Slow dynamics and equilibrium gelation in fractionated montmorillonite nanoplatelet dispersions

Abstract

Spontaneous temporal evolution of self-assembled phases of high aspect ratio (~ 250) fractionated nanoplatelets of Na Montmorillonite (FMMT) dispersions is systematically studied using rheology, light scattering, dilution, and electron microscopy experiments. Age-dependent viscosity of the suspensions shows a power-law behavior with the waiting time and scaled concentrations, beyond a threshold concentration, cg, below which phase separation occurs on long waiting times. The slow dynamics was monitored for more than 5 years which revealed the system evolved into different self-assembled phases, namely phase separation and equilibrium gels. Scanning electron microscope images show the highly porous nature of the gels. Gelation kinetics in the aging dispersion was modeled through percolation formalism which suggested that slow dynamics was responsible for the growth of the network. The formation of equilibrium gels was observed similar to the low aspect ratio (~ 25) Laponite® dispersions and the patchy particles. Thus, the present study provides experimental evidence of patchy nature of MMT clays, equilibrium, gelation, and empty liquids, which do not require an underlying phase separation to form as was predicted for the low-valance colloidal particles and multi-valent DNA nanostars.