Biomedical subjects

E Pefferkorn

Publications and source records attributed to E Pefferkorn.

7 recordsLinked to original sources

PubMed2000

Fragmentation of Kaolinite Aggregates Induced by Ion-Exchange Reactions within Adsorbed Humic Acid Layers.

Mixing of humic acids and kaolinite clay suspended in aqueous solution containing aluminum ions promotes aggregation of the clay instantaneously. To this fast clay destabilization succeed reversible fragmentation processes which display varying kinetic ranges for which the changes in the average masses of fragments are determined. We show the rate of formation of fragments to decay with the mass i as i(-2) and the maximal rate of aggregate breakup to increase with the fragment mass as i(lambda), where lambda lies between 0.35 and 0.40. At short terms the rate of variation of the average masses of the fragments is lower than expected from the theory. Aggregate fragmentation is attributed to slow progressive modifications in the positive and negative charge distributions within the amphoteric humic acid adsorbed layer, which are induced by the high density of positive charge existing on the clay surface after aluminum ion adsorption. Kaolinite-aluminum ion-humic acid complexes aggregate by forming initially linear small portions of stacked platelets. These portions are connected together to form larger agglomerates of fractal dimension close to 2.5. Copyright 2000 Academic Press.

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PubMed1999

Polyacrylamide at Solid/Liquid Interfaces.

In the literature, the term polyacrylamide refers to neutral, hydrolyzed, and chemically modified polyacrylamide and in this sense polyacrylic acid may likewise be considered to consist of hydrolyzed acrylamide groups. These hydrosoluble polymers are employed in wide range of applications where the polymer characteristics at solid-liquid interfaces play an important role. The interaction with solid adsorbents is very complex since electrostatic, hydration, van der Waals, and other forces operate simultaneously. As in most applications the polyacrylamide is brought into contact with oxides, clays or soils, synthetic alumino-silicates, and aluminum oxide may be used as model adsorbents. Thus, with reference to the two systems polyacrylamide/alumino-silicate and polyacrylamide/aluminum oxide, the author attempts to define some of the interfacial processes affecting the adsorbed macromolecules and to describe certain interfacial characteristics of these complex systems. The main results are as follows. -Neutral polyacrylamide. Hydrogen bonding between acrylamide and neutral aluminol groups is responsible for adsorption and the amount of polymer adsorbed parallels the surface density of aluminol sites. At ambient temperature, the kinetics of layer formation is governed initially by the random deposition of solution macromolecules and finally by tunneling. Thermodynamic and kinetic considerations lead to a model where the majority of the adsorbed macromolecule share identical dynamic characteristics under equilibrium conditions. Departure from equilibrium induced by changing the polymer concentration in the supernatant phase greatly modifies the dynamic features of the adsorbed macromolecules. -Hydrolyzed polyacrylamide. Charge-charge interactions superimpose on the persistent effects of hydrogen bonding to alter the amount of polymer adsorbed and the adsorption kinetics. The strong affinity of hydrolyzed polyacrylamide for positively charged surfaces induces interfacial spreading of the adsorbed macromolecules and in some instances desorption of part of this population (overshoots). -Complexed polyacrylamide. The presence of positively charged, negatively charged, and neutral chain segments confers amphoteric character on the polymer chain. Strongly complexed polymers display fast adsorption, while initially weakly complexed polymers show delayed adsorption. In addition, under certain circumstances, the presence of hydrophobic microdomains confers an amphiphilic character on the macromolecules, which profoundly modifies the nature of the interaction forces. The specific interaction typical of neutral polyacrylamide progressively disappears with increasing hydrolysis until for polyacrylic acid the surface coverage corresponds to nonselective interfacial deposition. Extremely slow displacement of hydrophilic and hydrophobic groups are found to occur within the adsorbed layer. Despite the promising use of macromolecules in environmental strategies, a consideration of the complexity of the underlying interfacial phenomena points to the difficulties which may arise at short or long terms in such applications. Copyright 1999 Academic Press.

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PubMed1999

Adsorption of Telechelic Poly(ethylene oxide) on Colloids: Influence on Colloid Stability.

Adsorption of telechelic polymers (hydrophobically end-capped poly(ethylene oxide)) on charged colloidal particles modifies the characteristics of the electrical double layer. These interfaces were found to be able to protect colloids against aggregation or to induce colloid destabilization depending on the molecular weight and type of surface coating. Colloid stability is observed when the polymer layer is thinner than the electrical double layer and when the adsorbed polymer adopts a tethered brush structure. Aggregation of coated latex particles induced by formation of interparticle polymer links is observed when (i) the distance of the random walk of the dangling paraffinic end group is larger than the thickness of the electrical double layer and (ii) the adsorbed polymer brush adopts a loopy structure. Aggregation is attributed to adsorption of hydrophobic end chains on two different latex particles. Copyright 1999 Academic Press.

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PubMed1998

Perikinetic Aggregation Induced by Chromium Hydrolytic Polymer and Sol.

We investigated the kinetics of destabilization of polystyrene latex particles bearing sulfate surface groups in the presence of aged chromium polymer and oxide sol particles of relatively smaller size employing a particle counting technique. From the mass distribution curve, we derived the weight S(t) and number N(t) average masses of the aggregates. From the variation with time of these characteristics we determined the scaling exponents z and w of the kinetic laws which serve to characterize the aggregation mechanism induced by the inorganic polymer and sol. From the slope of the self-preserved reduced mass distribution, we determined the exponent tau, and found the relationship tau = 2 - (w/z) to be valid. The usual value of tau = 1.5 was obtained for experiments performed in the presence of very small amount of polymer. Deviation from this behavior was determined in the presence of increasing amounts of polymer and sol. At short and long terms, we determined tau to be equal to 1.24 and 1.37, respectively. This was attributed to the diffusion-limited aggregation accompanying the prevailing reaction-limited process, which was found to provide an unusual mass distribution frequency characterized by a hump in the monotonically decreasing curve. From the comparison between the aggregation rates and the apparent aggregate reactivity at short and long terms, we concluded that (i) the short-term aggregation involving aggregates of high apparent reactivity is highly reversible and (ii) the long-term aggregation involving aggregates of lower apparent reactivity is irreversible (or less reversible). Copyright 1998 Academic Press.

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PubMed1998

Quantitative Description of Nonhomogeneous Mass Distribution in Controlled Aggregation and Fragmentation of Hydrated Colloids.

The destabilization of hydrated colloidal latex particles in an aqueous electrolyte solution was found to produce aggregates of unusual mass frequencies. Analysis of the mass frequency curves revealed the presence of at least three aggregate populations growing at different rates and allowed determination of the variation with time of the number and weight average masses of these populations and their relative concentrations within the suspension. The fragmentation of aggregates of large mass was induced by decreasing the ionic strength of the suspending medium at constant pH. This instantaneous fragmentation and the subsequent slow breakup of the resulting fragments were investigated by following the variation with time of the suspension characteristics. The aggregation mechanism presented the features of a reversible diffusion-limited process. On the other hand, since electrical interactions were of short range in the concentrated electrolyte medium and hydration forces could be considered to oppose attractive forces, it was possible to attribute the formation of three aggregate populations at least in part to the unusually low energy of the attractive forces involved in aggregate formation. Model structures were designed for each population: aggregates containing only linear interparticle bonds were assigned to population 1, aggregates containing double or triple interparticle contact points to population 2, and aggregates containing multiple contact points or circular links to population 3. Thus, the combined effects of internal stability and collision efficiency, both dependent on aggregate structure, could be assumed to be responsible for the well-differentiated growth of three distinct populations. The preservation of these populations during fragmentation was attributed to their different internal stabilities. In fact, the structure of large aggregates appeared to be very complex since their fragments could be ranked among different populations. Finally, the absence of self-similarity of the mass frequencies determined at different times led us to suppose that the aggregation mechanism would escape analysis on the basis of the mean field approach using Smoluchowski's equation with the usual form of the kernel. Copyright 1998 Academic Press.

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PubMed1996

Monte Carlo Simulation of Controlled Colloid Growth by Homo- and Heterocoagulation in Two Dimensions.

The kinetics of two dimensional homo- and heterocoagulation of particles of equal size were investigated by Monte Carlo simulation using a square lattice and particles with a functionality of four. In both processes, a size-dependent cluster aggregation ability gamma allowed simulation of diffusional and reactional limited processes. In heterocoagulation, determination of the effects of the relative concentration x of the particles A and B and the cluster aggregation ability on the aggregation rate provided information concerning the influence of the frequency of unsuccessful encounters between identical particles on the temporal increases in the weight S(t, x, gamma) and number N(t, x, gamma) average sizes of the clusters. Dynamic scaling exponents for the temporal variations of the average cluster sizes were found to be linear or power law functions of the composition x.

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PubMed1988

Steric protection of a modified glass surface with adsorbed polyacrylamide: interactions with blood proteins.

Glass beads chemically modified with aluminol (A1OH) groups were coated with a monolayer of high-molecular-mass polyacrylamide at different temperatures and exposed to 125I-labelled albumin and fibrinogen solutions for 1 h. No adsorption of albumin was observed, whereas fibrinogen adsorption increased as a function of temperature. These observations were interpreted in terms of the density and stability of the adsorbed polymer layer. At 25 degrees C, where fibrinogen adsorption was already important, it was shown, using 3H-labelled polyacrylamide, that fibrinogen partly displaced the polymer from its adsorption sites.

Acrylic Resins↗