THE FRONTIER BETWEEN ADSORPTION AND PRECIPITATION OF POLYACRYLIC-ACIDON CALCIUM-CARBONATE

Adsorption of polymers on mineral surfaces allowing colloidal stabilit y have widespread applications in industrial processes. The binding me chanism has been quite well described on oxide surfaces. Mainly in ter ms of hydrogen bonds and electrostatic interactions between charged si tes and polymer segments. This phenomenon has been modelized and the i nfluence of pH. lonic strentgh, and molecular weight can be calculated or predicted. In the case of sparingly soluble substrates such as BaS O4, CaCO3 or CaSO4, several problems arise: the difficulty for the ide ntification of surface sites and the interference of ions coming from the material's solubility. In the case of calcite, the solubility impo ses dissolved calcium ions in solution which could complex the polyele ctrolyte and reduce its solubility. For that purpose, we have measured the binding energy using microcalorimetry. Microcalorimetric measurem ents have shown that the adsorption enthalpy is weakly endothermic: ab out + 2 kj/mol. Interestingly, this value is very closed to that of ca lcium complexation with PANa. It is suggested that the driving force f or adsorption is the net gain in entropy of the system. The microcalor imetric 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 cal culated from the decrease of its concentration in the solution after s eparation of the solid by centrifugation. This procedure does not disc riminate therefore between real adsorption and phase separation. To an swer the question, we have performed adsorption experiments using a di alysis membrane to separate the solid particles from the solution. It has been established that in some circumstances, depending on the rela tive amount of calcite, calcium ions and polyelectrolyte, precipitatio n takes place rather than adsorption. This is especially the case at l ow polymer concentration (less than 2 g/l). Adsorption isotherms were correctly reproduced, taking into account all solution equilibria, inc luding calcium and sodium binding constants and a solubility limit for the calcium polymer complex: the latter was determined by turbidimetr ic titration for various polymer molecular weights. Our conclusion is that in the presence of divalent ions which is often the case with spa ringly soluble minerals, the surface binding of the polyelectrolyte re sults from a precipitation rather than an adsorption phenomenon.