Have you ever noticed that commonly used pills or vitamin tablets have a thin coating on their surface? This is an additive made from magnesium stearate, which is usually added to medicines as a lubricant. So why is this substance added to medicines?
What is Magnesium Stearate?
Magnesium Stearate is a widely used pharmaceutical excipient. It is a mixture of magnesium stearate (C36H70MgO4) and magnesium palmitate (C32H62MgO4) as the main ingredients, which is a fine white non-sanding powder with a slippery sensation when in contact with the skin. Magnesium stearate is one of the most commonly used lubricants in pharmaceutical production, with good anti-adhesive, flow-increasing, and lubricating properties. The addition of magnesium stearate in the production of pharmaceutical tablets can effectively reduce the friction between the tablets and the die of the tablet press, greatly reducing the tablet force of the pharmaceutical tablet press and improving the consistency and quality control of the drug.
Image from the Internet
The key property of magnesium stearate as a lubricant is its specific surface area, the larger the specific surface area, the more polar it is, the greater the adhesion, and the easier it is to distribute evenly on the particle surface during the mixing process, the better the lubricity. CIQTEK self-developed static volume method-specific surface and pore size analyzer V-Sorb X800 series can be used to test the gas adsorption of magnesium stearate and other materials, and analyze the material's BET surface area. The instrument is easy to operate, accurate, and highly automated.
Effect of Specific Surface Area on Magnesium Stearate
Studies have pointed out that the physical properties of the lubricant can also have a significant impact on the pharmaceutical product, such as the lubricant surface condition, particle size, size of surface area, and structure of the crystals. Through grinding, drying, and storage magnesium stearate can change its original physical properties, thus affecting its lubricating function.
Good magnesium stearate has a low shear lamellar structure  and can be properly mixed with the active component of the drug and other excipients to provide lubrication between the compacted powder and the mould wall and to prevent adhesion between the powder and the mould. The larger the specific surface area of magnesium stearate, the easier it is to distribute it evenly over the surface of the particles during the mixing process, and the better the lubrication. Under certain conditions of the mixture and the tablet press, the larger the specific surface area of magnesium stearate the lower the tensile strength of the tablets obtained, the higher the brittleness, and the slower the dissolution and disintegration. Therefore, the surface area is considered an important technical index of pharmaceutical-grade magnesium stearate. The specific surface area of magnesium stearate available on the market ranges from 3 to 54 m2/g, usually between 5 and 20 m2/g. The different surface areas are attributed to the different ways in which they are prepared. Degassing conditions can affect the specific surface area values of magnesium stearate and it has been reported in the literature  (see the picture below) that the specific surface area of magnesium stearate decreases as the degassing temperature increases and that the degassing temperature needs to be controlled to prevent magnesium stearate from sintering or melting.
Suppliers often set the specific surface area at twice the upper limit as the lower limit (e.g. 6 to 12m2/g). While variations within this range may not affect all products, they may affect products that are prone to over-lubrication, so an accurate measurement of specific surface area is essential.
Application Cases of Specific Surface Area Testing in Pharmaceutical Excipients
The CIQTEK Specific Surface Area & Pore Size Analyzer, V-Sorb X800 series, was used to test samples of magnesium stearate as follows.
The magnesium stearate sample was degassed and pre-treated by heating and evacuation before the test, the pre-treatment conditions were: 60°C vacuum heating for 4 hours; the BET graph and detailed test data were automatically generated after the test was completed as follows.
P/P0 was selected at 5 points between 0.05 and 0.35 to obtain a BET curve with a slope of 0.786537 and an intercept of 0.061886. The experimental results showed that the specific surface area of this magnesium stearate was 5.130077m2/g.
Both the US Pharmacopoeia and the 2020 version of the Chinese Pharmacopoeia require a linear fit of >0.9975 to the specific surface. As can be seen from the graph, the BET test results for magnesium stearate showed a linear fit of 0.999524 with C > 0 and reliable BET test results.
CIQTEK Specific Surface Area & Pore Size Analyzer
CIQTEK V-Sorb X800 series products can provide stable testing of specific surface area and micro- and mesopore pore size and distribution. The data is verified to meet the testing requirements of the Chinese Pharmacopoeia, providing a reliable testing means for research and quality control of raw and auxiliary materials, process mapping, tablet quality, and other pharmaceutical fields. The product has many advantages such as high testing efficiency, accurate results, high-cost performance, and easy-to-learn automated operation.
CIQTEK Fully Automatic Specific Surface Area & Pore Size Analyzer V-Sorb X800
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