Sodium Chloride Dihydrate Crystals: Morphology, Nucleation, Growth, and Inhibition

Sodium
chloride powders have a strong tendency to cake at freezing
temperatures, which is caused by the formation of sodium chloride
dihydrate solid bridges between the powder particles. Using the crystallographic
structure, the morphology of the dihydrate crystals was determined.
In addition, the growth rate of the most important facets of these
crystals was measured as well as the influence of a number of anticaking
agents and related additives on the growth rate. A likely candidate
for the inhibition of dihydrate growth was found: a mixture of iron­(III)
and l-tartaric acid, which probably forms a metal–organic
coordination complex. The nucleation rate of the dihydrate was found
to be extremely low and to increase with time. This is probably caused
by the formation of a metastable prenucleation phase, which inhibits
the nucleation of dihydrate crystals. The influence of the additives
on the nucleation rate was found to be limited.