Surface chemistry and rheology of Laponite dispersions — Zeta potential, yield stress, ageing, fractal dimension and pyrophosphate

• The initial state of the laponite gel for the ageing study must be well-defined.
• Power law models described the yield stress-volume fraction at different aged states.
• Fractal dimension is 2 at 0.01 M KNO3, independent of the aged state
• No dispersed pH region for the 3 wt% Laponite gel.
• Pyrophosphate additive created a dispersed pH region at pH > 7.

The ageing behaviour of Laponite gels at the phase boundary of attractive gel and flocculated state (ionic strength ~ 0.01 M 1:1 electrolyte) was investigated due to a lack of study in this region. Zeta potential and yield stress measurement revealed that freshly prepared Laponite dispersion took time to reach the surface chemical equilibrium (SCE) state. The higher the ionic strength, the shorter was the time needed. This well-defined structural state was employed at the commencement of ageing study. The ageing behaviour was characterised by a rapidly increasing yield stress region followed by a gradually increasing region and then by a plateau region. Both the initial and fully aged yield stress of Laponite gels increased with ionic strength and solid loading. Both the Leong and the two-parameter logarithmic time models described the ageing behaviour quite well. The relationship between the yield stress at the SCE state and at the fully recovered or rejuvenated state, and volume fraction obeyed a power law model. The fractal dimension of the two states was the same 2.0. This study also investigated the yield stress-pH behaviour of Laponite gel with and without pyrophosphate additive. Pure Laponite dispersion displayed a maximum yield stress at high pH. A drop in yield stress occurred at low pH region and eventually approaching zero yield stress. A totally opposite trend was observed with pyrophosphate additive. No yield stress was detected at high pH region and a maximum yield stress was located at pH 5 followed by a drop in yield stress until pH 2. This drop in the yield stress regardless of the presence of pyrophosphate, was due to the particle agglomeration promoted by low pH.