Aluminium stearate is a metallic soap – a salt formed from aluminum and stearic acid (a common fatty acid). It appears as a white, fine, fluffy powder. Unlike reactive aluminum salts (chloride, nitrate), aluminum stearate is mild, water‑repellent, and used primarily as a thickening agent, lubricant, and waterproofing additive in industrial and cosmetic formulations.
1. Basic Identification
- Chemical Formula: Al(C₁₈H₃₅O₂)₃
- Alternative Names: Aluminium(III) tristearate, aluminum soap, stearic acid aluminum salt
- Molecular Weight: 877.35 g/mol
- CAS Number: 637-12-7
- Appearance: White, fine, fluffy powder
2. Physical Properties
| Property | Aluminium Stearate |
|---|---|
| Melting Point | ~115 °C (239 °F) (decomposes) |
| Density | ~1.01 g/cm³ |
| Bulk Density | Very low (fluffy, lightweight) |
| Solubility | Insoluble in water; soluble in hot oils, turpentine, and aromatic solvents |
| Hydrophobicity | Strongly water‑repellent |
Aluminium stearate is a soft, greasy‑feeling powder. It is non‑hygroscopic and stable in air.
3. Chemical Behavior & Synthesis
3.1 Hydrophobic Nature
The long hydrocarbon chains (C₁₈H₃₅) from stearic acid make the molecule strongly lipophilic (oil‑loving) and hydrophobic (water‑hating). When added to paints or coatings, it aligns at surfaces to repel water.
3.2 Synthesis
Prepared by reacting aluminum salts (e.g., aluminum sulfate) with sodium stearate (soap) in a double‑displacement reaction:
Al₂(SO₄)₃ + 6 C₁₇H₃₅COONa → 2 Al(C₁₈H₃₅O₂)₃ + 3 Na₂SO₄
The precipitate is washed and dried to yield the fluffy white powder.
4. Industrial & Cosmetic Applications
4.1 Waterproofing Agent
Aluminum stearate is added to:
- Concrete and mortar to reduce water penetration
- Textiles and paper to impart water resistance
- Paints and varnishes as a flatting agent and to prevent pigment settling
4.2 Thickener for Lubricating Greases
When mixed with mineral oils, aluminium stearate forms a stable gel – a grease – used in high‑temperature and water‑resistant lubricants.
4.3 Cosmetics and Pharmaceuticals
- Acts as a thickener in creams, lotions, and ointments
- Used as a gelling agent in antiperspirants and deodorants (where it helps suspend active ingredients)
4.4 Anticaking Agent
Added to powders (e.g., in food packaging or industrial blends) to prevent clumping.
5. Safety & Hazard Management
General Safety: Aluminium stearate is considered low toxicity and safe for cosmetic use. It is not an oxidizer, not flammable, and not corrosive.
| Route of Exposure | Effect |
|---|---|
| Inhalation | Fine dust may cause mild mechanical irritation. Use dust mask. |
| Skin Contact | Non‑irritating; may feel greasy. |
| Eye Contact | Mechanical irritation (dust). Flush with water. |
| Ingestion | Low toxicity (used in some food‑contact applications). |
PPE: Dust mask, safety glasses, standard gloves (minimal hazard).
Storage: Normal dry storage; avoid excessive dust accumulation.
Disposal: Non‑hazardous; can be disposed as solid waste.
6. Environmental Impact
Aluminium stearate is derived from natural fatty acids and is biodegradable. It does not persist in soil or water. No significant ecotoxicity concerns.
7. Comparison with Other Aluminium Compounds
| Compound | Formula | Primary Nature | Primary Use |
|---|---|---|---|
| Aluminium Stearate | Al(C₁₈H₃₅O₂)₃ | Metallic soap (hydrophobic) | Waterproofing, grease thickener |
| Aluminium Hydroxide | Al(OH)₃ | Amphoteric powder | Antacid, flame retardant |
| Aluminium Chloride | AlCl₃ | Lewis acid | Catalyst, antiperspirant |
8. Frequently Asked Questions
Q: Is aluminum stearate a “soap” that cleans?
A: No. “Metallic soap” refers to its chemical structure (salt of a fatty acid). It does not lather or clean; it repels water and thickens oils.
Q: Is it safe in cosmetics?
A: Yes. Aluminum stearate is widely used in creams, lotions, and antiperspirants at low concentrations. It is not absorbed significantly through skin.
Q: Can I mix aluminum stearate with water?
A: It will not dissolve. It floats and clumps. It must be dissolved in hot oil or solvent to be effective.
9. Summary Data Sheet
| Property | Value |
|---|---|
| Chemical Name | Aluminum Stearate |
| Formula | Al(C₁₈H₃₅O₂)₃ |
| Appearance | White, fluffy powder |
| Melting Point | ~115 °C |
| Defining Trait | Hydrophobic, greasy texture |
| Primary Utility | Waterproofing, grease thickener, cosmetic additive |
| Hazard Note | Low toxicity; dust nuisance only |
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