P3-11: LANGMUIR EXPERIMENT
- p3-11
- Demonstrate a monomolecular layer of oleic acid molecules held together by surface tension, and to experimentally determine the length of the oleic acid molecule.
Place the clean projection tray on the overhead projector with the ruler underneath. Cover the tray with a layer of water (over 1/8") and allow the water to settle. Lightly dust the surface with lycopodium powder, and adjust the projector so the powder and ruler are both in focus (center photograph). Hold the dropper just above the center of the tray and carefully release one drop of oleic acid solution onto the water surface. A circular "hole" quickly appears in the powder film, reaching its fixed maximum diameter in a few seconds (photograph at right). This is the monomolecular layer of oleic acid molecules held together by surface tension. Measure the diameter of the film so that the approximate thickness can be determined.
The following comments on the nature of oleic acid that makes this experiment possible were taken from the Science Teachers' Resource Center, Chemistry section, laboratory #31.
Molecules that are repelled by water are called hydrophobic. Molecules that are attracted to water are called hydrophilic. Cooking oil is hydrophobic; it won't mix with water. Some molecules have one end that is hydrophobic and one end that is hydrophilic. There are such molecules in the cells in your body. They are used to take hydrophobic nutrients into the cell that is mostly water. Soaps are this way also so that they can dissolve both hydrophobic and hydrophilic substances and be washed away by water.
Oleic acid is a substance with one hydrophobic and one hydrophilic end. When a small amount of oleic acid is placed on the surface of water, it stands on end with the hydrophilic end towards the water and the hydrophobic end away. If you could see them, they would look like fans at a crowded concert.
In this lab, we will find the length of one oleic acid molecule by spreading a small amount over the surface of water and measuring the diameter of the circle. The oleic acid spreads itself into a one-molecule thick layer in the shape of a VERY flat cylinder.