Crystal, equilibrium moisture content

Makower, B. and Dye, W.B. 1956. Equilibrium moisture content and crystallization of amorphous sucrose and glucose. J. Agric. Food Chem. 4, 72-77. 

Moisture content slightly hygroscopic. A well-defined crystalline hydrate is not formed although surface moisture may be picked up or contained within small pores in the crystal structure. At relative humidities between about 15% and 65%, the equilibrium moisture content at 25°C is about 2.0%. At relative humidities above about 75%, tribasic calcium phosphate may absorb small amounts of moisture. Particle size distribution Tribasic calcium phosphate powder typical particle diameter 5-10 pm 98% of particles <44 pm. 

Hydrated crystals may show more complicated relationships, such as those of Fig. 12.4 for the system copper sulfate-water at 25 C. Three hydrates are formed in this system, as the figure indicates. The anhydrous salt shows a negligible equilibrium-moisture content, which would in any case consist merely of adsorbed water upon the surface of the crystals. If exposed to air containing a partial pressure of water less than 7.8 and more than 5.6 mmHg, the salt will... 

Other factors also impact the type of crystals formed upon cooling of hot soap. Water activity or moisture content contribute to the final crystal state as a result of the different phases containing different levels of hydration. Any additive that changes the water activity changes the crystallization pathway. For example, the addition of salt reduces the water activity of the mixture and pushes the equilibrium state toward the lower moisture crystal stmcture. Additionally, the replacement of sodium with other counter cations influences the crystallization. For example, the replacement of sodium with potassium drives toward the formation of 5-phase. 

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