Stearic acid is a saturated monobasic acid with a chain length of 18 carbons. It is synthesized by hydrolysis of animal fats or hydrogenation of cottonseed or vegetable oils. Commercial stearate acid is a mixture of stearic acid with palmitic and myristic acids. Depending on the ratio of stearic acid to palmitic acid, it can vary in structure from macrocrystalline (45:55 w/w) to microcrystalline (50:50 to 90:10 w/w) (Li & Wu, 2014). Stearic acid polymorphs A, B, and C (the most stable) were prepared using different organic solvents and crystallization conditions (Garti, Wellner, & Sarig, 1980). Thermal studies have shown little to no batch-to-batch or inter-manufacturer variation in stearic acid from different suppliers (Garti et al., 1980; Inaoka, Kobayashi, Okada, & Sato, 1988).
Due to its low surface area, stearic acid is used in concentrations of 1%–3% w/w. Magnesium stearate at a concentration of 0.25% w/w has been reported to soften tablets made with pregelatinized starch and may affect tablet strength and dissolution, so stearic acid is the lubricant of choice for pregelatinized starch agent. Starch undergoes plastic deformation during tableting and therefore has a high sensitivity to the concentration of magnesium stearate. Furthermore, stearic acid protected the drug (aspirin) from degradation, although magnesium stearate accelerated the degradation of aspirin, as reported by Fouda et al. (Fouda, Mady, & El-Azab, 1998). In addition, stearic acid can also play a role in the polymorphic phase transition of drugs, resulting in slower tablet dissolution (Wang, Davidovich, et al., 2010). Since stearic acid facilitates the transformation of the polymorphic form (Form II to Form I) of the drug, the tablet dissolves slowly (Wang, Davidovich, et al., 2010).