Studies of moisture exchange between stearic acid, calcium stearate, and magnesium stearate

Moisture content, hygroscopicity, and thermal desorption parameters of stearic acid, calcium stearate, and magnesium stearate, which are used as excipients for tableting, were studied. In contrast to stearic acid, calcium and magnesium stearates were found to be hygroscopic compounds which were able to absorb moisture in conditions of elevated air moisture content, this water being present in the bound and free states. Moisture was eliminated at temperatures of 40–105°C. Depending on the manufacturer, excipient samples differed in terms of moisture content and sorptive capacity, demonstrating the importance of studying these parameters. Endothermic effects for calcium stearate (Tmax  = 125.7°C) and magnesium stearate (Tmax  = 113°C) were induced by melting of the crystalline structure of these compounds and their transition to the amorphous state.