Emile Pefferkorn

Polyacrylamide-sodium kaolinite interactions: Flocculation behavior of polymer clay suspensions

Abstract The behavior of Na-kaolinite suspensions in presence of high-molecular-weight polyacrylamide is investigated experimentally. In analyzing the effect of polymer, three different domains are found: at low concentrations the polymer acts as a stabilizer, at intermediate concentrations enhanced flocculation is observed, and at high concentration the suspension is again stabilized. The mechanism of polymer action is analyzed in the light of adsorption data obtained using radiolabeled polymer. A correlation is established between adsorption strength and the mode of particles association in the three domains. The nature of the adsorption forces is discussed in terms of the surface chemistry of Na-kaolinite.

Adsorption—desorption processes in charged polymer/colloid systems; structural relaxation of adsorbed macromolecules

Abstract Interfacial structural relaxation of adsorbed polyelectrolytes was evidenced by controlling the rate of polymer supply in an adsorption experiment. During initial contact with the surface, the macromolecule adsorbed, preserving its solution conformation. Under conditions of slow transfer of solution macromolecules toward the sorbent surface, a slow structural change affected the adsorbed polymer, leading to a flat layer. This structure however was unstable and, via a slow adsorption process, reached a state of thermodynamic equilibrium. Under conditions of fast transfer, the sorbent was rapidly covered with polymers having a solution-like conformation. This polymer surface was also unstable and thermodynamic equilibrium was now attained via a desorption process. The amount of adsorbed polymer at adsorption equilibrium did not depend on the rate of the mass transfer.

Derivation by hydrodynamics of the structural characteristics of adsorbed polymers at liquid-solid interfaces

Abstract The adsorption of a polyacid, the alternated copolymer of maleic acid and ethylvinyl ether, from aqueous NaCl electrolyte solutions onto cellulose ester surfaces was studied as a function of molecular weight and solvent power by increasing the electrolyte concentration. The thickness of the adsorbed layer was derived by measuring the volume flow rate of solvent through narrow-pore cellulose ester filters coated with the adsorbed molecules. The apparent hydrodynamic thickness was interpreted on the basis of the characteristic layer width of an exponential distribution of beads in polymeric loops. We discuss the linear relation between the layer thickness and the intrinsic viscosity of the polymer solution in connection with previous published theories for interacting chains in contact with an adsorbing surface. We show that the solvent power most profoundly affects the layer thickness and we establish a well-defined correlation between the Flory polymer-solvent interaction parameter and the average layer thickness.

Polyelectrolyte induced aggregation of latex particles : influence of the structural relaxation of adsorbed macromolecules on the colloid aggregation mode

Abstract An experimental procedure allowing reproducible mixing of colloids and polymers in dilute suspension was used to study perikinetic aggregation resulting from polymer adsorption onto colloids. Changes of the temporal variation of mean characteristics of the colloid suspension (N0 and N2, zeroth and second moments of the aggregate size distribution) and of the shape of the colloid size distribution (cn(t) versus n, the number of elementary particles constituting an aggregate), were related to the slow structural relaxation of adsorbed macromolecules. At the time of initial contact of polymer and colloid, the adsorbed macromolecules had characteristics of solution macromolecules. Then, this coil-like conformation developed into a more flattened form. The parameter followed in the study was the relative polymer/latex concentration, which governed the initial surface coverage θ(0) and the rate of the temporal evolution of θ(t) toward equilibrium. At low polymer dosage, the destabilizing process at short times was interparticle bridging which developed with features of reaction-limited aggregation (RLA). After a given flocculation period depending on the polymer dosage, diffusion-limited aggregation (DLA) was found to describe the observed phenomenon. At higher polymer concentrations, “instantaneous” flocculation occurred when the polymer was continuously supplied to the stirred suspension. The aggregates formed during this RLA process thus became sterically protected against further agglomeration by the polymer layer on the surface and the suspension remained unchanged for days.

Kaolinite/humic acid interaction in the presence of aluminium ion

Abstract The present study concerned the modifications of the interfacial electrical characteristics of a kaolinite clay induced by adsorption of aluminium ions and humic substances. The strong interaction of aluminium ions with clay and humic acids strongly increased the humic acid adsorption. The extent and nature of the kaolinite surface modifications were determined to be functions of the initial doses of kaolinite and humic acids. Weakly complexed humic acids behaved as polyacids and highly complexed humic acids behaved as amphoteric polyelectrolytes. For the latter situation, the anisotropic distribution of positive and negative charges within the adsorbed layer was found to depend on the surface charge of the adsorbent. The ζ potential of kaolinite–aluminium ion–humate complexes depended in a complex manner on all parameters of the system including the initial pH, the area developed by the clay and the volume of the liquid phase, and evolved from negative to positive values with increasing degrees of the relative concentration of aluminium ions and humic acids.

An experimental study of kaolinite crystal edge-polyacrylamide interactions in dilute suspensions

Abstract With regard to the adsorption mechanism of a neutral hydrosoluble polymer on Na kaolinite, three crystallographically different surfaces are exposed by the platelike clay, each bearing a different type of chemical function. By the use of well-defined samples of different sizes, experimental evidence for edge adsorption of polyacrylamide on Na kaolinite was obtained in the pH range 4–10. No adsorption was detected on the silicate and aluminate cleavage faces. The limiting value of the adsorbed amount per unit area obtained for a well-dispersed suspension of nonaged clays permitted a description of the interfacial structure of the polymer hydrogen-bonded to the kaolinite clay.

Dynamics of latex aggregation. Modes of cluster growth

The kinetics of aggregation of polystyrene latices occuring in the presence of an electrolyte or poly(4‐vinylpyridine) in aqueous media are examined. The aggregate size distribution was recorded at different times by counting the flocs containing a given number of elementary colloids. The moment of order zero ( the total number of clusters per unit volume of the suspension), as well as the second moment of the size distribution are computed. In the presence of an excess of electrolyte or an optimum amount of polymer, a dynamic scaling function of the form cg (t)=t−2 F[g/t] and a narrow time invariant polydispersity is found. Under slow flocculation conditions, a large time dependent polydispersity sets in. Large aggregates then react preferentially with equally large ones. This behavior is discussed in term of a sticking probability , using the formalism of the reaction limited cluster–cluster aggregation.

POLYELECTROLYTIC NATURE OF HUMIC SUBSTANCES-ALUMINUM ION COMPLEXES. INTERFACIAL CHARACTERISTICS AND EFFECTS ON COLLOID STABILITY

Abstract The present study concerned the modifications of interfacial characteristics of aluminum oxide (hydrophilic substrate) and a polystyrene latex particle (less hydrophilic substrate) induced by adsorption of humic substances, polyacrylamide, hydrolyzed polyacrylamide or polyacrylic acid in the presence of aluminum ions and aimed at determination of the mode of action of humic substances in the colloid stabilization and destabilization. On the basis of electrophoretic mobility measurements, polyacrylamide and hydrolyzed polyacrylamide interact differently with the oxide and synthetic latex particles, whereas humic and polyacrylic acids similarly modify the surface characteristics of the two substrates. The relative concentration of acid groups and aluminum ions determined the electrokinetic potential of the adsorbed layer. The instability domain of the colloidal suspension was found to depend similarly on adsorption time and zeta potential. When the zeta potential ranged between +20 and −20 mV, the domain where positively and negatively charged chain elements coexist, the suspensions became unstable and flocculated rapidly. Excess of complexed positively charged chain segments present at small polymer dosage or dissociated negatively charged chain segments present at high polymer dosage promoted colloid stabilization.

Stability of polymer—clay suspensions. The polyacrylamide—sodium kaolinite system

Abstract Kaolinite is characterized by three exposed surfaces of different chemical composition. Adsorption measurements in dilute suspensions of radioactively labelled polyacrylamide as a function of surface area on several well-fractionated kaolinite samples, indicate that polyacrylamide is preferentially adsorbed on the platelet edge. The effect of polymer addition on the stability of the clay suspension was determined in concentrated medium. The stability and/or destabilisation behavior of the suspension in presence of polymer reflects the edge face—polymer association. The nature of adsorption forces and the mode of clay particle aggregation are discussed.

Polyacrylamide Na-kaolinite interactions: effect of electrolyte concentration on polymer adsorption

The variation in polyacrylamide adsorption on Na-kaolinite as a function of the electrolyte concentration of the clay suspension, was determined under three pH conditions, where the clays display varying charge characteristics. Interpretation of the results is based on two arguments: non-charged polyacrylamide adsorption is restricted to the edge faces of the colloidal platelets and hydrogen bonding between the amide groups of the polymer and the “isolated” hydroxyl sites of these faces is the mode of surface attachment. At constant pH, when Na-kaolinite bears charged surface groups, the polymer adsorption, which is related to the density of the anchoring sites, parallels the state of ionization of the edge surface. The mechanism by which the salt modifies the adsorption properties of the kaolinite in neutral medium is not established with certainty. Nevertheless, no polymer-clay association occurs if the surface is entirely uncharged. This result is in line with the fact that while hydroxyl groups are engaged in internal hydrogen bonds, they cannot hydrogen-bond to the polyacrylamide. Variations in the clay-polymer affinity are attributed to a modification of the interfacial structure of the adsorbed polymer, associated with changes in the surface density of the anchoring groups.