Thixotropic Clay Gels: Microstructure and Role of Colloidal Forces

Gels of NaMt and hectorite displayed prominent time-dependent or ageing behaviour. This behaviour may last for days or even months. The microstructure showed flexible nano-sized platelets interacting attractively to form the network junctions and repulsively opening up the structure (Fig 1a,b,c,d). These junctions are formed by heterogeneous charged and vdW attraction in the jagged and curled edges with faces of nearest platelets. The EDL repulsive force governs the development of the 3-D structure as it moves to attain a minimum free energy state and this force is responsible for the time-dependent behaviour. At high ionic strength, 0.3M KCl, the bentonite gels became time independent behaviour (Fig. 1e) - vdW dominates the interaction force. Leong model described the ageing behaviour well for the low salt the gel (Fig 1e). For the nanodiscotic Laponite gels, the 3-D structure is formed from sheets (Fig. 1d) made from primarily disks interacting in the overlapping face-face interaction at the edge. The sheet charge property is the same as the disk; face negative and edge positive.

With this microstructure knowledge, a new model may be needed to explain behaviour like layer stacking and large dried platelets, low permeability, intercalation property. Overlapping face-face attraction at the edge enlarged the platelet size in the dried powder. Capillary force and layered cations mediating face-face repulsion allowed layer stacking to occur upon drying. In the wet state, the open cellular structure can accommodate any additive molecules in the relatively large pore spaces negating their need for interaction with the platelet surfaces prior to platelet stacking and intercalation upon drying.The stacking tendency of the platelets intercalates the trapped molecules in-between. The complete interpenetration of the structure with nano-pores and channels (Fig. 1a) trapped the water molecules by strong capillary forces imparting low permeability to the clay gels.