorbed PDMDAAC necessary for changing the surface charge of the substrates from negative to positive increased with increasing specific surface area. The adsorption of PDMDAAC and P(MS-α-MeSty) from their mixed solutions on to the inorganic fine particles showed a synergistic effect with respect to the amount of cationic surface charge. This effect increased with the surface charge density of the solid particles. The interaction of P(MS-α-MeSty) with preadsorbed PDMDAAC led to a change in the polarity of the modified particles and did not show a synergistic effect. The obvious difference between the adsorption from the mixed solutions and sequential adsorption is discussed in terms of the configuration of adsorbed polyelectrolyte complex particles formed in the solution or at the solid/liquid interface.