G.J. Fleer

Kinetics of polymer adsorption in stagnation point flow

Abstract The kinetics of adsorption of poly(ethylene oxide) from water onto silica in stagnation point flow are studied using a reflectometric technique. It is shown that with reflectometry an absolute and continuous determination of the adsorbed mass per unit area can be obtained. For the stagnation point flow, the maximum rate of mass transfer to the surface is theoretically calculated. This rate is compared to the observed adsorption rate and it is concluded that mass transfer is the rate-determining step up to 75% or more saturation, depending on the molecular weight. Plateau adsorbed amounts obtained are in good agreement with those found by others. The molecular weight dependence of the diffusion constant agrees with scaling theory and the plateau adsorbed mass is in accordance with the theory for polymer adsorption from good solvents.

Reflectometry as a tool for adsorption studies

Abstract We discuss optical reflectometry as a tool for studying adsorption from dilute solutions. In particular, we elaborate on the kind of reflectometry where the reflecting substrate carries a thin dielectric film on which the adsorption takes place. The role of this film is to enhance the sensitivity of the method by producing a suitable phase shift in the reflected beam. The experimental set-up, calibration procedure and some conditions for successful determination of the surface excess are described. By means of an optical model we then calculate sensitivity and linearity of the technique for various solvents and adsorbents and we determine the optimum thickness of the dielectric film and the most appropriate angle of incidence. We find that, provided such optimum conditions are chosen, the technique allows sensitive and accurate detection of the adsorbed mass per unit area in all cases where the refractive index of the adsorbate film is sufficiently different from that of the solvent. For a typical example (organic solutes in water), adsorbed amounts as low as 0.01 mg m 2 can be detected.

Adsorption of weak polyelectrolytes from aqueous solution

Abstract A theoretical model for the adsorption of weak polyelectrolytes from aqueous solution is developed by extending recent theories for uncharged polymers at interfaces with electrostatic contributions. The segment concentration profile is found from the optimal compromise of several factors, such as the conformational entropy of the chains, the configurational entropy of the solvent, the adsorption energy, the solvent quality, the electrostatic repulsion between the segments, and electrostatic interactions due to the surface charge. The degree of dissociation of the polyelectrolyte charges and the dielectric permittivity are allowed to vary with the distance from the surface. Some typical results are presented for the adsorption as a function of polyelectrolyte concentration, solvent quality, pH, and surface charge. A simple equation is derived that relates the critical value of the surface charge σ cr 0 above (or below) which no adsorption takes place, to the nonelectrostatic adsorption energy χ s . As far as comparison with experimental data is possible, the predictions of the model are at least semiquantitatively in agreement with experiment.

Displacement of polymers. I. Theory. Segmental adsorption energy from polymer desorption in binary solvents

Abstract The complete removal of high molecular weight polymers from adsorbent surfaces is proved to be possible. A novel method to desorb polymers through displacement by a low molecular weight component, a so called displacer, is shown. A critical displacer concentration may be defined, above which the polymer desorption is complete. This desorption method may be important for a number of practical applications. Theoretical analysis is employed to determine which parameters govern the desorption process. A simplified model leads to an analytical expression for the critical displacer concentration. Additional information on the displacement process (i.e., effects of polymer concentration, molecular weight, and solvent quality) is obtained from more elaborate model calculations. It is shown that it is possible to determine the segmental adsorption energy if displacement data are combined with adsorption data for the corresponding displacer, provided solvency effects are properly taken into account.

Displacement of polymers. II. Experiment. Determination of segmental adsorption energy of poly(vinylpyrrolidone) on silica

The displacement of poly(vinylpyrrolidone) (PVP) from silica in two different solvents (water and dioxane) by a number of low molecular weight organic displacers was studied. The experimental data compare favorably with a theory developed in a previous paper. Using this theory we determined the segmental adsorption energy for PVP to be 4 kT, both in dioxane.and in water. This result is in good agreement with inferences from calorimetric and spectroscopic-bound fraction studies.

The Adsorption of Poly (vinyl-pyrrolidone) onto Silica. I. Adsorbed Amount

The adsorption of the flexible, linear, and nonionic homopolymer poly(vinyl pyrrolidone) from water and from 1,4-dioxane onto pyrogenic silica was studied. Results are reported for the adsorbed amount as a function of adsorption time, molecular mass, and molecular mass distribution (polydispersity). It is found that the adsorption of fractionated samples can be qualitatively explained by the recent theory of Scheutjens and Fleer. However, the influence of the solvent type is larger than predicted by this theory, and an extension of the model to account for this influence is suggested. The polydispersity effects encountered in adsorption isotherms are satisfactorily accounted for by a theory published by Cohen Stuart, Scheutjens, and Fleer.

Isolation and physical characterization of an exocellular polysaccharide.

The physical properties of a polysaccharide produced by the lactic acid bacterium Lactococcus lactis subsp. cremoris strain NIZO B40 were investigated. Separation of the polysaccharide from most low molar mass compounds in the culture broth was performed by filtration processes. Residual proteins and peptides were removed by washing with a mixture of formic acid, ethanol, and water. Gel permeation chromatography (GPC) was used to size fractionate the polysaccharide. Fractions were analyzed by multiangle static light scattering in aqueous 0.10 M NaNO3 solutions from which a number- (Mn) and weight-averaged (Mw) molar mass of (1.47 +/- 0.06).10(3) and (1.62 +/- 0.07).10(3) kg/mol, respectively, were calculated so that Mw/Mn approximately 1.13. The number-averaged radius of gyration was found to be 86 +/- 2 nm. From dynamic light scattering an apparent z-averaged diffusion coefficient was obtained. Upon correcting for the contributions from intramolecular modes by extrapolating to zero wave vector a hydrodynamic radius of 86 +/- 4 nm was calculated. Theoretical models for random coil polymers show that this z-averaged hydrodynamic radius is consistent with the z-averaged radius of gyration, 97 +/- 3 nm, as found with GPC.

Theoretical progress in polymer adsorption, steric stabilization and flocculation

Abstract The range of validity of scaling and mean field models for polymers at interfaces is discussed. According to a recent analysis by Schaefer, mean field theories describe the properties of polymers in solution correctly over a wider range of concentration and solvency than does scaling. Only for flexible polymers in very good solvents at concentrations below a few percent are scaling methods preferred. In polymer adsorption, high segment densities occur and therefore mean field theories are more appropriate than scaling. The most detailed mean field theory for polymers at interfaces has been proposed by Scheutjens and Fleer. The distribution of segments as a function of ranking number and, consequently, end effects (tails) are taken fully into account. Several results are given for adsorbing and nonadsorbing polymers at a surface and for the effect of polymers on colloidal stability (bridging, steric stabilization and depletion flocculation). In some cases, a comparison with the outcome of other theories is made. Experimental results, available so far, corroborate the main predictions.

Kinetic aspects of polymer bridging: equilibrium flocculation and nonequilibrium flocculation.

Abstract We carried out a detailed study of the flocculation of polystyrene latices by poly (ethylene oxide) with a specially designed single particle optical sizer. With this instrument the aggregate size distribution of a flocculating system can be determined very accurately. We found clear evidence for two types of flocculation mechanisms. In the first, which we denote as equilibrium flocculation, the time of reconformation is small with respect to polymer attachment and particle collisions. Consequently, the adsorbed layer is relaxed before two particles meet. In this case the flocculation process obeys normal second-order kinetics. On the other hand, in nonequilibrium flocculation the adsorbed polymer is still extended when particles collide. The flocculation is now more efficient and the overall process follows quite different kinetics. The time needed for the initially adsorbed polymer to flatten within twice the double layer thickness was found to be, for long polymers and low salt content, of the order of several seconds. We discuss the conditions under which these two mechanisms occur as a function of polymer molecular weight, salt concentration and particle concentration. In some cases both equilibrium and nonequilibrium flocculation take place simultaneously.

Adsorption of poly(vinyl pyrrolidone) on silica. II. The fraction of bound segments, measured by a variety of techniques

The conformation of poly(vinyl pyrrolidone) (PVP) adsorbed on pyrogenic silica was studied by measuring the fraction of segments bound directly to the solid/liquid interface. Two solvents, water and 1,4-dioxane, were used. Results obtained with infrared spectroscopy and with microcalorimetry are compared with IR, NMR, and EPR data from the literature. The experimental results are critically discussed in the context of recent theoretical predictions. It turns out that bound fractions derived from infrared spectroscopy agree rather poorly with theoretical data, whereas magnetic spectroscopy (NMR, EPR) seems a rather reliable way to determine the amount of polymer in trains. Microcalorimetry also yields reasonable results, but only for relatively short polymer chains. Explanations for the observed discrepancies are presented. With respect to the most probable conformation of the adsorbed molecules, it is concluded that at low adsorbed amounts the PVP molecules are in both solvents nearly flatly adsorbed with a high fraction of segments immobilized on the surface. As the adsorbed amount increases, loops and tails develop, and bound fractions of about 0.50 are reached at high adsorbed amount. The theoretical prediction that the bound fraction for a given polymer—solvent substrate combination is almost a unique function of the adsorbed amount, independent of molecular weight and concentration, is also supported by the present observations.