Pranab Bagchi

Effect of pH on the adsorption of immunoglobulin G on anionic poly(vinyltoluene) model latex particles

Abstract The adsorption of a set of goat and rabbit immunoglobulin Gs (IgG) on the surface of a set of well-characterized, pH-independent, negatively charged poly(vinyltoluene) model latices was studied as a function of pH. Highest adsorption occurred at the zero-point of charge (ZPC) of the IgG molecules at pH 7.8 and decreased linearly with both an increase and a decrease of pH. This indicated that the major driving force for the adsorption of the IgG molecules is due to hydrophobic interaction rather than to charge interaction between the protein molecules and the latex particles. Consideration of saturation area per adsorbed IgG molecule at different pH values led to the conclusion that ionization-induced conformational changes in the IgG molecule determine the extent of IgG adsorption. Based on this, a model is proposed for the conformations of adsorbed IgG molecules on latex surfaces at the ZPC and away from it. In accordance with this model, the saturation area per molecule of adsorbed IgG as a function of pH correlates quantitatively with an end-on arm-collapsed “Y” conformation at the ZPC of the protein, and an end-on arm-extended “T” conformation at pH values of 4.0 and 10.0, where the IgG molecule is fully charged, cationically and anionically, respectively. Results of intrinsic viscosity measurements of rabbit IgG solutions as a function of pH, at constant ionic strength, seem to support the proposed configuration changes of the IgG molecule as a function of pH. Although the adsorption of IgG molecules was irreversible at fixed pH values, it seemed to be quasi-reversible with pH cycling. A mechanism for this process is also suggested, based upon conformational considerations.

Theory of stabilization of spherical colloidal particles by nonionic polymers

Abstract In the stabilization of suspensions by polymers, at low surface coverage when two particles approach each other at close distances, mixing of the adsorption layers takes place with the displacement of some of the solvents from the adsorption layers. The free energy change for such a process has been calculated to predict the stabilizing potentials as a function of the distances of separation between the two particles. Such a mechanism has been termed the mixing mechanism. At high surface coverages, however, the adsorption layers will be dense and mixing would be improbable. It is proposed that in such a situation, on the close approach of the two particles, the polymer layers will be compressed with the exclusion of solvent from the adsorption layers. This mechanism has been termed the denting mechanism. Potentials due to such a process as a function of the distances of separation between the two particles have been calculated. Calculations based on these mechanisms were successfully used to predict the stability behavior of the AgI-poly(vinyl alcohol)-water system, which was experimentally investigated by Fleer et al. as a function of the surface coverage. The prediction of the stability of latex suspensions as a function of particle size and solvent-polymer interactions was also found to be in agreement with the experimental observations of Napper.

Preparation of model poly(vinyl toluene) latices and characterization of their surface charge by titration and electrophoresis

Abstract A set of monodisperse model poly(vinyl toluene) latices was prepared in the presence of sodium dodecyl sulfate as an emulsifier, with particle radii varying from 0.04 to 0.08 μm and with a constant surface charge density of about 0.5 to 0.6 μC/cm 2 arising solely from the complete and pH-independent dissociation of strongly attached sulfate groups. Surface tension measurements indicated that the adopted purification procedure removed all the emulsifier from the continuous phase. A quick and novel microelectrophoretic technique used for the determination of the electrokinetic characteristics of these very small latices involved the determination of the electrophoretic mobilities of a strongly scattering substrate covered with a layer of adsorbed latices, as has been used for biomacromolecules. Results of such experiments were compared with those obtained from extremely time-consuming conventional moving-boundary experiments. Determinations of ψ 0 ( eψ 0 kT ∼ 2 ) of these latices by ion exchange and subsequent titration and of ψ ζ from electrophoresis show that for these model latices ψ 0 ≈ ψ ζ , indicating the absence of a Stern layer. The low surface potential and the absence of a Stern layer indicate that the latex systems can be used as ideal Gouy-Chapman model colloids.

Test for particle size dependence in the theory of slow flocculation at a secondary minimum

SummaryThe theory of slow flocculation at secondary minima proposed earlier (9) has been tested for particle size dependence against the recently published experimental stability data ofOttewill andWalker (5) for polystyrene latex dispersions in water and in the presence ofn-dodecyl hexaoxyethylene glycol monoether. The agreement between theory and experiments seems excellent, which suggests that the proposed theory may have general validity, but additional data are necessary to completely substantiate this.ZusammenfassungDie Theorie über langsame Flockung am sekundären Minimum (9) wurde an der Teilchenabhängigkeit überprüft. Dazu wurden jüngst veröffentlichte experimentelle Stabilitätsergebnisse vonOttewill undWalker (5) an Dispersionen von Polystyrollatices in Wasser und in der Gegenwart vonn-Dodecyl-hexa-oxyläthylen glycolmonoäther verwendet. Die Übereinstimmung zwischen Theorie und Experiment erscheint ausgezeichnet, was allgemeine Gültigkeit der vorgeschlagenen Theorie andeutet. Jedoch sind weitere experimentelle Ergebnisse nötig, um dies vollständig zu bestätigen.

On the assumption of a step function for the adsorption-layer thickness in the nonionic stabilization of dispersions

Abstract The stability of poly (styrene) glycol (PS) latex dispersions in water and in the presence of n -dodecyl glycolhexaoxyethylene monoether (C 12 E 8 ), as investigated by Ottewill and Walker, has been interpreted quantitatively from the depth of the secondary minimum, assuming that a steep nonionic repulsion potential gives rise to a secondary minimum at a distance of separation of the particle surfaces equaling twice the experimentally determined adsorption-layer thickness. In view of Hesselinks tail and loop size distributions for the case of longchain molecules and polymers, respectively, such correlations for the PS latex-C 12 E 6 -water system and the AgI-poly(vinyl alcohol)-water system (as observed previously), suggest either that such distributions could be quite narrow or that any hydrodynamically measured adsorption layer thickness is a good indication of half the minimum distance of approach between two particles with adsorption layers. Consequently, the assumption of a step function for the adsorption-layer thickness seems adequate for the interpretation of the stabilization of lyophobic dispersion by nonionic additives.

Nonionic denting and mixing potentials between two flat plates

Abstract The nonionic denting and mixing potentials have been defined in a previous publication and have been used to predict the stability of nonionic-polymer-stabilized dispersions of spherical particles. In this paper, models for the calculation of such mixing and denting potentials between nonionic polymer-coated flat plates are proposed.

Poly(Vinyl Toluene) Model Latices

A set of monodisperse model poly(vinyl toluene) latices was prepared in the presence of sodium dodecyl sulfate as an emulsifier, with particle radii varying from 0.04 to 0.08 μm and with a constant surface charge density of about 0.5 to 0.6 μC/cm2 arising from the complete and pH-independent dissociation of strongly attached sulfate groups. Surface tension measurements indicated that the adopted purification procedure removed all the emulsifier from the continuous phase. A quick and novel microelectrophoretic technique used for the determination of the electrokinetic characteristics of these very small latices involved the determination of the electrophoretic mobilities of a strongly scattering substrate covered with a layer of adsorbed latices, as has been used for biomacromolecules. Results of such experiments were compared with those obtained from extremely time-consuming conventional moving-boundary experiments. Determinations of ψo (eψo /KT ~ 2) of these latices by ion exchange and subsequent titration and of ψζ from electrophoresis show that for these model latices ψ0 = ψζ, indicating the absence of a Stern layer. The low surface potential and the absence of a Stern layer indicate that the latex systems can be used as ideal Gouy-Chapman model colloids.