This deep comparison analyzes the differences between Ricinoleic Acid and Undecylenic Acid. While both are unsaturated fatty acids derived from the castor bean, they represent different stages of chemical processing—one is the primary fatty acid found in nature, while the other is a specialized “cracked” derivative.
In the oleochemical industry, Ricinoleic Acid and Undecylenic Acid are two of the most functional molecules available. One is the “mother acid” of castor oil, while the other is a high-value derivative produced through thermal decomposition.
Choosing between them depends on whether your application requires the high viscosity and lubrication of a C18 chain (Ricinoleic) or the antimicrobial and polymer-building capabilities of a C11 chain (Undecylenic).
Executive Summary
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Ricinoleic Acid (RA): A C18 hydroxy fatty acid that makes up about 90% of castor oil.2 It is unique due to its hydroxyl group, which provides high polarity and lubricity.
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Undecylenic Acid (UA): A C11 unsaturated fatty acid produced by the pyrolysis (cracking) of castor oil. It is a much smaller molecule, highly valued for its antifungal properties and as a precursor to high-end polymers.
Technical Specification Comparison Table
| Parameter | Ricinoleic Acid (C18) | Undecylenic Acid (C11) |
| Chemical Formula | $C_{18}H_{34}O_3$ | $C_{11}H_{20}O_2$ |
| Molecular Weight | 298.46 g/mol | 184.28 g/mol |
| Appearance | Yellowish, Viscous Liquid | Pale Yellow Liquid / Waxy Solid |
| Processing | Hydrolysis of Castor Oil | Pyrolysis (Cracking) of Castor Oil |
| Iodine Value | 82 – 90 | 135 – 140 (More Unsaturated) |
| Acid Value | 175 – 187 mg KOH/g | 296 – 301 mg KOH/g |
| Hydroxyl Value | 150 – 160 | None |
| Specific Gravity | 0.940 – 0.950 | 0.910 – 0.913 |
Key Differentiators: From A to Z
1. Molecular Structure (C18 vs. C11)
Ricinoleic Acid is a long-chain fatty acid (18 carbons).4 It contains a hydroxyl group at the 12th carbon, which gives it its famous “stickiness” and solubility in alcohol.5 Undecylenic Acid is a medium-chain fatty acid (11 carbons).6 It loses the hydroxyl group during the cracking process but gains a terminal double bond, making it highly reactive for polymerization.
2. Antimicrobial Potency
While Ricinoleic Acid has some antimicrobial benefits, Undecylenic Acid is a recognized Pharmaceutical Active Ingredient (API).8 Its shorter chain length allows it to penetrate fungal cell membranes much more effectively, making it the industry standard for treating skin infections.
3. Viscosity and Lubricity
Ricinoleic Acid is highly viscous and an excellent natural lubricant.9 It adheres well to metal surfaces. Undecylenic Acid is much thinner (lower viscosity) and is rarely used as a standalone lubricant; instead, it is used as a chemical intermediate to build other molecules.
4. Solubility and Odor
Ricinoleic Acid has a mild, oily odor and is soluble in most organic solvents and alcohols.10 Undecylenic Acid has a very sharp, pungent, “fatty-sweaty” odor that is characteristic of medium-chain acids. This odor is a key factor when formulating consumer-facing products.
Industry Applications
Where to use Ricinoleic Acid (C18):
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Soaps & Detergents: Used to make high-lathering, transparent “Castile” soaps.
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Textile Chemicals: Acts as a finishing agent and dye carrier (Turkey Red Oil).
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Industrial Lubricants: Used in cutting oils and hydraulic fluids where high polarity is needed.
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Pigment Wetting: Exceptional at dispersing pigments in paints and inks.
Where to use Undecylenic Acid (C11):
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Pharmaceuticals: The main ingredient in anti-fungal creams, powders, and ointments.
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Nylon-11 (Rilsan): The essential building block for manufacturing high-performance, bio-based Nylon for the automotive and aerospace industries.
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Personal Care: Used in anti-dandruff shampoos and deodorants for its germicidal properties.
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Fragrances: Used to synthesize “Peach” and “Apricot” scent notes (Undecalactone).
Pros and Cons
Ricinoleic Acid
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Pros: Excellent lubricant, highly biodegradable, mild odor, great pigment dispersant.
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Cons: Too thick for some applications; lacks the strong antifungal power of shorter-chain acids.
Undecylenic Acid
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Pros: Powerful antifungal agent, critical for bio-plastics (Nylon-11), highly reactive terminal double bond.
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Cons: Very strong/unpleasant odor, corrosive in concentrated form, more expensive due to complex processing (pyrolysis).
The Verdict: Which one should you choose?
If your application involves lubrication, soap making, or pigment dispersion, Ricinoleic Acid is the natural, cost-effective choice.
If your application is medicinal (antifungal), involves perfume synthesis, or requires the production of high-performance polymers (Nylon), Undecylenic Acid is the indispensable chemical grade.
