Estimating Molecular Size Using the Oil Film Method
Drop oleic acid onto a water surface to form a monomolecular film, then estimate the molecular diameter by measuring the film area
Experiment Controls
Oleic Acid–Alcohol Solution Concentration1:10000
1:1000001:1000
Volume per Drop0.05 mL
0.01 mL0.10 mL
Procedure
- ① Prepare the oleic acid–alcohol solution (known concentration)
- ② Measure a fixed volume of solution with a graduated cylinder
- ③ Lightly dust the water surface with a thin, even layer of lycopodium powder
- ④ Drop one drop of the solution onto the water surface
- ⑤ Trace the outline of the oil film after it stabilizes
- ⑥ Calculate the film area using graph paper
Principle
d = V / S
Assuming oleic acid molecules are spherical and form a close-packed monomolecular layer on the water surface, the thickness of the oil film equals the diameter d of one molecule. Given the volume V of pure oleic acid and the film area S, the molecular diameter is d = V / S. The molecular diameter measured by this method is typically on the order of 10⁻¹⁰ m.
Experimental Tips
• Spread the lycopodium powder evenly—do not apply too thick a layer
• Add the drop gently to avoid splashing the water
• Wait until the oil film has fully spread and stabilized before tracing the outline
• Count partial grid squares as one-half