Ageing behaviour spanning months of NaMt, hectorite and Laponite gels: Surface forces and microstructure – A comprehensive analysis

Abstract

Recent “artifact-free” microstructure information enabled the development of a more credible model for explaining the ageing time-dependent rheological behavior of NaMt, hectorite and Laponite gels. These gels displayed pronounced ageing behavior lasting months. The platelet morphology and the localised nature of the negative and positive charges produced the essential forces and interaction configurations to form the time-dependent structure. This structure is composed of nano-sized flexible platelets interacting both attractively to form the network junctions and repulsively to open up the structure. These junctions were formed by heterogeneous charge and van der Waals attractions between the jagged and curled edges, and the face of the interacting platelets. The force that is responsible for the time-dependent behaviour is the EDL repulsive force. This force governs the development of the 3-D network structure and drives it towards a minimum free energy state. This process takes a long time to complete. The resultant microstructure is completely interpenetrated with interconnected nanopores trapping as much as 99% of water in volume. At high ionic strength or 0.3 M KCl, the NaMt or bentonite gels became time independent as the van der Waals force formed the structure. The Leong model described the initial ageing behavior well. For the nanodiscotic Laponite gels, the 3-D structure is formed from sheets instead of primarily disks contradicting prevailing model. With the latest microstructure information, new mechanisms were proposed to account for the high yield stress at low solids, low permeability, intercalation behavior, layer stacking and enlarged dried platelets.