Effect of Degassing Temperature on the Specific Surface Area and Other Physical Properties of Magnesium Stearate

Abstract

The effect of degassing temperature on the magnitude of the BET specific surface area and other physical properties of magnesium stearate was investigated. Magnesium stearate samples from two manufacturers were degassed using helium at 23°, 30°, 40°, 50° and 60°C. The specific surface area of the degassed samples was measured using both single- and multi-point BET methods. The moisture content and thermal properties of the degassed magnesium stearate samples were also examined. The specific surface area values decreased with an increase in the degassing temperature. The Karl Fischer and thermogravimetric analysis results also showed a decrease in the moisture content with an increase in the degassing temperature. The differential scanning calorimetry thermograms of the degassed samples showed changes in the endotherm characteristics along with changes in the melting point and heat of fusion values. It is hypothesized that the lower specific surface area values obtained after degassing at higher temperatures may be due to a partial conversion of the magnesium stearate dihydrate crystals into a monohydrate, an anhydrate or a mixture of the two forms. These results suggest that magnesium stearate samples should be degassed only at room temperature prior to the BET surface area analysis. The results of this study also show that it is important to consider the degassing conditions used when comparing specific surface area data from different sources.