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Aluminum monostearate (also spelled aluminium monostearate) is a dibasic aluminum salt of stearic acid widely used as a thickening agent, gelling/structuring agent, opacifier and anti-caking additive in pharmaceuticals, cosmetics, food-contact materials, coatings and industrial formulations.
This review consolidates physicochemical properties, synthesis and manufacturing routes, analytical methods, material specifications, applications across industries, formulation behavior (rheology and gel-formation mechanisms), interactions with active ingredients and excipients, regulatory and safety profiles, environmental fate, quality control, and recent advances in research and functional derivatives.
CAS number: 7047-84-9
Primary synonyms: Aluminum monostearate; Aluminium monostearate; Dihydroxy(stearoyloxy)aluminium; Dihydroxyaluminum stearate; Dibasic aluminum stearate; Octadecanoyloxyaluminum dihydrate; Aluminum dihydroxide stearate; Stearic acid aluminum dihydroxide salt
Introduction and historical background
Aluminum monostearate is a metal soap—an organometallic salt produced from the reaction between stearic acid (octadecanoic acid) and aluminum salts or aluminum hydroxide.
Metal soaps such as aluminum, calcium, zinc, and magnesium stearates have been known since the early 20th century and have been industrialized for uses ranging from lubricant/bonding agents to rheology modifiers.
Aluminum monostearate, as the dibasic aluminum salt of stearic acid, has been used particularly where a combination of mild thickening, opacification and gelling is required (e.g., anhydrous gelling of oils and creation of cream/ointment bases).
Historically, its adoption in pharmaceutical and cosmetic formulations stems from its ability to produce stable oil gels and semisolid systems with minimal reactivity toward many actives.
Chemical identity, synonyms, and nomenclature
Systematic name (IUPAC-type): Dihydroxy(stearoyloxy)aluminium
Common names: Aluminum monostearate; Aluminium monostearate; Dibasic aluminum stearate; Dihydroxyaluminium stearate; Dihydroxy(octadecanoato-O-)aluminium
CAS registry number: 7047-84-9
EC / EINECS: 230-325-5
Other identifiers: PubChem CID and various MDL/UNII identifiers used in regulatory and commercial databases.
Note: the term "aluminum stearate" generically may refer to a mixture of mono-, di- and tristearates; manufacturers often specify "monostearate" or use assay grades (e.g., ~75% Al content) for technical specifications.
Structure, molecular properties and physical characteristics
Molecular structure
Aluminum monostearate is commonly described as Al(OH)₂(OCOC₁₇H₃₅) (a dibasic aluminum salt where two hydroxyl groups remain coordinated to the aluminum center and one stearate ligand is bound through the carboxylate oxygen).
In reality, commercial samples are often complex mixtures, with variable degrees of hydration and distribution between mono-, di-, and polymeric metal carboxylate species.
The long hydrocarbon chain of stearate gives the molecule amphiphilic properties that favor self-assembly in nonpolar media and the creation of three-dimensional networks via metal–oxygen bridging and hydrogen bonding.
Physical properties (typical, grade dependent)
Appearance: white to off-white waxy powder or granules
Odor: faint fatty
Melting point / softening range: not sharply defined (softening owing to long-chain crystallinity); behaves more as a waxy solid
Solubility: practically insoluble in water; soluble or dispersible in long-chain alcohols and oils; swelling/gelation behavior in mineral oils/vegetable oils depending on concentration and processing
Density: typically ~0.9–1.1 g·cm⁻³ depending on grade and porosity
pH: not directly applicable to nonaqueous systems; aqueous suspensions are alkaline to neutral depending on impurities
Molecular weight and formula
Reported molecular weight ~344.47 g·mol⁻¹ for the monomeric formula; commercial materials are mixtures so formal molecular weight is an approximation.
Synthesis and manufacturing methods
Aluminum monostearate can be prepared by several industrial routes:
Direct neutralization of stearic acid with aluminum alkoxides or aluminum alcoholates.
Controlled hydrolysis of aluminum isopropoxide in the presence of stearic acid yields the desired dibasic stearate with release of the alcohol by-product.
Reaction of stearic acid with aluminum hydroxide or aluminum oxide at elevated temperature.
This solid–solid or solid–liquid reaction forms aluminum carboxylates with water removal; reaction conditions (temperature, stoichiometry, mechanical shear) control the ratio of mono- to di- and tristearate species.
Metathesis using sodium stearate and aluminum salts (e.g., AlCl₃) followed by washing to remove salts.
This aqueous route forms insoluble aluminum stearates which are then filtered, washed and dried.
It is sometimes used for specialty grades but generates stoichiometric inorganic salts that must be removed.
Solvent-based or melt processes where stearic acid is heated with an aluminum reagent until formation of the organometallic salt and removal of volatile by-products.
Key process parameters include stoichiometry (excess stearic acid favors mono-species), temperature (higher temperatures favor conversion and faster reaction), water content (affects hydration and hydroxyl content), and mechanical shear (affects particle size and dispersion behavior).
Many commercial producers control these variables to meet NF (National Formulary) or other pharmacopeial grade specifications.
Purity, grades and commercial specifications
Commercial grades vary by intended use: pharmaceutical/NF grade, cosmetic grade, technical grade, and industrial grade. Typical specifications may include:
Assay (Al content) or stearate content — e.g., "~75% (Al)" indicates aluminum content basis used by some suppliers for technical materials.
Moisture content — controlled to reduce variability in gelling behavior.
Acid value / free stearic acid — upper limits for free fatty acid to ensure consistent performance.
Particle size distribution — influences rate of gel formation and final rheology.
Residues/inorganic salts — chloride and sodium residuals must be controlled for pharmaceutical or food-contact grades.
Pharmacopeial monographs for aluminum stearates may exist indirectly through excipient monographs or vendor statements (manufacturers sometimes supply Certificates of Analysis confirming compliance with NF or other standards).
Analytical methods and characterization techniques
Given the complexity of aluminum carboxylates, multiple complementary techniques are used:
Fourier-transform infrared spectroscopy (FTIR): identifies carboxylate binding modes (monodentate vs bidentate) via CO stretching shifts; hydroxyl bands indicate bound water or Al–OH groups.
Solid-state NMR (27Al, 13C CP/MAS): provides information on aluminum coordination environment (octahedral vs tetrahedral) and carbonyl/carboxylate environments of stearate chains.
X-ray diffraction (XRD): crystalline phases and long-chain packing; many commercial samples are partially amorphous/waxy but show lamellar or layered packing in some grades.
Thermogravimetric analysis (TGA) / Differential scanning calorimetry (DSC): moisture content, decomposition profiles, and melting/softening transitions.
Elemental analysis / ICP-OES or ICP-MS: aluminum content and detection of inorganic impurities (Cl, Na, heavy metals).
Gas chromatography (GC) or HPLC after derivatization: for residual free stearic acid and low molecular weight organic impurities.
Surface area and particle size analysis (laser diffraction, BET): critical for dispersion behavior.
Saponification and acid value titrations: for routine QC of free fatty acid content.
SAFETY INFORMATION ABOUT ALUMINUM MONOSTERARATE
First aid measures:
Description of first aid measures:
General advice:
Consult a physician.
Show this safety data sheet to the doctor in attendance.
Move out of dangerous area:
If inhaled:
If breathed in, move person into fresh air.
If not breathing, give artificial respiration.
Consult a physician.
In case of skin contact:
Take off contaminated clothing and shoes immediately.
Wash off with soap and plenty of water.
Consult a physician.
In case of eye contact:
Rinse thoroughly with plenty of water for at least 15 minutes and consult a physician.
Continue rinsing eyes during transport to hospital.
If swallowed:
Do NOT induce vomiting.
Never give anything by mouth to an unconscious person.
Rinse mouth with water.
Consult a physician.
Firefighting measures:
Extinguishing media:
Suitable extinguishing media:
Use water spray, alcohol-resistant foam, dry chemical or carbon dioxide.
Special hazards arising from the substance or mixture
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas
Advice for firefighters:
Wear self-contained breathing apparatus for firefighting if necessary.
Accidental release measures:
Personal precautions, protective equipment and emergency procedures
Use personal protective equipment.
Avoid breathing vapours, mist or gas.
Evacuate personnel to safe areas.
Environmental precautions:
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Methods and materials for containment and cleaning up:
Soak up with inert absorbent material and dispose of as hazardous waste.
Keep in suitable, closed containers for disposal.
Handling and storage:
Precautions for safe handling:
Avoid inhalation of vapour or mist.
Conditions for safe storage, including any incompatibilities:
Keep container tightly closed in a dry and well-ventilated place.
Containers which are opened must be carefully resealed and kept upright to prevent leakage.
Storage class (TRGS 510): 8A: Combustible, corrosive hazardous materials
Exposure controls/personal protection:
Control parameters:
Components with workplace control parameters
Contains no substances with occupational exposure limit values.
Exposure controls:
Appropriate engineering controls:
Handle in accordance with good industrial hygiene and safety practice.
Wash hands before breaks and at the end of workday.
Personal protective equipment:
Eye/face protection:
Tightly fitting safety goggles.
Faceshield (8-inch minimum).
Use equipment for eye protection tested and approved under appropriate government standards such as NIOSH (US) or EN 166(EU).
Skin protection:
Handle with gloves.
Gloves must be inspected prior to use.
Use proper glove
removal technique (without touching glove's outer surface) to avoid skin contact with this product.
Dispose of contaminated gloves after use in accordance with applicable laws and good laboratory practices.
Wash and dry hands.
Full contact:
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
Splash contact
Material: Nitrile rubber
Minimum layer thickness: 0.11 mm
Break through time: 480 min
Material tested:Dermatril (KCL 740 / Aldrich Z677272, Size M)
It should not be construed as offering an approval for any specific use scenario.
Body Protection:
Complete suit protecting against chemicals, The type of protective equipment must be selected according to the concentration and amount of the dangerous substance at the specific workplace.
Respiratory protection:
Where risk assessment shows air-purifying respirators are appropriate use a fullface respirator with multi-purpose combination (US) or type ABEK (EN 14387) respirator cartridges as a backup to engineering controls.
If the respirator is the sole means of protection, use a full-face supplied air respirator.
Use respirators and components tested and approved under appropriate government standards such as NIOSH (US) or CEN (EU).
Control of environmental exposure
Prevent further leakage or spillage if safe to do so.
Do not let product enter drains.
Discharge into the environment must be avoided.
Stability and reactivity:
Chemical stability:
Stable under recommended storage conditions.
Incompatible materials:
Strong oxidizing agents:
Hazardous decomposition products:
Hazardous decomposition products formed under fire conditions.
Carbon oxides, Nitrogen oxides (NOx), Hydrogen chloride gas.
Disposal considerations:
Waste treatment methods:
Product:
Offer surplus and non-recyclable solutions to a licensed disposal company.
Contact a licensed professional waste disposal service to dispose of this material.
Contaminated packaging:
Dispose of as unused product
