Oleic Acid

The dominant fatty acid in olive oil — surface chemistry and saponification

Molecular structure

Formula: C₁₈H₃₄O₂ — (9Z)-octadec-9-enoic acid
Appearance: Colorless to pale yellow oily liquid
Hazard: Low hazard · Skin/eye irritant

Properties

Colorless to pale yellow viscous liquid with a faint fatty odor. A monounsaturated fatty acid with 18 carbons and one cis double bond at position 9 (hence the shorthand C₁₈:1). It is the most abundant fatty acid in olive oil (~70%), avocado oil (~60%), and almond oil (~70%). Melting point 13°C — liquid at room temperature. Immiscible with water but soluble in ethanol and organic solvents. Very weakly acidic (pKa ≈ 9.85); the sodium salt, sodium oleate, is the principal component of Castile soap.

Being a cis-unsaturated fatty acid, the molecule has a pronounced kink at the double bond. This kink prevents tight packing, keeping olive oil liquid at room temperature — saturated fats like lard pack more tightly and are solid.

Historical Context

The name comes from the Latin oleum (oil). Michel Eugène Chevreul isolated oleic acid from beef tallow in 1823 and established that fats are glycerol esters of fatty acids — foundational work for which he coined the word saponification. His careful analysis of dozens of fats gave chemistry a new class of compounds: the fatty acids.

In 1890, Lord Rayleigh noticed that a drop of olive oil spread spontaneously across a pond in Noyce, calming the ripples in an expanding circle. He measured the area and calculated the film thickness at about 1.6 nm — without realizing he had estimated the length of a single molecule. Around the same time, Agnes Pockels, working alone at her kitchen sink in Göttingen, built a wooden trough to measure surface tension and wrote to Rayleigh about her findings. He helped her publish in Nature (1891), and her apparatus became the prototype for the Langmuir trough.

Irving Langmuir developed the full theory of monomolecular films in the 1910s and 1920s, showing that amphiphilic molecules stand upright at the water surface with their polar heads in the water and their hydrocarbon tails pointing up. Langmuir received the Nobel Prize in Chemistry in 1932.

Obtaining

  • Cosmetic/aromatherapy suppliers: Pure oleic acid (>90%) sold for cosmetics formulation.
  • High-oleic sunflower oil: Contains ~85% oleic acid — suitable as an approximation for the film experiment, though less consistent than pure material.
  • From olive oil: Olive oil is ~70% oleic acid in triglyceride form. Saponification followed by acidification can liberate the free acid, but pure oleic acid is simpler to work with.

Experiments

Oil Film (Langmuir Monolayer): A dilute ethanol solution of oleic acid is dropped onto a water surface dusted with lycopodium powder or chalk dust. The ethanol dissolves instantly; the oleic acid spreads into a single molecule-thick layer that pushes the powder to the edges. Measuring the cleared area and knowing the volume of oleic acid gives the film thickness — the length of one molecule — and from that Avogadro’s number can be estimated. This is one of the most accessible molecular-scale measurements possible in a home laboratory.

Saponification / Soap Making: Reacting oleic acid directly with sodium hydroxide is simpler than saponifying oil, because there is no triglyceride to hydrolyze — the reaction is a straightforward acid-base neutralization. The product, sodium oleate, is the active ingredient of Castile soap and an excellent emulsifier.

Emulsification: Dissolve oleic acid in a small volume of oil, then add sodium hydroxide solution — the soap forms in situ and instantly emulsifies the oil into a milky white dispersion. This demonstrates why soap cleans: the carboxylate head is hydrophilic, the hydrocarbon tail is lipophilic.

Ferrofluid Surfactant: In ferrofluid synthesis, oleic acid coats freshly precipitated magnetite (Fe₃O₄) nanoparticles while they are still hot. The carboxylate head binds to iron sites on the particle surface; the C₁₈ hydrocarbon tail faces outward, making the particle hydrophobic and compatible with mineral oil. Without this coating the particles clump and settle — oleic acid is what makes a ferrofluid possible.

Experiments using this chemical:

Safety

Note

Low hazard — similar to cooking oil.

Mild skin and eye irritant at high concentration; no significant acute toxicity. Flammable at high temperature (flash point ~190°C). Incompatible with: Strong oxidisers; concentrated mineral acids (may cause spattering).