Influence of organic and inorganic salts on the coagulation of montmorillonite dispersions

The colloidal state (stable, coagulated, or gel-like) and the rheological p roperties of Na-rich montmorillonite (Wyoming) dispersions are strongly inf luenced by organic cations. This effect is shown for homologous organic cat ions: alkyl trimethylammonium ions, paraquat, diquat, alkyl bispyridinium i ons, and the triphenylmethane dyes crystal violet, methyl green, and tris ( tri-methylammonium phenyl) methane chloride. The critical coagulation conce ntrations, c(K), are small (often <1 mmol/L) because the cations are enrich ed in the Stern layer and influence the solvent structure near the surface. The strong adsorption of the counterions at the clay-mineral surface cause s c(K) values to increase with the solid content. Charge reversal (rechargi ng) of the particles was observed with the longer chain alkyl trimethylammo nium ions, dodecyl bispyridinium ions, and crystal violet. Other cations re duced the electrophoretic mobility to zero but positive particle charges we re not observed. The plastic viscosity increased sharply at the critical coagulation concent ration and showed a minimum slightly below c(K), which was caused by the el ectroviscous effect. Yield values were developed at concentrations above c( K). In most cases, yield values reached a plateau where the amount of organ ic cations was similar to 0.5 mmol/g, i.e., about half of the cation-exchan ge capacity. The c(K) values decreased with increasing hydrophobicity of ho mologous compounds, but the yield value showed maxima at intermediate chain lengths. The yield value of several 0.5% dispersions was high, e.g., dodec yl trimethylammonium ions, 71 Pa; paraquat, 100 Pa; diquat. 42 Pa; hexyl bi spyridinium ions, 53 Pa (vs. Ca2+, 0.2 Pa; Al3+, 0.7 Pa). The storage modul us as a function of the number of organic cations changed in a similar way as the yield value, and high values were observed (e.g., dodecyl trimethyla mmonium ions, hexyl bispyridinium ions: 1000 Pa, paraquat: >4000 Pa). Thus, dispersions with high viscosity, yield value, and pronounced viscoelastici ty are obtained by coagulating Na-rich montmorillonite dispersions with org anic cations.