Oil Film — Measuring a Molecule

Estimate the length of an oleic acid molecule by spreading a thin film on water

Difficulty: Easy | Time: 30 minutes | Visual Impact: Medium–High

Historical Context

In 1890, Lord Rayleigh published a brief note observing that a very small amount of olive oil, dropped onto a pond, spread into an enormous thin film that calmed the surface ripples. He calculated the film to be about 1.6 nm thick — a plausible length for a single molecule — but did not make that connection explicitly.

Around the same time, Agnes Pockels was running similar experiments at her kitchen sink, using a wooden trough she had built herself to study surface films. She had no formal scientific training and no laboratory, but her results were rigorous. Her letter to Rayleigh led to a landmark 1891 paper in Nature on surface tension of contaminated water surfaces. Her improvised “Pockels trough” became the direct ancestor of the Langmuir trough.

Irving Langmuir, from the 1910s onward, showed that amphiphilic molecules form a monomolecular layer (a monolayer) at the air-water interface, with polar heads pointing into the water and hydrocarbon tails pointing upward. His quantitative theory of these Langmuir films won him the Nobel Prize in Chemistry in 1932.

This experiment repeats Rayleigh’s observation, but with a measured volume and a measured area — giving a genuine experimental estimate of molecular length.

Materials

  • Oleic acid — pure or high-oleic sunflower oil
  • Ethanol (rubbing alcohol, ≥70%) — 100 mL
  • Large flat tray or baking dish — at least 30 × 40 cm
  • Lycopodium powder, chalk dust, or baby powder — small amount (for visualizing the film)
  • 1 mL syringe or calibrated dropper (to measure drop volume)
  • Ruler and tape measure
  • Distilled or tap water — to fill tray

Procedure

Part 1 — Calibrate your dropper (5 min)

  1. Fill the syringe or dropper with ethanol
  2. Dispense drops slowly onto a sheet of paper, counting them, until you have released exactly 1 mL (note how many drops make 1 mL — typically 20–25)
  3. One drop volume = 1 mL ÷ (number of drops). Record this as \(V_\text{drop}\)

Part 2 — Prepare the oleic acid solution (5 min)

  1. Measure 1 mL of oleic acid into 500 mL of ethanol and mix thoroughly
  2. This gives a 0.2% (v/v) solution. Each drop contains:

\[V_\text{OA} = V_\text{drop} \times 0.002\]

For a typical dropper (\(V_\text{drop}\) ≈ 0.05 mL): \(V_\text{OA}\) ≈ 1 × 10⁻⁴ mL = 10⁻¹⁰ m³

Part 3 — Form the film (10 min)

  1. Fill the tray with water to within 1 cm of the rim; let it settle for 1 minute
  2. Very lightly dust the surface with lycopodium powder or chalk dust — just enough to see it (too much and it will suppress spreading)
  3. Hold the dropper 1–2 cm above the surface and release one drop into the center
  4. The powder is pushed outward as the oil film spreads; the cleared circle is the film boundary
  5. When spreading stops (5–15 seconds), measure the diameter of the cleared area in at least two perpendicular directions and take the average — call this \(d\)

Part 4 — Calculate (5 min)

Film area: \(A = \pi (d/2)^2\)

Film thickness (= molecular length, assuming a monolayer):

\[\ell = \frac{V_\text{OA}}{A}\]

Example: \(V_\text{OA}\) = 1 × 10⁻¹⁰ m³, \(d\) = 25 cm = 0.25 m

\[A = \pi \times (0.125)^2 \approx 0.049 \text{ m}^2\]

\[\ell = \frac{1 \times 10^{-10}}{0.049} \approx 2 \times 10^{-9} \text{ m} = 2 \text{ nm}\]

The actual length of an oleic acid molecule is approximately 2.0 nm.

The Science

Why does the oil spread?

Oleic acid is amphiphilic: the carboxylate head (–COOH) is polar and attracted to water, while the 17-carbon hydrocarbon tail is non-polar and repelled by water. At the air-water interface, the molecules find their minimum-energy configuration by standing upright — heads in the water, tails pointing up — forming a single molecular layer.

Once the surface is fully covered, spreading stops. There is no room for more molecules in the monolayer; any excess would pile on top. This is the basis of the Langmuir model.

Why this gives molecular length

The volume of oil equals the area of the film multiplied by the length of one molecule (assuming vertical packing, one molecule thick):

\[V = A \times \ell \implies \ell = \frac{V}{A}\]

This is the same argument Lord Rayleigh made in 1890, though he was estimating rather than measuring precisely.

Estimating Avogadro’s number

Knowing \(\ell \approx 2\) nm, the cross-sectional area of one molecule at the surface is roughly \((0.5 \text{ nm})^2 = 0.25 \text{ nm}^2\).

The molar mass of oleic acid is 282 g/mol; its density is 0.895 g/mL.

Volume of one mole: 282 / 0.895 ≈ 315 mL = 315 cm³ = 3.15 × 10⁻⁴ m³

Volume of one molecule: \(\ell \times \text{area} = 2 \times 10^{-9} \times 0.25 \times 10^{-18} = 5 \times 10^{-28}\) m³

\[N_A \approx \frac{3.15 \times 10^{-4}}{5 \times 10^{-28}} \approx 6 \times 10^{23}\]

A rough estimate of Avogadro’s number from a kitchen experiment.

Tips and Troubleshooting

  • Film doesn’t spread: The dropper may be contaminated with previous oleic acid. Clean it between attempts.
  • Powder clumps or sinks: Use less powder — just a visible film across the surface, not a thick layer.
  • Film too small to measure: Increase the oleic acid concentration (use a 1:250 dilution instead of 1:500).
  • Water surface contaminated: Dust and skin oils contaminate water quickly. Refresh the water and lightly blow or skim the surface before each trial.
  • Elliptical rather than circular film: Normal — oil spreads in the direction of any air currents. Measure along multiple axes and average.

Extensions

  1. Compare oils: Try extra virgin olive oil, high-oleic sunflower oil, and refined mineral oil. Do they give the same monolayer thickness? (Mineral oil is non-polar and non-amphiphilic — it will not form a true monolayer.)

  2. Vary concentration: Repeat with 1:250 and 1:1000 dilutions. Does the calculated thickness stay constant? This tests whether the monolayer assumption holds.

  3. Surface tension measurement: Without powder, time how fast a floating toothpick is pushed when a drop is added beside it — the Marangoni effect drives it away. Compare different oils and concentrations.

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