Philippe Déjardin

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.

Streaming potential measurements related to fibrinogen adsorption onto silica capillaries

Abstract Fibrinogen adsorption from solutions onto silica capillaries was studied by the streaming potential technique under varying conditions of fibrinogen concentration, pH and ionic strength. It was found that a silica surface covered by adsorption from fibrinogen solutions at the isoelectric point of the protein exhibited a practically zero value of streaming potential even at the lowest ionic strength studied. This observation suggests some charge regulation process in the protein layer, probably with a contribution from the low dielectric constant of the protein screening the charges of the support. Assuming a transport-controlled process, the initial relative variation of ξ potential versus pH suggests that, compared with the isoelectric point situation, the average area per molecule on the surface is higher when molecules are adsorbed below their isoelectric point in the presence of an oppositely charged support, while conformation changes and/or electrostatic repulsion are less strong above the isoelectric point when molecules and surface bear charges of the same sign. This phenomenon could be correlated with a weakened globular structure of the molecule at lower pH.

Influence of a preadsorbed terpolymer on human platelet accumulation, fibrinogen adsorption, and ex vivo blood activation in hemodialysis hollow fibers

Results are presented on kinetics of platelet accumulation in charged polyacrylonitrile (AN69) hollow fibers by continuous data recording under flow conditions (wall shear rate 108-1050 s-1), using suspensions of washed 111In-labeled human platelets in Tyrodes-albumin buffer, containing washed red blood cells (0-40%). Preadsorption of a terpolymer of acrylonitrile, poly(ethyleneoxide) methacrylate and trimethylaminoethyl chloride methacrylate leads to very efficient passivation with respect to platelet accumulation and fibrinogen adsorption. In human ex vivo tests, evaluation of complement peptide C3a, platelet beta-thromboglobulin, leucocyte-polymorphonuclear neutrophile elastase and fibrinopeptide A shows no detectable activation. Furthermore, preadsorption appears to result in simultaneous improvement in hemocompatibility of the blood lines leading to and from the dialysis module. This single pretreatment of dialysis membranes should allow injection of lower doses of anticoagulant to patients submitted to hemodialysis.

Coupling between bulk diffusion/convection and interfacial adsorption/desorption: The linear gradient approximation with Langmuir excluded surface effect

Abstract We describe the kinetics of adsorption when stirring of the bulk solution occurs. It is assumed that the range of the solute/surface interactions is significantly smaller than the size of the macromolecular or colloid solute molecules. Using the approximation of a quasi steady-state regime near the interface, an analytical solution relating the adsorption rate to the various parameters is proposed at initial times. We compare this solution to the numerical solution of a model which takes into account a Langmuir excluded surface effect. We apply finally the results to the problem of convective diffusion mechanism with laminar flow.

Hydrophilic graft polyacrylonitrile copolymers: 1. Synthesis and characterization in dilute solution

Abstract Radical copolymerization of methoxy-poly(ethylene glycol) monomethacrylate (A(8); M n ∼470 ), a water-soluble macromonomer, with acrylonitrile (B) was studied in homogeneous dimethylsulphoxide solution at 60°C in the following conditions: [A(8)]0+[B]0=0.5–1.5 mol l−1; molar fraction of A(8) in the monomer feed, f0A(8)⩽0.75. The process obeys the terminal unit model (rA(8)=1.35, rB=0.38), and readily yields graft copolymers over a broad range of composition and molecular weight (Mw∼(0.5–9.0)×105). The reactivity ratio of a macromonomer of higher molecular weight (A(21); M n ∼10 3 ) may be estimated to be about 0.66, a significantly larger value than those of the shorter-chain (A(8)) and ordinary alkyl methacrylates. Light scattering measurements in dimethylformamide (DMF) solution at room temperature only lead to apparent values of the second viral coefficient A2 and of the radius of gyration ( s 2 z ) 0.5 for these heterogeneous copolymers. Although DMF is a poorer solvent for the polyether than for polyacrylonitrile and in spite of the branched structure, the apparent dimensions of the copolymers seem only slightly lower than those of polyacrylonitrile of the same molecular weight.

Simulations of adsorption from flowing solutions in a slit or capillary with a finite adsorption constant at the walls

Abstract Simulations of adsorption from flowing solutions of a solute with diffusion coefficient 2.0 × 10 −7 cm 2 s −1 , typical for fibrinogen, were performed for a slit or capillary of dimension 0.2–10 mm and computational results compared to the approximate relation k −1 = k a −1 + k Lev −1 , where k is the apparent kinetic constant, k a is the kinetic adsorption constant at the interface and k Lev is the kinetic constant relative to a purely transport controlled process (Leveque model). The variations of k −1 with distance from the slit or capillary entrance were examined. When approaching “chemical” control, a good estimation of k a could be obtained whereas the diffusion coefficient was overestimated by about 30%, while near the Leveque regime we obtained, as expected, a good evaluation of the diffusion coefficient.

Influence of polymer preadsorption on the performance of an electrochemical serum biosensor

Abstract We compare the performance of platinum and glassy carbon electrodes with and without polymer pretreatment, in detecting the presence of H2O2 after passage of blood serum through an enzymatic minireactor. The simple pretreatment is performed by physisorption from a dilute aqueous solution of the polymer, leading to a diffuse layer of polymer segments which protects the electrode from large entities such as proteins, while allowing the small electroactive species to reach the electrode surface. Three polymers, all with a high polyoxyethylene content, were tested and the results were evaluated. The signal on starting sequences of serum injections is less high in the pretreated electrode than in the untreated electrode. However, the loss of signal on increasing the number of injections is lower and leads to a more stable response and finally, after 20 injections, to a higher signal than at untreated electrodes.

Transient adsorption of a tri(ethyleneglycol) polyacrylate on a plasmapheresis membrane

We study the adsorption of polyacrylate of tri(ethyleneglycol) in water on a plasmapheresis membrane, whose main constituent is a copolymer of acrylonitrile and methylmethacrylate. The amount of adsorbed polymer has a strong dependence on pH. Adsorbance is high at low pH and very low at high pH, while in the intermediate range the interfacial concentration Γ is not a monotonic function of the solution concentrationC. Viscosimetry andUV adsorbance measurements in solution suggest the existence of plurimolecular structures, aggregates, or micelles. We propose a kinetic model, where these latter objects act as detergents at highC and, therefore, contribute to decreasing the interfacial concentration Γ when increasingC.

Adsorption of an amphipatic copolymer on a kidney dialysis membrane

We have studied the adsorption of a copolymer of acrylonitrile and methacrylate of nonaethyleneglycol on a kidney dialysis membrane, which contains mostly nitrile groups. The kinetics study reveals at first a rapid increase of the adsorbance, followed by slow variations suggesting rearrangements at the interface. The isotherms do not present a true plateau at high concentration. The high adsorbance observed at low pH or high salt concentration could be related to the ability of polyethyleneoxide to form a complex with cations.