Saving Lives with Solubility
Grades: 7th-8th
Time: About 2 hours per activity
Objectives: Students will learn and apply fundamental physical science principles to
design a carrier for a hydrophobic drug. Additionally, students will be introduced to the
applications of controlling solubility in everyday items including medicine, food, and
cosmetics. Concepts illustrated to students in these activities include solubility, surface
tension, phase change, phase separation, the engineering design process, and polar
vs. nonpolar compounds.
This lesson is intended to integrate basic physical science and chemistry
principles with the National Academy of Engineering's Grand Challenges; specifically,
engineering better medicines.
Arizona 8th Grade Science Standards:
• S1C1: Observations, Questions, and Hypotheses (PO 1, 3)
• S1C2: Scientific Testing (PO 1,3,4,5)
• S1C3: Analysis and Conclusions (PO 1,3,5)
• S2C2: Nature of Scientific Knowledge (PO 1,2,3,4)
• S3C2: Science and Technology in Society (PO 1,2,3)
• S5C1: Properties and Changes of Properties in Matter (PO 2,4,5,6,7)
Materials:
Activity #1:
Water
Oil
Test tubes
Hot Plate
Salt
Sugar
Hand Soap
Milk (Skim & Whole)
Baking Soda
Toothpaste
Hot Sauce
Vaseline
"Bubbles" solution
Activity #2:
Craft supplies, sticky tack
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Procedures:
Activity #1: Making Bubbles ... of Water
Make and show students a stabilized emulsion (Water, Oil, and Soap) or (better
yet) some polymer or glass microspheres suspended in water. Use a microscope to
prove the shape of the spheres if they are not easy to see with the naked eye. Conduct
a discussion on how the particles or bubbles could be made to be so small and yet
perfectly round. If necessary, provide the hint that they can be made using oil and
water, which usually will separate into two layers. Ask them what could possibly make
spheres without this separation occurring. Likely, many of the suggestions proposed by
students will be included in the activity.
For the activity itself, students need to prepare a test tube with 1:1 water:oil by
volume (about 5 mL each is good). Provide the different treatment options for materials
that can be added to the tubes (or others depending on the suggestions of the class)
and let students choose 3 or 4 to test on their own. Options include salt, sugar, soap,
toothpaste, milk, vaseline, hot sauce, "bubbles," or whatever else you have available. A
small amount of the substance should then be added to the oil and water.
After adding one material, students will stir or shake the solution for a set amount
of time. Encourage students to use the same shaking or stirring method and time for
each sample. After each trial, students will record data on a worksheet. It is important to
wait at least 2 minutes after mixing to tell if the bubbles are truly stable or if the water
and oil are separating slowly. The students should also try first without adding any
material.
Following the lab, lead a discussion either with the class or in small groups
discussing the chemical structure of each additive and then relating that to the results of
the students' experiments. Questions could include: What does the salt "like"? What
does the vaseline "like"? Why do the ones that make bubbles work? Why would we
want to make microspheres? Why would we want water and oil to not separate? What
could we use that for?
There are basically 4 classes of materials: polar, non-polar, mixtures of both, and
surfactants (the ones that make oil and water "mix"!).
1) Polar materials, like salt, prefer water to oil because they have charges. An
analogy for students could be that water is like a battery or a magnet and it has positive
and negative ends to it. Salt dissolves because water dissolves materials which also
have charges. This is why magnets stick to other magnets but not to something inert
like modeling clay.
2) Non-polar materials, like vaseline. Students will describe these as greasy or
oily. Exactly opposite of the salt, these will mix with oil but not with water.
3) Mixtures of both, like hot sauce. Hot sauce has some salt and spices which
are polar, and some chili oil which is non-polar. Students will see the components (by
their color) extract into both phases.
4) Surfactants. These should lead to bubbles and the water and oil will blend into
one big phase containing lots of very tiny bubbles.
This is a good lab to introduce polar vs. non-polar bonding. The compounds that
make bubbles, such as soap and toothpaste, are special because they have molecules
in them that have a polar segment and a non-polar segment. For example, sodium
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lauryl sulfate (shown in the figure at left) is a common component of many soaps,
detergents, and toothpastes. The molecule works by the left non-polar side (full of nonpolar C-H bonds) dissolving in the oil/dirt/grease and the right polar side (containing
charges) dissolving in water. The oil then becomes mixed with the water in a little
bubble structure as shown in the figure at right.
We use this type of molecule (called a surfactant) for many things. Some key
examples are soap, detergents, toothpaste, lotions, ointments (such as Neosporin to get
polar antibiotics into oily skin), and foods such as milk. In fact, the reason that wet
ingredients for baking often include milk and eggs is that these materials prevent oily
ingredients and watery ingredients from separating. Perhaps the most important
application is that cell membranes is made up of a double surfactant layer, with water
on either side and an oily phase in the middle. These layers prevent small polar
particles (such as sodium and potassium ions) from traveling freely in and out of cells.
Another analogy that may be useful for students for this activity is raindrops on
the window of a car. As they fall down the window, drops will quickly combine with other
drops and become bigger. Similarly, after shaking oil and water, similar droplets "find
each other", combine, and settle out. Surfactants create a protective barrier around
each droplet or bubble, so the droplets just bump into each other and do not combine.
A related topic is that when two phases do not like each other, surface tension
causes phases to separate to minimize the contact between the phases. Including video
demonstrations from the internet of liquid-liquid or liquid-air interfaces (such as water
bubbles in zero gravity) are visual and interesting.
Assessment for this activity can be based on the detail and accuracy of students'
worksheets as well as their participation in the pre and postlab discussions.
There is a worksheet available for this activity.
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Activity #2: Designing a Drug Carrier
After discussing the "like-dissolves-like" concepts in Activity 1, present students
with the following design/engineering problem. Pretend that you have to make a carrier
for an oily cancer drug molecule so that the drug can dissolve in the body which is
mostly made of water. How can you "trick" the water in the body into dissolving the
drug? There's no limit to what you can put on your carrier or whether it is possible. How
big do you think it would really have to be? Draw the molecules as lines or shapes. You
don't have to say what the molecules are, but label which parts are polar and what parts
are nonpolar. What has to be near the drug? What about the water part?
Some optional guidance or hints for students: Drug must be dissolved in an oily
phase. The carrier has to "make the body think" that water "likes it" or will dissolve it.
If a student finishes quickly, challenge them to add other functions in that we
might want in a drug for treating a specific disease. For example, targeting and imaging
are also often incorporated so the drug knows to go to a particular place or can be
tracked by X-rays to see if it went where it was supposed to go.
Arrange students in groups and have them discuss their designs. Let the groups
pick a design together and then build the carrier out of craft supplies. Groups will then
present to the rest of the class on how the carrier works and any special features their
carrier has.
Peer assessment can be used for this activity. Each group can grade another
group's design and write 2-3 sentences explaining their thoughts on the design
(advantages/disadvantages).
A postlab discussion can follow on the real world applications of drug carriers.
Examples for applications of these drug carriers already being used include liposomal
drug formulations (for cancer), nanoemulsions being used as vaccines, micelles (for
cancer), polymer-drug conjugates (for macular degeneration), and microspheres (drug
capsules, pore-forming agents, cosmetics).
This problem is real. Cancer drugs already exist which can kill cancer cells very
effectively. The problem is that it is difficult to get cancer drugs to get into the watery
environment of the body and then to only go to the tumor because the drugs are also
good at killing normal cells.
Depending on the students, you might also introduce some figures about jobs,
funding, and expected growth in the field of biomedical engineering. Many problems in
this field remain unsolved.
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