Proton saturation and rheological properties of smectite dispersions

Proton-saturated dispersions of the <2-μm fractions of five smectites were prepared. Potentiometric titration data of freshly saturated and autotransformed samples were analysed by calculating proton affinity distribution curves to distinguish different proton interacting sites. Sites with pK values of ∼2.8 and 11.3 were assigned to protons exchanged for sodium ions and deprotonisation of silanol groups, respectively. Hydrated aluminum ions in freshly proton-saturated dispersions were characterized by pK∼6. This group of weakly acidic centers also included oligomeric hydroxoaluminum cations because the amount of these sites increased during autotransformation and was accompanied by a shift to pK∼5.5. The freshly prepared proton-saturated dispersions showed low pH values (∼2.6), and the particles interacted by edge(+)/face(−) contacts. This increased the viscosity in comparison to the sodium forms at pH∼7, and, depending on the smectite, yield values between 10 and 400 mPa were observed. Autotransformation removed all sites with pK∼2.8, reduced the viscosity, the yield value (mostly <10 mPa), and the flow behaviour approached that of the sodium smectite dispersions at pH∼7. The cause is enrichment of aluminum and/or oligomeric hydroxoaluminum cations in the solution and Stern layer, which leads to fragmentation of the networks into smaller stacks of more densely packed particles.