Due to the uncertain chemical composition of magnesium stearate, magnesium stearate has different physical properties, which affect its lubricating function. This includes particle size, particle morphology, moisture content, density and specific surface area.
The average particle size of magnesium stearate on the market is less than 5 to 10 microns, and it can also be as low as 1 micron in average particle size and as high as 20 microns in average particle size. Generally, the smaller the particle size, the lower the bulk density, so monitoring the change in bulk density of magnesium stearate from batch to batch is a cost-effective way to monitor the change in particle size.
One of the reasons why magnesium stearate can be used as a lubricant is because of its platelet-like crystal shape. These lamellar crystal structures are layered together and separated during the mixing process to coat individual particles or device surfaces. While stacked lamellar crystal shapes are more common, circular and even needle-like shapes also exist. The trihydrate is a needle-shaped particle with significantly reduced lubricity compared to other particle forms. It should be noted that all forms of magnesium stearate can be converted to trihydrate under high humidity conditions.
Commercial batches of magnesium stearate are a mixture of amorphous and crystalline hydrates. After the anhydrous absorbs moisture under ambient humidity, the water content reaches 3% to 4%, and most of the water is weakly bound inside the crystal lattice. Anhydrous will have obvious hygroscopicity when the relative humidity is above 80%. The Pharmacopoeia specifies that the loss on drying of magnesium stearate is less than 6%, and changes in the loss on drying may lead to a decrease in lubricity.
The bulk density of commercial grade magnesium stearate is generally between 0.15g/cm3 and 0.30g/cm3. Using magnesium stearate with a 2-fold difference in bulk density as a lubricant may produce different final products. You can use bulk density to determine the particle size and specific surface area of magnesium stearate (usually, the finer the particle size, the larger the specific surface area, and the smaller the bulk density).
In addition, the attribute that affects the lubricating function of magnesium stearate is the specific surface area. The larger the specific surface area, the better the lubricity. Under certain conditions of the mixture and tablet press, the larger the specific surface area of magnesium stearate, the lower the tensile strength of the tablet, the higher the friability, and the slower the dissolution and disintegration. The specific surface area of magnesium stearate on the market is usually between 5~20m2/g, which is a considerable range. Some of the variation may be due to the way the test samples were prepared and tested. Specific surface area data should be determined at several different relative pressures (ie, multipoint method). Usually suppliers set the specific surface area at twice the lower limit (for example: 6~12m2/g). While changes within this range may not affect all products, it may affect products that are prone to over-lubrication.