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.

The Gibbs energies of different oligoethers (oligomers of tetrahydrofuran and ethylene oxide and a block cooligomer of these ethers) with hydroxyl and acetyl end groups in tri- and tetrachloromethane are obtained by vapour pressure measurements. A group contribution theory, based on a quasichemical equilibrium between contacting segment surfaces, which was used so far only for calculating mixing enthalpies, is extended to Gibbs energies. The Gibbs energies predicted with this theoretical approach show a good agreement with the measured data of the tested polymer solutions. With this theory it is not only possible to describe but even to predict gaps in the mixing behaviour of polymer solutions.

In order to describe the influence of cationic polyelectrolytes (PE) on the stability and flocculation of dispersed particles, the polymer adsorption on particles of colloidal silica and quartz powder Sikron was investigated. Poly-methacryloyloxyethyltrimethyl-ammoniumchloride (PM) and poly-methacryloyloxyethyldimethylbenzylammonium chloride (PMBQ) of different molar mass have been used. On silica and Sikron, the adsorbed amounts increase with the ionic strength. The zero point of charge (zpc) and the shape of the zeta potential–pH plots vary with the surface coverage by the polycations. The zeta potentials, diffusion coefficients, and flocculation rates are influenced at different salt concentrations by the adsorbed amounts and characterize the flocculation of the particles. Stabilization is accomplished at low salt concentration for PM and PMBQ near the plateau of the adsorption isotherm by electrostatic repulsion of the covered particles with reverse charge. At high ionic strength, only PM stabilizes the particles sterically by osmotic repulsion of the long tails. For PMBQ, we found a large flocculation range with no stabilization at high coverage.

The mixing enthalpies of different oligoethers (oligomers of tetrahydrofuran and ethylene oxide and a block cooligomer of these ethers) with hydroxyl and acetyl end groups in various solvents (2-propanol, tri- and tetrachloromethane) are obtained by microcalorimetry. An extended group contribution theory, based on a quasichemical equilibrium between contacting segment surfaces, is applied successfully fur the description and prediction of the mixing enthalpies. The theory can be used fur every kind of molecular interaction, for polar and non-polar interactions as well as for hydrogen bridges. In addition, it is possible to apply the theory to mixing enthalpies of polymer blends.

In order to describe the influence of cationic polyelectrolytes (PEs) on the stability and flocculation of dispersed particles the adsorption of statistic copolymers of diallyl-dimethyl-ammonium-chloride (DADMAC) and of N-methyl-N-vinylacetamide (NMVA) and of the corresponding homopolymers was investigated on particles of colloidal silica, quartz powder Sikron and polystyrene latex. The adsorbed amounts and the hydrodynamic thicknesses of the polymer layers and furthermore the surface charge densities and the electrophoretic mobilities of the bare and covered particles have been measured. On silica and latex the adsorbed amounts and the layer thicknesses increase with ionic strength and pH (only silica), with decreasing chain charge density of the polycations and at high electrolyte concentration with the molar mass of the polymers. The zero point of charge (zpc), and the shape of the zeta potential-pH plots vary with the coverage of the surface by the polycations. The zpc-values are independent of the ionic strength. Zeta potentials, diffusion coefficients and flocculation rates measured in dependence of the adsorbed amount of the different charged copolymers at various salt concentration and pH demonstrate fast flocculation by van der Waals attraction at small zeta potentials because of screening of the repulsive surface charges at high ionic strength and by adsorbed polycations. Additional with high molecular polycations slow mosaic flocculation is observed at low PE concentrations at still higher surface charge and slow bridging flocculation at approach of plateau adsorption. Stabilization is accomplished at low salt concentration at full coverage by electrostatic repulsion because of charge reversal. At large ionic strength high molar mass PEs are necessary to stabilize the particles sterically by osmotic repulsion of the long tails. For Sikron particles optimum flocculation is obtained at about 30% of plateau adsorption of poly(diallyl-dimethyl-ammonium-chloride) (PDADMAC) and the size of the flocs increase with molar mass.

Isotherms of the cationic polyelectrolyte poly(diallyl-dimethyl-ammoniumchloride), poly(diallyl-dimethyl-ammoniumchloride-co-N-methyl-N-vinylacetamide) and neutral poly(N-methyl-N-vinyl-acetamide) on polystyrene latex were determined by measuring the concentration of the polymer in the supernatant solution with polyelectrolyte titration. All isotherms are of the high affinity type. The adsorbed amount rises with the concentration of salt and with the decrease of polyelectrolyte charge density. At high concentrations of salt the adsorbed amount increases with the molar mass. The procedure for the polyelectrolyte titration of polyelectrolyte solutions with high salt concentrations was performed after dialysis against hydrochloric acid.

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.

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.

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.

The adsorption of cationic polyelectrolytes on colloidal silica-particles is investigated. The adsorbed amounts and hydrodynamic layer thicknesses of the cationic polyelectrolytes poly(diallyl-dimethyl-ammoniumchloride) PDADMAC of different molar masses and statistic copolymers of DADMAC and N-methyl-N-vinyl-acetamide (NMVA) of various contents have been measured. Furthermore, the surface charge densities of the bare and polyelectrolyte covered silica particles have been determined by potentiometric titration. The adsorbed amounts as well as the hydrodynamic layer thicknesses of the adsorbed polyelectrolyte layer increase with the ionic strength and the pH of the suspension and at high salt concentration with the molar mass of the macroions. At low salt concentrations the adsorbed amounts rise with decreasing charge density of the polyelectrolyte. During polyelectrolyte adsorption the surface charge density of colloidal silica is increased because of the dissociation of silanol groups.

The adsorption of poly-L-lysine (PL) and poly-L-glutamic acid (PG) was determined by FTIR-ATR-spectrometry on negatively charged hydrophilic and neutral hydrophobic silica surfaces of flat silicon crystals. Position and shape of the amide bands correspond to a coil like adsorption of PL at all pH values and of PG at pH > 5. High affinity adsorption isotherms are obtained for both polyelectrolytes. On the hydrophobic surface the plateau amounts of PL and PG decrease with their ionization degree demonstrating flat adsorbed macromolecules in the highly charged state because of the repellent electrostatic segmental interactions. The plateau amounts of PL increase at increasing pH and of PG at decreasing pH and nearly the same maximal amounts are reached at the neutral state corresponding to a dominant role of hydrophobic interactions. On hydrophilic surfaces the additional attraction by the negatively charged SiOH groups increases the adsorption of PL, whereas the adsorbed amount of PG decreases by repulsion. Decreasing surface charge with decreasing pH reduces these effects. Screening by the electrolyte KBr has no effect at the neutral surface, whereas at the charged surface rite adsorbed amounts of PL and PG increase distinctly at C-KBr > 0.4 mol/l.

With photon correlation spectrometry (PCS) the diffusion coefficients, average diameters and polydispersities of colloidal particles can be determined in dilute aqueous suspensions. In this study PCS is used to follow the coagulation and flocculation of silica particles. Electrolyte solution added to suspensions of bare particles and of particles covered with adsorbed polyethylene oxide layers induces aggregation. The rate constants of aggregation are evaluated by the second-order Smoluchowski theory with the assumptions of spherical aggregated particles and volume proportional light-scattering amplitude. Adsorbed PEO layers of molar mass lower thanMw=160000 decrease the critical flocculation concentration and the flocculation states and rate constants for bare and covered particles are the same at high electrolyte concentrations. Polymer layers of high molar mass (Mw=325000, 900000) reducved at full coverage the rate constants and stabilize the suspensions even at high electrolyte concentrations. At low coverage adsorption of high molar mass polymers results in the same values as of low molar mass PEO. The correlation between rate constants and hydrodynamic PEO layer thicknesses demonstrates the steric influence of the tails of the adsorbed macromolecules on stability and flocculation.

The adsorption of statistic and block copolymers of ethylene oxide (EO) and propylene oxide (PO) and of corresponding homopolymers on silica (Aerosil 200) from CCl4 and CHCl3 was investigated by IR spectroscopy. The adsorption isotherms were determined from the concentration difference in the solution, and the fraction of adhered segments from the IR absorption of free and hydrogen-bonded SiOH groups. Competition and displacement studies demonstrate that the EO segments are preferentially adsorbed compared with the PO segments. The fraction of adhered segments decreases with increasing amount adsorbed, molar mass, EO fraction and blocklength. The dependences are interpreted by conformational models of the block copolymers with central PO or EO blocks.

AbstractA survey will be given on the experimental methods to characterize the structure of adsorbed polymer layers at solid/liquid interfaces. Characterization parameters accessible by experimental methods are: adsorption isotherm mostly determined from the difference in the solution concentration; fraction of adhered segments, obtained from IR‐, NMR‐ ESR‐spectrometry and from microcalorimetry; layer thicknesses of different type from ellipsometry, diffusion, sedimentation, viscosimetry, electrophoresis, force‐distance measurements between contacting layers and from segment density profiles obtained by SANS; adsorption enthalpies from microcalorimetry and from displacer‐technique. Own experimental data of the adsorption of olystyrene, poly(ethylene oxide)s (PHD), poly(N‐vinyl‐2‐pyrrolidone), poly‐(butyl methacrylate)s, poly(vinyl alcohol)s as well as copolymers from ethylene oxide and propylene oxide on interfaces of colloidal silica‐ and latexparticles and partially on flat chromium‐, platinum‐and gold surfaces from organic and aqueous solutions and characteristic results from the literature will be compared and discussed in order to get insight into the conformation of the adsorbed macromolecules.

Hydrodynamic layer thicknesses of adsorbed polyethylene oxides (PEO), polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO) block copolymers, and polyvinyl alcohols (PVA) of different molar mass and composition are determined by photon correlation spectrometry (PCS) and ultracentrifugation (UC). The adsorbents used are precipitated silica and polystyrene latex. The increase of the diffusion coefficient, D, of the bare silica particles with reduction of the electrostatic double layer is related to the existence of immobilized water near the charged interface. At high PEO adlayer thicknesses no influence of the extension of the double layer exists. PCS allows precise determination of the critical flocculation concentration (c.f.c.). An influence of the particle curvature on the hydrodynamic adlayer thicknesses is to be seen. The conformation of adsorbed polymer is determined by the surface-polymer interaction; hydrogen bonds on the polar silica surface and hydrophobic interactions on the apolar latex surface are decisive.

With a Laserphoto sedimentometer, parameters are obtained which enable the characterization of the turbidity and hindered sedimentation of silica suspensions flocculated by the adsorption of polyethylene oxides. By continuous addition of the polymer solution to the silica suspension, the influences of time of addition, stirring time, stirring intensity, polymer coverage, polymer molar mass, solution age, silica concentration, pH and ionic strength on the flocculation state can be quantified. In contrast to pyrogenic silica, precipitated silica is only flocculable by decreasing the pH below 3.5 or by addition of salt above 10−2 moll−1. At high pH and without salt, the orthokinetic flocculation of pyrogenic silica is markedly dependent on stirring, showing the existence of optimum mixing conditions. The molar mass of the polymer, the age of the polymer solution and the ageing conditions are of decisive importance in determining the efficiency and state of flocculation. At high salt concentrations and pH < 3.5, the flocculation of precipitated and pyrogenic silicas becomes independent of mixing conditions.