Revealing the microstructure of sodium-montmorillonite aqueous suspensions

TL;DR

This study uses advanced scattering and microscopy techniques to reveal that sodium-montmorillonite suspensions form a hybrid microstructure with both attractive and repulsive domains, resolving a long-standing debate.

Contribution

It provides the first comprehensive in-situ analysis showing that Na-Mt gels have ordered, large-scale structures with both nematic domains and aggregation, challenging previous models.

Findings

Na-Mt suspensions contain entities much larger than individual particles.

The microstructure includes nematic domains and particle aggregation.

The gel microstructure is hybrid, with both attractive and repulsive interactions.

Abstract

Aqueous suspensions of geometrically anisometric (2D) sodium-montmorillonite (Na-Mt) particles display a sol-gel transition at very low solids concentrations. The underlying microstructure of the gel has remained a point of contention since the time of Irving Langmuir. An in-situ investigation encompassing length scales much larger than the individual particles is required to provide support for one of the two models proposed in the literature: 1) a percolated network governed by electrostatic attraction between platelets; and 2) a jammed suspension stabilized by repulsive electrostatic forces between particles. We settle this debate by comprehensively probing the microstructure of Na-Mt suspensions using ultra-small angle neutron/X-ray scattering and found that it is ordered and contains entities that are at least an order of magnitude larger than the individual particles. Complementary cryo-electron microscopy showed both the presence of domains having strong particle-particle ordering and regions of particle-particle aggregation. These data indicate 1) the presence of nematic domains, which refutes a purely attractive nature, and 2) assembly of particles, which refutes a purely repulsive nature. Na-Mt gels appear to have a hybrid microstructure with both attractive and repulsive domains.