Anticaking Activity of Ferrocyanide on Sodium Chloride Explained by Charge Mismatch

Sodium chloride crystals have a strong tendency to cake, which can be prevented by treating them with the anticaking agent ferrocyanide. Using surface X-ray diffraction, we show how the ferrocyanide ion sorbs onto the {100} face of the sodium chloride crystal where it replaces a sodium ion and five surrounding chloride ions. The coverage is about 50%. On the basis of the determined atomic structure, we propose the following anticaking mechanism. Because of the charge of the ferrocyanide ions sorbed on the surface, the crystal can only continue growing by leaving an energetically unfavorable sodium vacancy, or by desorbing the ferrocyanide ion. Therefore, the ferrocyanide effectively blocks further growth of sodium chloride crystals, thereby preventing them from agglomerating and caking