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A Thin Film on Water

STEM ED/CHM Nanotechnology at UMass Amherst

When Ben Franklin was a passenger on large sailboats, he learned that cooking oil was often poured overboard. The oil would spread out and calm the ocean waters. He conducted experiments on a pond and discovered that a small volume of oil created a thin film spread out over a large area. You can do a similar experiment by pouring a very small volume of oleic acid onto the surface of a tray of water.

Oleic acid is an organic compound found in many oils, including olive oil. A group of atoms called a “carboxylic acid” group is at one end of an oleic acid molecule. A diagram of an oleic acid molecule is shown below. One end of the oleic acid molecule is a long chain of carbon atoms that are chemically bonded to other carbon atoms and to hydrogen atoms. The long chain is symmetrical and does not contain oxygen atoms. The long chain end of the molecule is hydrophobic because it is nonpolar and is not attracted to polar water molecules.

Source: acid.pdf -

Two oxygen atoms in the carboxylic acid group at the other end of the molecule cause atoms at that end of the oleic acid molecule to develop positive and negative charges. That end of an oleic acid molecule is polar and is attracted to polar water molecules. That end of the molecule is called hydrophilic because that end is attracted to water molecules.

When a small volume of oleic acid is put on the surface of water, the oleic acid molecules form a thin layer. Because the polar end of the oleic acid molecules are attracted to water molecules, the oleic acid molecules stand vertically in a thin film on the surface of water similar to the diagram shown below.

Your Goal: Calculate the thickness of the thin film of oleic acid when you put one drop of a solution of oleic acid onto a tray of water.

Materials:

  • Either a small plastic tray or a large plastic tray
  • Two graduated cylinders to measure volumes to make a solution.
  • Bottles of isopropyl alcohol will be used as the solvent to make a solution.
  • Small cups of oleic acid will be used as the solute to make a solution.
  • Plastic cups
  • A dropper
  • Plastic cups
  • There may be some water in a circular tray. More water will added.
  • Paper towels to wipe up spills
  • Groups will share bottles of baby power.

Directions for groups who have a large plastic tray

Step One: Make Solution #1

  1. Large Tray Groups use a 10 cm3 graduated cylinder and a 25 cm3 graduated cylinder to make solutions.
  2. Use a 10 cm3graduated cylinder to measure 1.0cm3of oleic acid.
  3. Put 1.0 cm3 of pure oleic acid into a 25 cm3graduated cylinder.A suggestion: Oleic acid sticks to the walls of the 10 cm3graduated cylinder. Add approximately 5cm3 of alcohol to remove any oleic acid that is in the 10cm3cylinder. Transfer the liquid to the larger graduated cylinder. Put 5 more cm3 of alcohol into the 10 cm3 graduated cylinder to remove any more oleic acid. Pour that liquid into the 25 cm3graduated cylinder. Then read the next step.
  4. Add alcohol to the oleic acid in the 25 cm3 graduated cylinder until the totalvolume of Solution #1 is 25 cm3.
  5. Transfer the solution back and forth between a clean plastic cup and the 25 cm3graduated cylinder to mix the solution. In the last step of the mixing process, Solution #1 should be in the plastic cup.

Step Two: Make a More Dilute Solution #2

  1. Put 1.0cm3 of Solution #1into a 25 cm3graduated cylinder.
  2. Large tray groups add alcohol to the 1.0 cm3of Solution #1 until the total volume of the Solution #2 is 25cm3
  3. Transfer the solution back and forth between a clean plastic cup and the 25 cm3 graduated cylinder to mix the solution.
  4. In the last step of the mixing process, Solution #2should be in the plastic cup.
  5. Save Solution #2. All other liquids will no longer be needed and will be collected.

Go to Steps 3 and 4 at the Bottom of Page 3 to continue.

Directions for groups who have a small plastic tray

Step One: Make Solution #1

  1. Small Tray Groupsuse a 10 cm3 graduated cylinder and a 50 cm3graduated cylinder to make solutions.
  2. Use a 10 cm3graduated cylinder to measure 1.0 cm3of oleic acid.
  3. Put 1.0 cm3 of pure oleic acid into a 50cm3graduated cylinder. A suggestion: Oleic acid sticks to the walls of the 10 cm3 graduated cylinder. Add approximately 5 mL of alcohol to remove any oleic acid that is in the 10cm3cylinder. Transfer the liquid to the larger graduated cylinder. Put 5 more cm3 of alcohol into the 10 cm3 graduated cylinder to remove any more oleic acid. Pour that liquid into the 50 cm3 graduated cylinder. Then read the next step.
  4. Small tray groups add more alcohol to the oleic acid in the 50 cm3graduated cylinder until the total volume of Solution #1 is 40cm3.
  5. Transfer the solution back and forth between a clean plastic cup and the 50 cm3graduated cylinder to mix the solution. In the last step of the mixing process, Solution #1 should be in the plastic cup.

Step Two: Make a More Dilute Solution #2

  1. Put 1.0cm3of Solution #1into a 50 cm3graduated cylinder.
  2. Small tray groups add alcohol to the 1.0 cm3 of Solution #1 until the total volume of the Solution #2 is 40cm3.
  3. Transfer the solution back and forth between a clean plastic cup and the 40cm3 graduated cylinder to mix the solution.
  4. In the last step of the mixing process, Solution #2 should be in the plastic cup.
  5. Save Solution #2. All other liquids will no longer be needed and will be collected.

Step Three: Count Drops

  • Be sure that you are working with Solution #2.
  • Use a medicine dropper and a 10 cm3 graduated cylinder to determine and record how many drops (N) are in 1.0 cm3of Solution #2.

Step Four Make an oleic acid thin film.

  • Ask for a demonstration of how to do this after you have read directions for this procedure.
  • Bottle of baby powder will be shared by groups.
  • Have a ruler ready to make several diameter measurements when the oleic acid spread out over the surface of the water. This happens very quickly.
  • Lightly and evenly cover the surface of the water with a dusting of baby powder.
  • Use a medicinedropper to drop just one drop of Solution #2 onto the center of the surface of the waterin the tray. The drop should fall approximately 10 centimeters.
  • Record the averageradius of the area.

Step Five: Calculate the Depth (thickness) of the Layer of Oleic Acid

Use the Calculation Worksheet to Record Data.

Refer to the Sample Calculation Document if you need to.

Important Note: You recorded volume measurements in cubic centimeters (cm3) instead of milliliters (mL). A milliliter and cubic centimeter have the same volume. You used cm3so that you can use algebraic formulas to calculate the thickness of the thin film of oleic acid.

Step 1: Write a fraction thatrepresents how much pure oleic acid is in each cm3 of Solution #1. You started with 1 cm3of pure oleic acid. The solution had a total volume of either 25 cm3 or 40 cm3.

Step 2: Change the fraction in Step 1 into decimal form. This is how many cm3of pure oleic acid are in 1.0 cm3of Solution#1.

Step 3: Calculate how many cubic centimeters of oleic acid are in just 1cm3of Solution #2. The amount of oleic acid that was in 1.0cm3 of the Solution #1 was dissolved in enough alcohol to make a solution with a total volume of either 25 cm3 or 40cm3.

Step 4: You recorded the number of drops in 1.0 cm3of Solution #2. Calculate the volume of pure oleic acid that is inone drop of Solution #2. Record that value on the worksheet with a proper unit for volume.

Record the average radius of the somewhat circular area of the film of oleic acid.

Step 5: Calculate the area of the oleic acid thin film. Use the formula for the area of a circle. Include the proper unit for area.

Now you know the volume and the area of the thin film of oleic acid.

Step 6:Calculate the thickness of the thin film of oleic acid.Use the formula for the volume of a disc. Record the thickness of the oleic acid thin film. Include units as you make your calculation.

Step 7: Change the thickness of the thin film from centimeters to meters. Record the thickness using scientific notation.

Step 8: Change the thickness of the thin film to nanometers. Record the thickness of the thin film in nanometers.

Go to Step Six on the Next Page.

Step Six: Answer a few questions.

When a small volume of oleic acid is put on the surface of water, the oleic acid molecules form a thin layer. The polar end of the oleic acid molecules are attracted to water molecules. The oleic acid molecules stand vertically and form a thin film on the surface of water similar to the diagram shown below.

Question 1: The length of an oleic acid molecule is approximately 2.0 nanometers.

How does your value for the thickness of the thin film compare with generally accepted the length of an oleic acid molecule?

Question 2: Why might there be a difference between the thickness of the thin film that you made and the length of an oleic acid molecule?

Question 3: What are some sources of experimental error in this activity?

Question 4: How can you minimize experimental error in this activity?

Question 5: Imagine that 1.0 liter of pure oleic acid is accidentally spilled onto the surface of a lake. Based on your experimental results, what area of the lake’s surface would be covered by a thin film of oleic acid?

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