Kinetics and interfacial interactions in the adhesion of colloidal calcium carbonate to glass in a packed-bed

Abstract

The adhesion of colloidal particles of calcium carbonate flowing through a packed-bed of glass, at different ionic strengths of the aqueous solutions has been studied. The experimental results are compared with those predicted by the Levich–Smoluchowski kinetic model and discussed in terms of the interfacial contributions to the adhesion between the particles and the substrate. Both materials have a negative zeta potential ranging from −75 to −10 mV (glass) and from −30 to 0 mV (calcium carbonate) in the range of electrolyte (NaCl) concentration used (10 −4–0.5 M). Thus the electrostatic component of free energy of interaction of glass–calcium carbonate is always repulsive. The surface free energy components of the two surfaces were studied by contact angle measurements (glass) and thin-layer wicking method (calcium carbonate). Glass behaves as an electron-donor monopolar surface (Lewis sense, see the work of C.J. van Oss, Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994) at the lowest NaCl concentrations, but manifests a slight dipolar character (electron-acceptor component around 1.7 mJ/m 2) at [NaCl]>0.1 M. Calcium carbonate surface is also strongly monopolar at the lowest concentrations but the electron-acceptor component of its surface free energy predominates over the electron-donor component at the NaCl concentration 0.5 M. The determination of the free energy changes associated to interactions between surfaces in aqueous media demonstrated that, for [NaCl]≤10 −2 M, the adhesion between calcium carbonate and glass is a more favourable process than the aggregation between calcium carbonate particles. The contrary occurs for higher NaCl concentrations, i.e., the interaction between calcium carbonate particles is the dominant process and a further deposition of aggregates on glass could be the mechanism for the adhesion phenomena. The stability factors determined from the Levich–Smoluchowski theory allow the kinetic description of the coagulation phenomena involved in the adhesion processes in terms of efficiency of collisions between the interacting surfaces.