Colloidal forces, microstructure and thixotropy of sodium montmorillonite (SWy-2) gels: Roles of electrostatic and van der Waals forces

A model for explaining the effect of ionic strength on the yield stress and thixotropic behaviour of sodium montmorillonite or NaMt (SWy-2) gel was presented. NaMt gels displayed thixotropic behaviour at low to 0.1 M KCl concentration. This behaviour is reflected by an increasing yield stress with rest time after preshearing. The increasing yield stress measured the strengthing of the gel structure as it recovered at rest. The microstructure was formed by highly flexible nanosized platelets interacting both attractively via heterogeneous charge attraction in the edge-face and overlapping edge-edge configurations, and repulsively between the faces via electric double layer (EDL) force. This EDL force opened up the microstructure and was responsible for both the strength and thixotropic behaviour of the gel. The increasing gel strength with ionic strength up to 0.1 M KCl was a result of the weakening of the EDL force between the faces (interacting at an angle) particularly at the platelet bond forming the network junctions. The heterogeneous charge attraction forming the bond effectively increased in strength. In addition, weak platelet bonds were also able to form. The EDL force governs the microstructure development and this process takes a long time for every platelet to experience a minimum net force. The Leong model described the ageing or thixotropic behaviour quite well. At higher KCl concentrations, the van der Waals attractive force dominated the platelet-platelet interactions. These suspensions displayed time independent behaviour and was found to belong to the flocculated phase state.