Effect of electrolyte on the microstructure and yielding of aqueous dispersions of colloidal clay

TL;DR

This study investigates how varying salt concentrations influence the microstructure, viscoelasticity, and stability of aqueous clay dispersions, revealing morphological transitions and providing microscopic visualization of gel networks.

Contribution

It offers direct microscopic visualization of microstructural changes in clay dispersions due to salt, highlighting a transition from honeycomb-like networks to face-face coagulation.

Findings

Non-monotonic viscoelasticity and yield stress with salt variation

Identification of a morphological transition in gel microstructure

Visualization of honeycomb and face-face coagulation networks

Abstract

Na-montmorillonite is a natural clay mineral and is available in abundance in nature. The aqueous dispersions of charged and anisotropic platelets of this mineral exhibit non-ergodic kinetically arrested states ranging from soft glassy phases dominated by interparticle repulsions to colloidal gels stabilized by salt induced attractive interactions. When the salt concentration in the dispersing medium is varied systematically, viscoelasticity and yield stress of the dispersion show non-monotonic behavior at a critical salt concentration, thus signifying a morphological change in the dispersion microstructures. We directly visualize the microscopic structures of these kinetically arrested phases using cryogenic scanning electron microscopy. We observe the existence of honeycomb-like network morphologies for a wide range of salt concentrations. The transition of the gel morphology, dominated by overlapping coin (OC) and house of cards (HoC) associations of clay particles at low salt concentrations to a new network structure dominated by face-face coagulation of platelets, is observed across the critical salt concentration. We further assess the stability of these gels under gravity using electroacoustics. This study, performed for concentrated clay dispersions for a wide concentration range of externally added salt, is useful in our understanding of many geophysical phenomena that involve the salt induced aggregation of natural clay minerals.