5.2.4: Properties of soap

Bubbles lab

In this lab, we will explore intermolecular interactions of polar, nonpolar and amphipathic molecules.

Safety notice

During the Covid-19 pandemic, blowing bubbles is a safety hazard. While exhaled air (especially outside) is diluted quickly, it is contained in a bubble and gets released when and where the bubble bursts. In these experiments, we are not blowing bubbles by exhaling air. We are using the pipettes and syringes which are filled with air from the room, avoiding this safety hazard.

Materials

There are multiple suggestions for experiments; hopefully, you will have the materials to do any of them, but everyone will be able to do some.

Materials from the kit

• Empty microcentrifuge tube
• 5 mL syringe
• toothpicks
• soap (choose lemon or coconut)

Other materials needed

• Water
• Container for solutions (glass, bowl, etc)
• Detergent (shampoo, dish washing detergent, laundry detergent)
• Hand sanitizer (less than one squirt)
• a penny

Nice to have

• Permanent marker ("sharpie")
• Nail polish remover
• Cooking oil
• Spices (pepper, cinnamon)

Beginning question

What kind of observations can we explain considering the intermolecular interactions of detergents with water, air, oil, and solids?

Experiments

Preparation

Open one of the two centrifuge tubes containing the soap, give it one last sniff, and remove it from the lid using a tooth pick. Take about one half or third of the soap, "smoosh" it between your fingers, make a ball and poke it with the flat side of the toothpick so that the soap stays attached. You now have a "magic wand" with soap on one end. Put the remainder of the soap back into the tube, fill to about 1 mL, cap the tube and gently mix to make some soapy water.

Basic like dissolves like experiment

Using a permanent marker, draw four dots on a glass. Try the following solvents to remove the dots (you can wet a paper towel, napkin or tissue with the solvent, and then use it as a wipe):

1. Water
2. Hand sanitizer (should contain 50% or more alcohol)
3. Soapy water
4. Oil (if you have)
5. Nail polish remove (if you have)

If you don't have a permanent marker but have nail polish and nail polish remover, you can use those instead.

Basic soap experiment

Choose one of these basic experiments, depending on the materials you have available.

1. Penny trick. Carefully, using the syringe, place 0.8 mL of water on the penny. Take a picture. Poke the water with a clean toothpick and make sure the water stays on the penny. Then, take your magic wand and gently touch the surface of the water. What happens?
2. Spice trick. Fill a mug or (even better) a shallow bowl with water, and spray some pepper or other finely ground spice on the water. If you don't have these, any water-insoluble powder will work as well (baby powder, various makeup powders). Once the surface has settled and nothing is moving anymore, touch the middle of the liquid with your magic wand. What happens?
3. Bubbles. Open the centrifuge tube with the soapy water after shaking it vigorously. Using a syringe filled with air, blow bubbles under the surface of the solution or into an existing bubble. How big of a bubble can you make. After you made a bubble, observe closely how the appearance changes until it bursts.

Explain your observations by describing what happens at the molecular level. The soap is amphipathic. When in contact with air, the non-polar side will face the air and the polar side will face the water. At higher concentrations, soap will also make micelles, and other non-polar substance such as oil will collect inside the micelles. Your explanation should include a figure depicting the soap molecules as "head and tail" cartoon.

Advanced experiment and video challenge

Pick one of these experiments and make a 10 sec video showing your observations

1. Soap boat: For this, you need a bowl of water and a piece of plastic that floats on the water (e.g. the wrapper of the syringe works). Suggestions e.g. at this site
2. Oil spill: For this, you need a bowl of water and some cooking oil. Place the minimal amount of oil in the center of the water surface and observe what happens. Then, add some detergent to the oil and record what happens on video. If you use very small amounts of oil and detergent and have the right lighting, you might see color effects like in the picture below. You can use your magic wand to see how the oil interacts with soap. Here is some context: https://edu.rsc.org/download?ac=11854
3. Anti-bubbles. For this, you need a glass of water (where you can see from the side), detergent and the syringe. Here is a colorful intro. Here is another instructive video. First, adjust the detergent concentration so that drops released from about 1/4 inch above the surface float for a while before the join the rest of the solution. Then, try to make some antibubbles by releasing solution from the syringe at about 1 inch above the surface (the trick is not to have a stream of water but instead a steady drip). Once you have an antibubble, try keeping it below the surface by blowing liquid at it with the syringe.

Once you have a nice 10 sec video, turn it into an animated GIF (e.g. ezgif) and post it on Piazza to the rest of your lab group. For the report, explain what is going on at the molecular level, and what intermolecular interactions are at play.