Surface chemistry, rheology and microstructure of purified natural and synthetic hectorite suspensions

Natural (N-) and synthetic (S-) hectorite suspensions were found to display significant time-dependent rheology or ageing behaviour and shear thinning flow behaviour. The ageing behaviour was characterised by an increasing yield stress with rest time. The yield stress continued to increase even after a week of rest, a reflection of a long process. An open sponge-like cellular microstructure formed by platelet particles interacting attractively in the overlapping edge-face configuration was captured by cryo-SEM of gel samples prepared at high pressure (∼2000 bar) and subjected to rapid cryo-freezing, for both N- and S-hectorite gels. Even nano-discotic S-hectorite particles formed platelet particles hundreds of nanometres in length in the overlapping coin configuration. This structure, displaying a cell size ranging from tens to several hundred nanometres, is formed by strong attractive and repulsive forces. The platelets showed deformations such as bending and curling of the edges in response to these forces. The S-hectorite platelets are smaller and more rigid. During ageing the particles in the structure experience a net force. These particles will move in response causing force imbalance to be experienced by neighbouring particles and they will move in response. This action and reaction percolate through the network structure causing a high concentration of particles to respond. As a consequence the ageing process takes a long time to reach equilibrium. Various ageing models were used to fit the ageing data. The N-hectorite gels displayed a maximum yield stress at pH ∼ 8 and a particle zeta potential of -35 mV. This suggests the presence of critical positive and negative charge density is needed to form a structure with maximum strength. The zeta potential is negative and quite insensitive to pH from pH 4 to 12.