Surface Chemisty, Microstructure, and Rheology of Thixotropic 1-D Sepiolite Gels

The rheological properties of sepiolite gels in relation to solution chemistry, fiber charge, and microstructure are poorly understood. The purpose of this study was to bring more clarity to this topic by quantifying the effects of solution pH, ionic strength, and adsorbed tetrasodium pyrophosphate (TSPP) additive on rheological properties. The electrical charge on sepiolite fibers was investigated to explain the fiber interaction configuration observed in the microstructure. Fiber interaction forces and dynamics explained the ageing behavior of the gel. Sepiolite gels of only a few percent solids displayed long-time ageing behavior, which was manifested by an increasing yield stress with wait time and continued for weeks. The gel microstructure showed randomly orientated rigid fibers with cross configuration attraction. Each fiber experiences both attractive (van der Waals and heterogeneous charge) and repulsive (electric double layer) forces, and initially a net force. The repulsive force causes these fibers to orientate or move continually to achieve a state of force equilibrium and this process takes a long time. The Leong model describes this ageing behavior. For good fiber separation, high intensity probe sonication of the suspension was required. The yield stress increased with sonication time, solids loading, and temperature. The yield stress was absent at pH > 11 and increased to a maximum value at pH < 8. This maximum was insensitive to pH between 4 to 8, and ionic strength up to 1 M KCl. TSPP reduced this maximum and shifted the zero yield stress region to a lower pH, similar to 7. The zero yield stress state corresponded to a zeta potential with a minimum magnitude of 30 mV.