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.
This article investigates the relationship between additive induced creeping and anticaking activity in sodium chloride. Through a series of creeping experiments and powder flow analysis, we establish a clear correlation between the amount of creeping and the anticaking effect of an additive. Habit modification is found not to be a sufficient condition for an anticaking agent. The correlation is explained by the fact that both creeping and anticaking require blocking of crystal growth.Creeping pattern of saturated brine with 1% (w/w) nitrilotriacetamide (NTAA).
AbstractThis article investigates the relationship between additive induced creeping and anticaking activity in sodium chloride. Through a series of creeping experiments and powder flow analysis, we establish a clear correlation between the amount of creeping and the anticaking effect of an additive. Habit modification is found not to be a sufficient condition for an anticaking agent. The correlation is explained by the fact that both creeping and anticaking require blocking of crystal growth.
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.
A multitude
of ultrathin crystal needles are formed during the
evaporation of saturated aqueous NaCl solution droplets in the presence
of amide containing additives. The needles are as small as 300 nm
wide and 100–1000 μm in length. Heating experiments,
X-ray diffraction, and energy dispersive X-ray spectroscopy showed
that the needles are cubic sodium chloride crystals with the needle
length direction pointing toward [100]. This shape, not expected for
the 43̅m point group symmetry of NaCl, has
been explained using a model, based on tip formation by initial morphological
instability followed by time dependent adsorption of additive molecules
blocking the growth of the needle side faces. The latter also suppresses
side branch formation, which normally occurs for dendrite growth.
We have investigated the use of polymers and monomers as habit modifiers and anticaking agents for sodium chloride. We show that amide functional groups cause the {111} faces to propagate on sodium chloride crystals and that polymer amides give a 1−2 orders of magnitude greater effect than the corresponding monomers on the habit modification of sodium chloride. We have also shown that the alcohol functional group does not have an effect on the surface or habit modification but that, when in a polymer form, leads to macrostep formation and acts as a nucleation inhibitor. The latter also holds true for other polymers that we have also tested. We have also found that there is no evidence of these polymers having anticaking abilities. Finally, we found that no amino acids, apart from glycine, have an effect on the morphology of NaCl crystals.
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