Oleylamidopropyl Dimethylamine Oxide (OAPO) (also referred to as Oleylamidopropylamine Oxide; CAS No. 25159-40-4) is a high-performance, long-chain amphoteric/zwitterionic surfactant.
With its long, monounsaturated C18 oleyl hydrophobic tail and an amidopropyl spacer group connected to a polar amine oxide head, OAPO exhibits unique rheological, conditioning, and surface-active properties. It behaves as a nonionic surfactant in neutral and alkaline media, and transitions to a weakly cationic surfactant in acidic environments (pH< 7.0).
Technical Specifications
| Parameter | Standard Value |
| Chemical Name | Oleylamidopropyl Dimethylamine Oxide (OA-PO) |
| CAS Number | 25159-40-4 |
| Molecular Formula | C23H46N2O2 |
| Appearance (at 25°C) | White to off-white paste / viscous liquid |
| Active Matter (Solid Content %) | 23.0% – 27.0% |
| pH (10% aqueous solution) | 5.5 – 7.5 |
| Free Amine (%) | 1.0% |
Key Physico-Chemical Properties
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Exceptional Thickening & Viscosity Modification: Due to the long oleyl (C18:1) chain, OAPO easily forms worm-like micelles in aqueous solutions, especially when paired with anionic surfactants (like SLES). This creates highly viscoelastic systems at low active concentrations.
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Mildness & Irritation Mitigation: Extremely gentle on keratinous substrates (skin and hair). When co-formulated with harsh anionic surfactants, OAPO associates with them to form mixed micelles, dramatically reducing skin and eye irritation.
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Conditioning & Substantivity: Under acidic conditions, the amine oxide head group protonates, gaining a positive charge. This makes it highly substantive to negatively charged hair fibers and skin, providing excellent wet-combing, anti-static, and softening properties.
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Rich, Dense Foam Stabilization: Unlike shorter-chain amine oxides (like Lauryl/Coco-amidopropyl amine oxides), the oleyl variant stabilizes dense, creamy, micro-bubble lathers rather than flash foams, enhancing the sensory profile of cleansing products.
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Hard Water Tolerance: Highly stable across a broad pH range and tolerant to extreme water hardness (calcium and magnesium ions), preventing soap scum precipitation.
Primary Applications
1. Hair Care & Premium Shampoos
OAPO is a widely favored additive in professional hair care formulations, conditioning shampoos, and hair masks.
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Function: Acts as a 2-in-1 conditioning agent and secondary surfactant.
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Benefits: Smooths the hair cuticle, reduces static flyaway, improves wet/dry combability, and adds a natural, healthy sheen to hair fibers.
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2. Viscoelastic Gelling Agents in Oilfield Fracturing
Beyond cosmetics, OAPO has emerged as an essential industrial chemical in oil and gas stimulation.
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Function: It serves as a key component in Viscoelastic Surfactant (VES) systems for hydraulic fracturing fluids.
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Benefits: When mixed with specific inorganic salts, OAPO builds highly viscous, shear-thinning gels that carry proppant deep into rock fractures. Once oil or hydrocarbons flow back, the gel rapidly breaks down without leaving residue, avoiding reservoir formation damage.
3. Mild Skin Cleansers & Body Washes
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Function: Foam booster and rheology modifier.
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Benefits: Used in sensitive skin washes, facial cleansers, and baby products to minimize barrier disruption while building a luxurious, non-stripping skin feel.
4. Hard Surface Detergents & I&I Cleaners
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Function: Wetting agent, degreaser, and thickener in acidic or alkaline cleaning systems.
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Benefits: Stable in bleach (sodium hypochlorite) and acid-based formulations, making it ideal for thickening automatic dishwashing liquids, bathroom cleaners, and exterior wall washes.
Synthesis Overview
OAPO is synthesized via a two-step, organochlorine-free pathway:
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Amidation: Oleic acid (from natural vegetable or animal lipids) is reacted with N,N$-Dimethylaminopropylamine (DMAPA) to yield Oleylamidopropyl Dimethylamine.
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Oxidation: This tertiary amine intermediate is oxidized using hydrogen peroxide (H2O2) in a polar solvent (like water or isopropanol) under controlled temperatures (55℃ – 65℃) to yield the final amine oxide.