The impact of deliquescence lowering on the caking of powder mixtures

• Deliquescence lowering in mixture leads to a high sensitivity to variations of RH.
• Loss of crystallinity and high amount of residual water in NaCl-sucrose mixtures.
• Phase diagram predicts a critical behavior of solid mixtures at the eutonic point.
• Multiplying the number of cycles reinforces caking of mixtures.

The first step in caking of hygroscopic powders is usually the condensation of water vapor followed by deliquescence of solid particles. If water evaporation subsequently occurs (induced, for example, by the change of ambient conditions) the phenomenon of efflorescence will lead to the formation of solid bridges between particles. Deliquescence takes place at the so-called deliquescence relative humidity (DRH), which corresponds to the vapor pressure of saturated solution of the substance. When two or more deliquescent substances are present, a singular behavior known as “deliquescence lowering” is observed. It was shown that the lowering of deliquescence point favors the caking of powders mixture. Binary blend of NaCl and sucrose was chosen as a model system to investigate the impact of deliquescence lowering on the stability and flowability of powder. Deliquescence lowering was evidenced by ESEM observations and by the quantitative analysis of water uptake. It was shown that mixtures take a large amount of water up while drying step is inhibited. Caking tests, coupled with kinetic measurements of water uptake and loss, are carried out in order to relate the residual water content to the reinforcement of caking of the mixture. The influence of the proportion of two substances in a blend is also analyzed. It was found that the most sensitive composition with respect to caking is close to the one of the eutonic point i.e. the mixture showing the lowest critical relative humidity at a given temperature. This work also shows that the presence of two (or more) solutes deteriorates the mechanical properties and the stability of powders compared to the situation when only one deliquescent substance is present. The determination of the eutonic composition allows anticipating the conditions of reinforced caking.

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