Structure Rebuilding Time-Dependent Behavior of NaMnt Gels in Aging and Stepdown Shear Stress Response: Hectorite Comparison and the Layer Charge Hypothesis

Abstract The NaMnt (SWy-2) gels displayed structure rebuilding (SB) time-dependent behavior in the stepdown shear stress response at pH from 3 to 11. The electric double layer (EDL) face–face repulsion remained strong even at pH 3, to slow the edge-face bonding in forming the network. The strong EDL repulsion at pH 3 is solely due to its high permanent layer charge density of −0.11 C·m–2. Hectorite gel commenced to show time-independent SB behavior at pH ∼ 8. Its silica face charge density was at least as high as −0.11 C·m–2 at pH 8 with the layer charge contributing −0.08 C·m–2. The negative silica face charge density was postulated to be moderated by the relatively high positive edge charges of the lath-like hectorite platelet because of its high edge-to-face area ratio of 0.24 compared to only 0.0081 for NaMnt. Salt increased the structure rebuilding speed by creating more low-energy paths to edge-face bonding. It increased the bond density and strength reflected by the larger yield stress and shear stress. It also made the time-dependent behavior in the stepdown shear stress response milder and caused it to disappear at 0.1 M KCl for NaMnt and 0.01 M KCl for hectorite. Increasing the platelet charge density by adsorbed P2O74– made the 0.1 M KCl NaMnt gel display the time-dependent behavior. In aging where an imposed shear is absent, the 0.1 M KCl NaMnt gel continued to display time-dependent structure rebuilding behavior. The faster structure rebuilding speed at higher salt concentrations is also reflected by the smaller time constant of the Leong model used to fit the aging data. The pH-dependent charge contribution to NaMnt face charge density at pH 8 is small, ∼27%. NaMnt gel behavior is more stable and less sensitive to pH change.