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.