Measuring the Length of a Molecule

Introduction

Is it possible to measure the length of a single molecule using a quick and easy procedure? Yes! Irving Langmuir received the Nobel Prize in chemistry in 1932 for his work on monomolecular films, which are films that are assumed to have a thickness of one molecule. By using a method similar to his, the approximate length of a single oleic acid molecule can be determined.

Concepts

• Monomolecular films
• Polarity
• Hydrophilic/hydrophobic interactions
• Organic chemistry

Materials

Safety Precautions

Ethyl alcohol is flammable and a dangerous fire risk. Lycopodium is a highly flammable powder and a possible allergen. Avoid contact with skin. Wear chemical splash goggles, a chemical-resistant apron and chemical-resistant gloves. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Flush the oleic acid/ethyl alcohol/water mixture in the tray down the drain according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

1. Clean all equipment thoroughly with detergent and water, rinse well, and allow it to dry completely. Clean, dry equipment is a must!
2. Calibrate the pipet by counting the number of drops of the oleic acid/ethyl alcohol solution needed to produce a volume change of 1 mL in a 10-mL graduated cylinder. For greatest accuracy, make sure that the bottom of the meniscus is at the 1-mL mark. You may want to show this technique to your students.
3. Calculate the volume of oleic acid in one drop. To do this, divide the number of milliliters of oleic acid in one milliliter of oleic acid/ethyl alcohol solution by the number of drops of solution per milliliter. For example, if it takes 25 drops of solution to produce a volume change of 1 mL, then the volume of oleic acid per drop of solution is 0.0002 mL.

Procedure

1. Add enough cold tap water to the pan to cover it to a depth of 0.5 cm. Place the pan on a flat, stable surface. Allow it to stand for a minute or two so that any motion caused by the pouring has subsided.
2. Put a light dusting of lycopodium powder on the surface of the water. The powder should be evenly distributed across the surface of the water.
3. Carefully add one drop of the oleic acid/ethyl alcohol solution to the center of the tray. The addition of the oleic acid will cause the lycopodium powder to spread out creating a clear circle of oleic acid in the center surrounded by a ring of lycopodium powder. Gently tap the tray so that the oleic acid is spread out over the largest possible area without being broken. This area should be roughly circular.
4. Measure the diameter of this circle, taking at least three measurements. Calculate the average diameter, then the average radius.
5. Using the formula for the volume of a cylinder, solve for h, the height of the cylinder. In the formula for the volume of a cylinder, V = π r² h, V is the volume of oleic acid per drop of solution (calculated in Step 3 of the Preparation section) and r is the average radius of the circle (determined in step 4). The height of the cylinder is equal to the length of an oleic acid molecule, or the thickness of the monolayer.

Student Worksheet PDF

11888_Student1.pdf

Teacher Tips

• Enough materials are provided to perform the demonstration at least seven times.
• If enough pans or cafeteria trays are available, this demonstration makes an excellent hands-on laboratory activity for students.
• Use a ruler graduated in millimeters for greatest accuracy.
• It is important to hold the Beral-type pipet at the same angle when delivering drops. This ensures that all drops are uniform in size and thus have the same volume.
• Use a piece of filter paper to demonstrate a very short cylinder. Compare this piece of filter paper to the layer of oleic acid.
• Have students calculate the theoretical length of an oleic acid molecule and compare the theoretical value to the experimental value determined in this demonstration.
• Fine talcum powder may be substituted for the lycopodium powder in the event of allergies to lycopodium powder.

Sample Data

Plugging the above sample data into the formula for the volume of a cylinder, a length of 3.3 x 10^–7 cm is obtained for the length of the oleic acid molecule. In general, the length will turn out to be in the neighborhood of 10^–7 cm.

V = π r² h

Worksheet

Volume oleic acid in one drop = 0.0002 mL

Diameter of oleic acid circle:

Trial 1 = 26 cm Trial 2 = 30 cm Trial 3 = 28 cm

Radius of oleic acid circle:

Trial 1 = 13 cm Trial 2 = 15 cm Trial 3 = 14 cm

Average radius = 14 cm

Answers to Questions

1. Using the following formula for the volume of a cylinder, solve for h, the height of a cylinder.

2. Oleic acid does not dissolve in water, but when it is placed in a polar solvent such as water, it forms a single layer of oleic acid molecules, with a polar “head” that points directly downward and a nonpolar “tail” that sticks straight up (see Figure 2). This monolayer spreads out in a rough circle across the surface of the water. With this information, why do you think we use the height of a cylinder determine the length of an oleic acid molecule?

The circle of single-layer oleic molecules can be compared to a cylinder with a very small height. Since the molecules stick straight up in the air, forming the “wall” of the cylinder, the height of the cylinder is actually equal to the length of one oleic acid molecule.

Discussion

Oleic acid, C₁₈H₃₄O₂, is a fatty acid molecule that has two distinct parts. One end is a nonpolar, hydrophobic (water-fearing) carbon chain, resembling a “tail.”. The other end is a polar, hydrophilic (water-loving) “head.” Fatty acid molecules are commonly simplified using “head and “tail” drawings, such as the one in Figure 1.

Oleic acid is soluble in many organic solvents, such as ethyl alcohol, but not in water. When placed in the polar solvent water, it tends to form a monolayer with the polar “head” sticking down towards the surface of the water and the nonpolar “tail” sticking straight up away from the surface of the water (see Figure 2).

When the drop of oleic acid/ethyl alcohol solution is placed onto the surface of the water, the alcohol and the oleic acid separate. The ethyl alcohol is soluble in water, and it dissolves in the water as soon as the drop is placed on the surface of the water. But, because the oleic acid is not soluble in water, it does not dissolve in the water and instead spreads out on top of the water to form a layer that is roughly circular in shape. The length of the molecule is calculated by assuming that this “circle” is actually a cylinder with a very small height. The height of the cylinder is equal to the length of the oleic acid molecule.

References

Bilash, B; Gross, G.; Koob, J. A Demo A Day™, Volume 2—Another Year of Chemical Demonstrations; Flinn Scientific Inc.: Batavia, IL, 1996; p 5.