Physical aging in aqueous nematic gels of a swelling nanoclay: sol (phase) to gel (state) transition

Aqueous dispersions of geometrically anisometric nano sized sodium-montmorillonite (Na-Mt) display a sol–gel transition at very low solids concentrations. The microstructure of the gel formed at very low ionic strengths is considered electrostatically repulsive with a nematic character, and the gel state at ionic strengths where Debye length is of the order of particle size is conjectured to be physical aging free. Here, we investigate the true nature of osmotically prepared Na-Mt dispersions at low ionic strength (~10-5 M), below and above the gel point. The sol phase exhibited very low yield stress than the gel state, without any sign of physical aging, thus behaving as a equilibrium state. In contrast, the gel exhibited signatures of physical aging, that is, an evolving microstructure which consolidates with time when left undisturbed thus behaving as out of equilibrium state. The physical aging behaviour becomes more pronounced at Na-Mt concentrations far above the gel point. A critical shear rate existed, below which no stable flows were possible in the gel state representing the microstructural reorganization timescale. Overall, Na-Mt dispersions in the gel state behaves as out of equilibrium systems with an ever-evolving microstructure, in opposition to the assumption that low ionic strength Na-Mt gels are an equilibrium phase. The possible origin of physical aging such as reversible orientation of Brownian anisotropic particles, stiffening of an existing microstructure or reorganization of microstructure towards minimal energy configuration has been discussed in detail.