Multiscale Study on the Mechanism of a Bio-Based Anticaking Agent for NaCl Crystals

Caking constitutes a major problem for the flowability, transport, packaging, and consumption of hygroscopic granular crystalline materials such as salt. Sodium chloride is the most abundant salt on the earth and known to form strong lumps, mainly due to cycles of water uptake and water evaporation. We report on a multiscale study of the anticaking effect of the bio-based additive Fe-mTA, a metal-organic complex of iron (III) and meso-tartrate. Drying-deliquescence cycling experiments are performed to reproduce the situation in which the salt undergoes repeated humidity fluctuations. Our results show that Fe-mTA acts as a nucleation promoter and growth inhibitor by inducing roughness on the surface of crystals. To directly study the effect of Fe-mTA down to the micrometer scale, we study liquid capillary bridges between two macroscopic salt crystals by applying droplets of salt solution with various levels of additives. Scanning electron microscopy and three-dimensional (3D) laser scanning confocal profilometry results show that Fe-mTA produces a surface roughness at the micron scale. This roughness decreases the effective contact area between crystals and promotes the spreading of the liquid bridge; consequently, the formation of a solid bridge between grains with water evaporation is avoided, thus preventing the caking phenomenon and, in addition, preventing adhesion of the crystals to solid substrates.