Sol–gel transitions of sodium montmorillonite dispersions

The flow behavior of sodium montmorillonite dispersions at salt concentrations below the critical coagulation concentration is determined by the influence of the ionic double layers on the mobility of the particles (secondary electroviscous effect). At solid contents above 3% the dispersions became gel-like with the appearance of a yield value and viscoelastic properties. Increasing salt concentration reduced the thickness of the diffuse ionic layers and the immobilization of the particles. As a consequence, the yield value and the viscosity decreased to a minimum at about 2–20 mmol/l NaCl (depending on the montmorillonite). This behavior was virtually independent on the type of salt. Above the electroviscous minimum the values of the flow properties increased with the salt concentration. Again a gel formed because the interaction between the edges(−) and faces(−) and, at somewhat higher salt concentration, between the faces(−) became attractive. High yield values and storage moduli were observed. The reversible part of the compliance reached 60–70%. The gel-like dispersion showed pronounced thixotropy. At salt concentrations above 400 mmol/l NaCl and solid contents below 2–3% (depending on the montmorillonite), viscosity, yield value, storage modulus, and reversible compliance decreased again because the gel transformed into a sediment. The cause is the contraction of the network into distinct particles when the attraction between the silicate layers is too strong. Formation and properties of the attractive gel were influenced by the type of salt. Potassium and cesium ions enhanced the elasticity of the gel. Sulphate anions reduced the yield value and storage modulus. This effect was very strong with diphosphate which liquefied the gel to a sol. The different states of sodium montmorillonite dispersions: sol, repulsive and attractive gel, sediment, are represented in phase diagrams which were constructed on the basis of compliance (creeping) measurements.