Growth Inhibition of Sodium Chloride Crystals by Anticaking Agents: In Situ Observation of Step Pinning

We have determined how the growth of sodium chloride crystals is inhibited by the anticaking agents ferrocyanide and iron­(III) <i>meso</i>-tartaric acid. Using in situ atomic force microscopy, we show how steps flow on clean crystals and how the step flow is inhibited by the anticaking agents. At submonolayer coverages, steps are temporarily pinned, whereas at higher coverages, no step movement is observed at all. Finally, we found that the two anticaking agents influence the surface morphologies of treated crystals differently, providing a possible explanation for the observation that powders treated with ferrocyanide flow freely whereas those treated with iron <i>meso</i>-tartaric acid show slip–stick behavior.