Stable colloids made with inorganic molten salts

A colloidal solution is a uniform dispersion of two different types of components—particles or droplets of one phase, the solute, in a second (typically liquid) phase, the solvent. Milk, a colloid of butterfat in water, is one of the most familiar.

Colloids have nearly exclusively been formed in polar solvents such as water or in organic solvents. Dispersions of silica particles in molten inorganic salts have been made before, but they have been metastable—their colloidal form breaks down.

Now, Dmitri V. Talapin of the University of Chicago and coworkers report the first stable colloids of nanoparticles in molten inorganic salts and propose a mechanism behind the new colloids’ stability (Nature 2017, DOI: 10.1038/nature21041).

Such colloids could improve the heat-transport properties of molten salts, which are used as heat-transfer fluids in nuclear and solar-energy facilities. The work could lead to new nanomaterials by making it possible to synthesize them at higher temperatures than are currently feasible. And the new colloids could result in more-efficient inorganic thermoelectric materials.

Talapin and coworkers developed an improved understanding of the surface chemistry of materials that enabled them to select combinations of materials that form stabilized colloids. They made stable colloids by incorporating metal, semiconductor, rare-earth, and magnetic nanocrystals into AlCl₃-NaCl-KCl and six other types of molten inorganic salts.

Conventional colloids are stabilized by electrostatic or steric forces that prevent solute particles from aggregating. But Talapin’s group proposes a new mechanism for molten inorganic salt colloids: Solute-solvent chemical bonds that form at the nanocrystal surface induce partial ordering of the molten salt around each nanocrystal, which prevents the particles from combining.

Colloids expert Håkan Wennerström of Lund University remains skeptical about some technical details of the mechanistic proposal. However, he says, the study’s observations are “clear-cut and carefully made,” and the chemical bonding-based mechanistic proposal is overall “convincing.”