Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes

Montmorillonite is the most often studied swelling clay mineral. The layers have permanent negative charges due to isomorphic substitutions, and pH-dependent charges develop on the surface hydroxyls at the edges. Wyoming montmorillonite samples with different extents of isomorphic substitutions (Swy-1 and Swy-2) were studied. The acid–base titration of Na-montmorillonite suspensions between pH 4 and 9 at 0.01, 0.1 and 1 M NaCl was used to characterize pH-dependent charge development on amphoteric edge sites and to determine the point of zero charge (PZC) of edges. The evaluation of reversible net proton surface excess vs. pH functions revealed that the OH groups at edges having PZC at pH ∼6.5 are less basic than the Al–OH and less acidic than the Si–OH groups. Positive charges can develop in a protonation reaction of Al–OH sites at edges only at pHs below ∼6.5, and deprotonation of Si–OH then that of the Al–OH sites takes place with increasing pH of solution resulting in negative charges at edges. Therefore, patch-wise charge heterogeneity of montmorillonite, i.e. oppositely charged surface parts of layers, exists only under acidic conditions. Coagulation kinetics measurements resulted in reliable stability ratio data for fine montmorillonite sols at different pHs, and provided undisputable characterization of hetero- and homocoagulation. Edge-to-face heterocoagulation occurs above NaCl concentration 25–26 mmol l−1 at pH ∼4, where the hidden electric double layer (edl) of positively charged edge region has emerged. Edge-to-face attraction between the poorly charged edges and negatively charged faces of platelets around the pH of PZC of edges (pHPZC, edge∼6.5) in relatively low concentration of the indifferent electrolytes (typically around 50 mmol l−1 NaCl) is probable. The homocoagulation of uniformly charged lamellae at pH 8–8.5, formation of face-to-face aggregates requires much higher salt concentration (typically around 100 mmol l−1 NaCl) to compress the dominant edl on the highly charged faces of particles. XRD patterns of montmorillonite films prepared from slightly acidic suspensions proved that formation of well-ordered layer packages is hindered by the attraction between edges and faces. Characteristic changes in gel formation and in rheological properties induced by decreasing pH in dense suspensions containing 0.01 M NaCl provided experimental evidence for the structure of particle network. A significant increase in thixotropy and yield values, and also the formation of viscoelastic gels only below pH ∼6.5 verify that attractive interaction exists between oppositely charged parts of lamellar particles.