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Adsorption of polymers on mineral surfaces allowing colloidal stability have widespread applications in industrial processes. The binding mechanism has been quite well described on oxide surfaces. Mainly in terms of hydrogen bonds and electrostatic interactions between charged sites and polymer segments. This phenomenon has been modelized and the influence of pH. Ionic strentgh, and molecular weight can be calculated or predicted. In the case of sparingly soluble substrates such as BaSO4, CaCO3 or CaSO4, several problems arise : the difficulty for the identification of surface sites and the interference of ions coming from the material's solubility. In the case of calcite, the solubility imposes dissolved calcium ions in solution which could complex the polyelectrolyte and reduce its solubility. For that purpose, we have measured the binding energy using microcalorimetry. Microcalorimetric measurements have shown that the adsorption enthalpy is weakly enclothermic: about + 2 kj/mol. Interestingly, this value is very closed to that of calcium complexation with PANa. It is suggested that the driving force for adsorption is the net gain in entropy of the system. The microcalorimetric adsorption isotherm does not show any evidence for a strongly exothermic interaction between positive edges and negative segment of the polyion. Practically, in most cases, adsorption of polymers is calculated from the decrease of its concentration in the solution after separation of the solid by centrifugation. This procedure does not discriminate therefore between real adsorption and phase separation. To answer the question, we have performed adsorption experiments using a dialysis membrane to separate the solid particles from the solution. It has been established that in some circumstances, depending on the relative amount of calcite, calcium ions and polyelectrolyte, precipitation takes place rather than adsorption. This is especially the case at low polymer concentration (less than 2 g/l). Adsorption isotherms were correctly reproduced, taking into account all solution equilibria, including calcium and sodium binding constants and a solubility limit for the calcium polymer complex : the latter was determined by turbidimetric titration for various polymer molecular weights. Our conclusion is that in the presence of divalent ions which is often the case with sparingly soluble minerals, the surface binding of the polyelectrolyte results from a precipitation rather than an adsorption phenomenon. L'adsorption des polymères sur les surfaces minérales qui permet de stabiliser une suspension colloïdale a des applications largement répandues dans les procédés industriels. Le mécanisme de liaison a été bien décrit sur les surfaces d'oxyde, principalement en termes de lien d'hydrogène et d'interactions électrostatiques entre sites chargés et segments de polymère. Ce phénomène a été mis en modèle et l'influence du pH, de la résistance ionique et du poids moléculaire peut être calculée ou prédite. Dans le cas de substrats faiblement solubles tels que le BaSO4, CaCO3 ou CaSO4, plusieurs problèmes apparaissent : la difficulté d'identifier les sites de surface et l'interférence des ions provenant de la solubilité du matériau. Dans le cas de la calcite, la solubilité induit des ions calcium dissous dans la solution, ce qui peut complexer le polyélectrolyte et réduire sa solubilité. Dans ce but, nous avons mesuré l'énergie de liaison en utilisant la microcalorimétrie. Les mesures microcalorimétriques ont montré que l'enthalpie d'adsorption est faiblement endothermique: environ 2 kJ/mol. Il est intéressant de noter que cette valeur est très proche de celle de la complexion du calcium avec PANa. Il est suggéré que la force d'entraînement pour l'adsorption est le gain net en entropie du système. L'isotherme d'adsorption microcalorimétrique ne montre aucune évidence d'une interaction fortement exothermique entre les bords positifs et les segments négatifs du polyion. Pratiquement, dans la plupart des cas, l'adsorption des polymères est calculée à partir de la diminution de sa concentration dans la solution après séparation du solide par centrifugation. Cette procédure ne distingue cependant pas l'adsorption réelle et la séparation des phases. Pour répondre à la question, nous avons réalisé des expériences d'adsorption en utilisant une membrane de dialyse pour séparer les particules solides de la solution. Il a été établi que dans certaines circonstances, dépendant de la quantité relative de calcite, des ions calcium et du polyélectrolyte, une précipitation a lieu plutôt qu'une adsorption. Ceci est spécialement le cas pour les faibles concentrations de polymères (moins de 2 g/1). Les isothermes d'adsorption ont été correctement reproduits en prenant en compte tout l'équilibre de la solution, y compris les constantes d'association du calcium et du sodium et une limite de solubilité pour le complexe calcium-polymère. Ce dernier a été déterminé par titrage turbidimétrique pour différents poids moléculaires du polymère. Notre conclusion est qu'en présence d'ions divalents, ce qui est souvent le cas avec des minéraux difficilement solubles, la liaison surface/polyélectrolyte provient d'un phénomène de précipitation plutôt que d'un phénomène d'adsorption.

Casein micelle dispersions have been concentrated through osmotic stress and examined through rheological experiments. In conditions where the casein micelles are separated from each other, i.e., below random-close packing, the dispersions have exactly the flow and dynamic properties of the polydisperse hard-sphere fluid, demonstrating that the micelles interact only through excluded volume effects in this regime. These interactions cause the viscosity and the elastic modulus to increase by three orders of magnitude approaching the concentration of random-close packing estimated at Cmax≈178 g/l. Above Cmax, the dispersions progressively turn into “gels” (i.e., soft solids) as C increases, with elastic moduli G′ that are nearly frequency independent. In this second regime, the micelles deform and/or deswell as C increases, and the resistance to deformation results from the formation of bonds between micelles combined with the intrinsic mechanical resistance of the micelles. The variation in G′ with C is then very similar to that observed with concentrated emulsions where the resistance to deformation originates from a set of membranes that separate the droplets. As in the case of emulsions, the G′ values at high frequency are also nearly identical to the osmotic pressures required to compress the casein dispersions. The rheology of sodium caseinate dispersions in which the caseins are not structured into micelles is also reported. Such dispersions have the behavior of associative polymer solutions at all the concentrations investigated, further confirming the importance of structure in determining the rheological properties of casein micelle systems. [ABSTRACT FROM AUTHOR]

Here we investigate a colloidal suspension of practical interest, silica particles, which provides a mesoscopic system for the study of a dynamic near fluid–solid transition. Dynamic properties of concentrated solutions of nearly monodisperse charged colloids (standard deviation/mean radius≊0.11) at low ionic strength (≊6×10-4 M) have been studied by quasielastic light scattering. Below a volume fraction of the order of 15%, the solutions are viscous fluids and the refractive index fluctuations relax in two steps. Above a volume fraction of 20%, and over a macroscopic time window extending up to few seconds or more, the solutions are elastic and fluctuations in the refractive index become frozen. This is typical of systems which pass from a fluid to a solid state. The results are critically analyzed within the same framework used for similar macroparticle systems. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]