C. Geffroy et al., THE FRONTIER BETWEEN ADSORPTION AND PRECIPITATION OF POLYACRYLIC-ACIDON CALCIUM-CARBONATE, Revue de l'Institut francais du petrole, 52(2), 1997, pp. 183-190
Citations number
14
Categorie Soggetti
Energy & Fuels","Engineering, Chemical","Engineering, Petroleum
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.