Donald H. Napper

Depletion stabilization and depletion flocculation

Abstract An approximate theory of depletion stabilization and depletion flocculation (i.e., stabilization and flocculation of colloidal particles by polymers in free solution) is presented. This is based upon the rotational isomeric state-Monte Carlo procedures for calculating the conformation of macromolecules between two particles and the Flory-Huggins theory of polymer solutions. It is shown that reasonable agreement between theory and experiment can be achieved for the critical volume fractions of polymer required to induce both depletion flocculation and depletion stabilization. The presence of adsorbed or attached polymer at the interface may have a significant effect on the onset of both flocculation and stabilization; this is especially true if the molecular weight of the attached chains is comparable to or greater than that of the free polymer. The effects of the steric chains, however, are reduced if the free polymer is significantly higher in molecular weight. Free energy minimization procedures are then necessary to calculate the polymer distribution between the plates. The agreement obtained between theory and experiment suggests that the theory incorporates the dominant factors that control depletion flocculation and stabilization.

Flocculation studies of sterically stabilized dispersions

Abstract The incipient flocculation properties of aqueous polymer dispersions, which are stabilized solely by poly(ethylene oxide) moieties, have been investigated. Flocculation was induced by decreasing the solvency of the dispersion medium for the stabilizing chains. The influence of such factors as the anchor polymer, the disperse phase, the particle size, the surface coverage, and the molecular weight of the stabilizing moieties was examined. Flocculation occurs in dispersion media which are either Θ-solvents for the stabilizing moieties or of better solvency than Θ-solvents. Both aqueous and nonaqueous dispersions, which are stabilized solely by amphipathic nonionic macromolecules, exhibit remarkably parallel flocculation phenomenology. The stability of both types of dispersions is therefore amenable to a common theoretical approach. However, the phase diagrams for aqueous and nonaqueous polymer solutions may be very different. Measurements of the thermodynamic parameters which govern stability demonstrate that, whereas entropic effects may govern stability in nonaqueous systems, enthalpic interactions provide the stabilization mechanism in the aqueous dispersion studied. A general scheme which distinguishes seventeen different thermodynamic situations in the stability-flocculation domain is presented. Stability is shown to originate in entropic effects, enthalpic interactions, or a combination of both.

Studies of the steric stabilization of colloidal particles

Abstract The incipient flocculation of sterically stabilized dispersions has been investigated. It was found that when a steric stabilizing barrier was supplemented by an electrostatic barrier, no change in the incipient flocculation point was evident at high ionic strengths. Dispersions sterically stabilized by commercial Pluronic nonionic surfactants, which are based on poly(ethylene oxide), exhibited incipient flocculation behavior similar to model dispersions stabilized by poly (ethylene) oxide. It is shown that dispersions which undergo flocculation on heating must be enthalpically stabilized, at least immediately below the incipient flocculation temperature; conversely, those that flocculate on cooling are entropically stabilized, at least just above the critical flocculation temperature (c.f.t.). Thus poly(ethylene oxide) is shown to function as an enthalpic stabilizer in water and as an entropic stabilizer in methanol. Experiments with strongly anchored steric stabilizers and multivalent cations demonstrate unequivocally the existence of the phenomenon of displacement flocculation at neutral pH. The order of flocculation effectiveness of cations (monovalent > divalent > trivalent) towards sterically stabilized dispersions is the reverse of that found in electrostatic stabilization, if displacement flocculation is absent. Displacement flocculation is promoted thermodynamically by weak anchoring, and by attractive electrostatic interactions between the particle surface and the displacing hydrolyzed species. It is retarded kinetically by increasing the molecular weight of the surf actant.

Steric stabilization and the Hofmeister series

Abstract The relevance of ion hydration and the Hofmeister series to the flocculation of poly (vinyl acetate) hydrosols, stabilized solely by poly(ethylene oxide) chains, has been examined. For these enthalpically stabilized dispersions, the flocculation potency of an electrolyte is determined by its ability to convert water into a theta-solvent for the stabilizing moieties. The order of decreasing flocculation effectiveness for the anions studied (SO4= > Cl− > NO3− > CNS− > Br− > I−) parallels that of the Hofmeister series. The order for cations (Rb+ = K+ = Na+ = Cs+ > NH4+ = Sr++ > Li+ = Ca++ = Ba++ = Mg++) is in direct conflict with the postulate that the more highly hydrated ions are the more effective flocculants. In general, monatomic anions and cations are less potent flocculants if their effective primary hydration radius (region A) is large. The results have been interpreted in terms of the influence which the positively hydrated ions exert on the structure of the water in their immediate neighborhood (region B), rather than simply in terms of dehydration of the stabilizing species. Preliminary measurements of the entropy and enthalpy of dilution parameters for poly (ethylene oxide) in electrolyte solutions support this interpretation.

The direct determination of kinetic-parameters in emulsion polymerization systems

Because of the complexity of emulsion polymerization systems, unequivocal mechanistic deductions on data on one particular aspect of the process (e. g. , particle nucleation) perforce require reliable rate parameters from other aspects (e. g. , polymerization within the particles during and after the cessation of nucleation). It is the primary aim of this work to review techniques whereby unambigous values of the appropriate rate laws and rate parameters can be obtained from experiment. The relation of this empirically determined rate law to the competing theoretical models may then properly be considered.

Volume phase transitions of poly(N-isopropylacrylamide) latex particles in mixed water-N,N-dimethylformamide solutions

Abstract Volume phase transitions of poly(N-isopropylacrylamide) (PNIPAM) latex particles in mixed water-N,N-dimethylformamide (DMF) solutions were studied at different temperatures and crosslink densities using dynamic light scattering. A collapse-swelling behaviour was observed on the addition of N,N-dimethylformamide to an aqueous dispersion medium. The degree of swelling was found to be dependent on the temperature and crosslink density. The transition between the collapsed state and the swollen state was continuous. The cononsolvency of water and DMF for PNIPAM latex particles has been interpreted in terms of the changes in the interactions between their components.

Enhanced steric stabilization

Abstract An extensive series of experiments has previously demonstrated that for model sterically stabilized dispersions, 0-solvents for the stabilizing chains in free solution represent the limit of colloid stability. It has now been shown that by anchoring the stabilizing chains to the particle surfaces at many points along the chains, the pattern of flocculation behavior may be profoundly altered. Polystyrene latices stabilized by poly(ethylene oxide) have been prepared that are stable in dispersion media with solvency for the stabilizing moieties markedly worse than 0-solvents for the chains in free solution. Multipoint anchoring in these systems results from the hydrogen bonding between the ether oxygens of the stabilizer and surface carboxylic acid groups. The enhancement of steric stabilization is a function of the pH and the molecular weight of the stabilizing chains. It originates in the perturbation of the conformation of the stabilizing macromolecules that renders the free solution properties of the chains no longer relevant.

Analytical theories of the steric stabilization of colloidal dispersions

Analytical theories are developed for the steric stabilization of colloidal spheres and flat plates by monodisperse tails. It is proposed that the reason for the failure of previous theories to predict the correct distance dependence of the repulsion resides in their neglect of lateral interactions between adjacent stabilizing chains. Theories are therefore formulated that implicitly allow for these lateral interactions. All that need be specified is the mathematical form of the relevant segment density distribution functions. Two models are examined in detail: a constant segment density model and a combined constant segment density and near symmetrical gaussian model. The latter gives the better agreement with experimental results. Both models, however, describe the distance dependence of the repulsion reasonably well over quite a wide range of separations.

The thermodynamic limit to the flocculation stability of sterically stabilized emulsions

Abstract In was found experimentally that the thermodynamic limit to the stability of sterically stabilized emulsions could be achieved if the stabilizing polymer chains were strongly attached to the emulsion droplet. This was accomplished by the use of anchor polymers, which were required to exceed some critical minimum size and to exhibit solubility in the disperse phase. When the stabilizing polymer chains were strongly anchored, the pattern of flocculation behavior for both water-in-oil and oil-in-water emulsions closely resembled that observed for sterically stabilized latices. This confirms a recent prediction by Phillips of their equivalence. It also brings emulsion stability within the compass of recent theories of steric stabilization.

Light scattering studies of poly(N-isopropylacrylamide) microgel particles in mixed water-acetic acid solvents

Properties of poly(N-isopropylacrylamide) (PNIPAM) microgel particles were examined in mixtures of water and acetic acid at 20°C using dynamic light scattering in order to investigate the cononsolvency phenomenon. It was found that the cononsolvency exhibited by the PNIPAM-water-acetic acid ternary system results in five regions in response to changes in solvent composition. Upon the addition of acetic acid to the aqueous dispersion medium, the regions appeared successively: volume change from a swollen to a collapsed state, unstable microgel aggregates (in water rich range), precipitation, unstable microgel aggregates (in acetic acid rich range), and swelling. The volume change from swollen to collapsed states starts before the phase-separation composition is achieved. Both volume phase transition and swelling are continuous but undergo in a sharp way. In a very narrow range of solvent composition, the microgel aggregates formed are unstable, while the immiscible phase was found to span a wide range. The degree of swelling is dependent on the crosslinker concentration in the solutions of very higher acetic acid content. The cononsolvency of the mixed water-acetic acid solutions for PNIPAM microgel particles is not only involved in changes of the interactions between their components but also in the partial neutralisation of surface charges of the PNIPAM microgel particles.