I Physical Properties
Appearance and State: At room temperature, stearic acid is a white or off-white solid, typically in the form of flakes, beads, or powder. It has a waxy texture and a slight, characteristic fatty odor.
Melting and Boiling Points: It has a relatively high melting point, ranging from 69°C to 71°C, which allows it to remain solid at ambient temperatures-critical for maintaining the structure of creams and powders. Its boiling point is approximately 361°C (at standard pressure), ensuring stability during formulation processes.
Solubility: Stearic acid is insoluble in water due to its long hydrophobic alkyl chain (C₁₇H₃₅-). However, it is soluble in organic solvents such as ethanol, ether, chloroform, benzene, and fatty oils, facilitating easy integration into oil-based cosmetic formulations.
Density: Its density is around 0.84 g/cm³ (at 20°C), making it lighter than water, which influences its distribution in emulsions.
II Chemical Properties
Saponification Reaction: A key property for cosmetics, stearic acid reacts with alkalis (e.g., sodium hydroxide, potassium hydroxide) to form stearates (e.g., sodium stearate, potassium stearate)-natural emulsifiers. These stearates enable the stable blending of water and oil phases in creams, preventing separation and ensuring a smooth texture.
Formation of Metal Soaps: It readily reacts with metal oxides or salts (e.g., zinc oxide, magnesium chloride) to form metal stearates (e.g., zinc stearate, magnesium stearate). These derivatives are hydrophobic, enhancing the water resistance of powders and contributing to their anti-caking and skin-protective functions.
Hydrolysis: Under strong acidic or alkaline conditions (and with heat), stearic acid can undergo hydrolysis, though this is typically controlled in cosmetic formulations to maintain product stability.
Oxidation Stability: It is relatively resistant to oxidation under normal storage conditions, ensuring a long shelf life for products containing stearic acid.
These properties-from its solid state and hydrophobicity to its ability to form emulsifiers and water-resistant metal salts-make stearic acid an indispensable ingredient in cosmetics, enabling the texture, stability, and efficacy of creams, lotions, and powders.
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Miya
Shijiazhuang City, Hebei Province, China
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