|ETP Title:||Intermolecular forces and their effects on the wetting of solutions|
|ETP Type:||Creating a New Lesson|
Liquids adhere differently to surfaces depending on the intermolecular forces within the liquid and the nature of the surface. In this lesson students will observe the differences in adhesion for droplets of water (highly polar, strong hydrogen bonding), mineral oil (highly nonpolar, strong dispersion forces) and methanol (intermediate polarity, weak hydrogen bonding and weak dispersion forces) on a glass slide. Students will photograph the droplets, measure the degree of spreading, and measure the contact angles of the droplets using open-source software. They will then carry out an inquiry-based investigation to see how the adhesion of one liquid is changed when they add varying amounts of another liquid (e.g., acid or base) to alter its intermolecular bonding properties. After an open exploratory phase, students will then design and perform a structured experiment in which the variables are carefully controlled and a hypothesis can be tested. This lesson sequence will take four 50-minute periods to complete.
Scientific progress is made by asking meaningful questions and conducting careful investigations.
1.) Students will measure how the shape of droplets changes with differences in intermolecular forces, and state a rule about the relationship.
2.) Students will formulate a research question about the effect on droplet shape of changing one variable in the system, predict an answer to this question, and describe an experiment to test the prediction.
3.) Students will perform the experiment they designed, and analyze their results to determine whether their prediction was correct.
1.) Data table and analysis questions for the guided experimentation phase of the lesson. Students will use these to organize their observations and consider what the experiment is telling them. We will debrief this in the following class session, using it as a formative assessment.
2.) Prelab writeup for the scientific inquiry phase of the lesson. Students will complete this according to regular guidelines and procedures that will be established for the entire course. I will check it prior to their participation in the scientific inquiry experiment.
3.) Data table and conclusions for the scientific inquiry phase of the lesson. Here students will reflect on the results of their experiment and determine whether their predictions were correct, as well as identifying likely sources of error in their experiment.
The final writeup for the lab, including all three of the components above, will comprise the summative assessment for the lesson. Student lab reports will be graded using a lab report rubric that has been taught and used repeatedly throughout the course.
1.1: Select and use appropriate tools and technology (such as computer-linked probes, spreadsheets, and graphing calculators) to perform tests, collect data, analyze relationships, and display data.
1.2: Identify and communicate sources of unavoidable experimental error.
1.3: Identify possible reasons for inconsistent results, such as sources of error or uncontrolled conditions.
1.4: Formulate explanations by using logic and evidence.
21st Century Skill(s)
Critical Thinking and Problem Solving
Make Judgments and Decisions
21st Century Skill(s) Application
1.) Students will apply technology as a tool by using cameras to photograph their droplets, open-source software to analyze the images, and spreadsheets to organize and graph their data for communication to others.
2.) Students will make judgments and decisions by choosing which variable to use for their inquiry experiment (e.g., adding acid, or base, or both in different trials) and by making conclusions about whether their data confirm or refute their hypothesis.
The Subramanian Lab at UC-Berkeley is investigating the properties of a variety of materials that can be used to make thin-film circuit components. Their goal is to develop chemical formulations that can be used consistently and effectively to manufacture printable electronic circuits, which could then be integrated into paper, cloth, plastic and other materials that cannot easily serve as substrates for more traditional circuits. Their findings will have increasing significance as electronics are incorporated into more and more commonplace items and materials.
During my fellowship I have been studying the effects of different dielectrics (insulators) on the performance of thin-film capacitors. To make these capacitors, I create solutions containing a precursor salt (e.g., aluminum nitrate), dispense them on pieces of glass coated with a film of conducting material (e.g., indium tin oxide), and then spin them on a vacuum centrifuge so that the solution is dispersed over the surface of the liquid in a thin layer. Depending on the formulation of the solution (e.g., concentrations of precursor salt and other solutes, solvent used, and pH of the solution), the solution may spread out into a thin layer that wets evenly or it may "clump" via adhesion, forming undesirable streaks and spots that will render the capacitor ineffective. If the solution wets the surface well, it can then be fixed into place via heat, forming a thin layer of dielectric over the thin layer of conductor. Finally, a top-coat conductor can be deposited onto the surface of the dielectric using metal evaporation and deposition. This process sprays a thin layer of metal atoms through a vacuum and onto the target surface, which can be partially masked in order to create a wide variety of different patterns. The result is a capacitor: two layers of conducting material with an insulating layer of dielectric in between. After creating these capacitors, my task was to measure their capacitance and track the effects of changing the dielectric solution on their resulting performance.
This project has exposed me to a mixture of chemistry skills (formulating solutions and annealing compounds) and electrical engineering skills (metal evaporation, measuring capacitor performance). It has shown me the integrated, interdisciplinary nature of applied science in the 21st century and how a diverse mixture of knowledge and skills is required to solve present-day problems. It has shown me what the work of an applied scientist looks like in a research setting so that I can advise my students about what a career in this field would entail.
The connection between the ETP and Fellowship. :
The students I will be teaching in the fall will all be new to me, so I plan to start out the school year by giving a short presentation about myself and my experiences with science. I plan to show my students pictures of the research process and tell them about the work I was doing during the summer, and then show them examples of the cool and useful gadgets that can be made with printable electronics. If possible, I'd also like to show them a short video clip with my mentor explaining what the research was about.
When the time comes to teach this lesson, I will show the students some of the results that I got during my project and explain how the "spreadability" of the thin-film solution was a big challenge to overcome. I will explain how we used the principles of scientific inquiry to overcome that problem, and tell the students that now they have the chance to investigate a similar problem. I will show the students that the scientific inquiry skills they are learning in this lab are directly relevant to real jobs making something many of them care about: cool electronic gadgets that are light, portable and cheap.
Time Required: Four 50-minute sessions
Prerequisites: Students should already have been introduced to the concept of polarity and the types of intermolecular forces (hydrogen bonds, dipole-dipole interactions, and Van der Waals dispersion forces).
New Vocabulary: Cohesion, Adhesion, Wettability
1.) Draw the following structures on the board:
- water (H2O)
- propane (C3H8)
- chloromethane (a.k.a. methyl chloride) (CH3Cl)
- acetone (CH3-CO-CH3)
- ammonia (NH3)
- 2-methylpentane (CH3-C(CH3)H-CH2-CH2-CH3)
Opening/Hook (15 min)
1. Do Now (5 min): Project a photograph of a smartphone. Students write down responses to the following prompt:
Smartphones have large glass screens that respond when you touch them. How do you think the phone "knows" where you are touching it? Write down your ideas in your lab notebook.
2. (3 min) Solicit answers from the students (draw names or cold call) and write them on the board. Ask for any additional ideas.
3. (2 min) Show students illustrations of a touchscreen's structure. Point out that the screen, which looks like ordinary glass, actually contains many layers of transparent materials which coat the glass in thin films. Show micrograph of touchscreen's cross-section to point out how thin these layers are.
4. (2 min) Think-Pair-Share: How do you think they get these thin layers of material onto the glass?
5. (1 min) Solicit answers from students.
6. (2 min) Explain that most of these layers are put onto the glass as a sol (solid particles suspended in a solvent) and then spread out into a film, which is dried and hardened into place with heat. This is what I was doing in my research project at UC-Berkeley, and figuring out how to get these sols to spread evenly is the challenge we'll be looking at in this lesson.
Direct Instruction & Modeling (22-24 min):
Students copy all notes/illustrations in this section into their notebooks.
1.) (1 min) Review the concepts of POLAR and NONPOLAR substances.
2. (2 min) Have students work in pairs to classify the molecules on the board as VERY POLAR, SOMEWHAT POLAR, or NONPOLAR.
3. (2 min) Call on students (draw names or cold call) to classify molecules in these three categories.
Very Polar: water, ammonia
Somewhat Polar: acetone, chloromethane
Nonpolar: propane, 2-methylpentane
NOTE: Some students may classify the molecules differently. Ask them to explain their reasoning; name the good points they have made, and correct any misconceptions.
3. (5 min) Remind students that molecules are attracted to other molecules through intermolecular forces: all molecules are attracted by Van Der Waals dispersion forces, the attractions created by temporary electron imbalances in the molecules. Somewhat polar molecules have the added attraction of permanent dipoles, and very polar molecules have the strongest intermolecular forces, hydrogen bonding. These bonds affect how cohesive a liquid is -- how well it sticks to itself.
Define COHESION: the attraction that a substance has between its own molecules.
Explain that liquids also stick to their containers to some degree, because of these same intermolecular forces. The attraction between molecules of different substances is called ADHESION.
Define ADHESION: the attraction between the molecules of one substance and another substance.
4. (5 min) In order for a liquid to form a good film over a surface, the forces of adhesion must be stronger the forces of cohesion.
Cohesion>Adhesion: Liquid pulls together, "beads up" (rounded droplet, contact angle >90 degrees)
Cohesion=Adhesion: Liquid forms a half-dome droplet (contact angle = 90 degrees)
Cohesion< Adhesion: The liquid spreads out into a flat sheet (flattened, wide drop, contact angle <<90 degrees)
The balance of cohesion and adhesion depends on both the surface and the liquid. A liquid that spreads out evenly over a surface is said to be WETTING that surface. The WETTABILITY of a liquid on a surface is a measure of how well it spreads out over that surface. (Write this definition on the board.)
5. (2 min) Show the students the clean glass slides using document camera or overhead projector. Show them each of the liquids and tell them that they will place a single drop of each liquid on one of the slides. Ask them to predict whether each of the liquids will wet the glass or bead up.
6. (2 min) Carefully place a drop of water on one of the slides. Show students how to use a ruler to measure the diameter of the drop: place the ruler under the slide and measure across the widest point of the drop. If the drop is irregularly shaped, measure its widest and narrowest points (e.g., 1.2 x 0.7 cm). Use the magnifying glass to measure with greater accuracy and precision.
Your results will vary depending on the type of glass used in your slide and how clean it is.
Water is moderately wetting for a typical microscope slide; a drop may measure ~1 cm across.
Methanol is highly wetting; it is likely that the drop will spread out so thinly that it begins to evaporate before it has finished spreading. Final drop spread may exceed 2 cm.
Heavy-chain organic compounds like mineral oil show a lower degree of wetting than water; a drop may measure ~0.6 cm across.
Test all three liquids on your slides prior to the experiment, and repeat your measurements at least 3 times to get a sense of the range of likely answers.
7. (2 min) Show students how to photograph the drops from the side. They should use their camera phones, place the slide near the table's edge, and place a dark piece of paper behind it. Focusing on the droplet will be a little tricky and probably will take two or three tries. The drop doesn't have to make up a very large part of the image; they will be able to zoom in later when they import the photo. The important thing is that the edges of the drop should be clear.
8. (1-3 min) Answer any questions students have, then release them to perform the initial experiment. Point out that they must complete the data table before leaving. Every person in the group is responsible to complete their own data sheet so that they will all have a copy of the data when they go home.
If a drop spreads out so thinly that it cannot be seen from the side, take the photograph of the space where the drop was placed. The contact angle that you measure later will be 0 degrees for this drop.
Guided Practice (8 min):
Students place the droplets on the slides, take pictures of the drops and fill out the data sheet. Circulate through the classroom and answer questions. Make sure that each picture includes some way of identifying the type of liquid (e.g., making sure that the bottle with its label is visible in the shot).
If students finish early, have them begin answering the analysis questions on the handout.
Clean Up and Debrief (2-4 min):
Have students dry slides and return all equipment to the prep area. Before the next class, they must (1) email their photos to the instructor and (2) answer all analysis questions on the handout.
If time permits, ask teams to share out their observations about which liquids showed the highest wettability, and check to see if all groups' answers agree.
Open the photographs of the droplets taken by each group. Organize the photos into folders according to which liquid is present. Place these folders somewhere that is accessible to the students (e.g., a Google Drive, a shared server, or a wiki). If this is not possible, crop the photos to just show the enlarged droplets and print them out for the students to observe.
Print up copies of the contact angles data sheet, one per student.
On the board, draw an empty data table that matches the contact angles data sheet. Students will add their data to this during the Guided Practice phase.
Intro (16 min):
1. (3 min) As they come in to class, students take out their completed analysis sheets and place them on the desk or table to be stamped or initialed (using whatever homework-tracking system you have). Students who are missing some of the data must fill in this information with the help of the students around them.
2. (5 min) Ask a student to summarize the results from yesterday's experiment, either orally or writing the results on the board. Ask other students to verify whether they got similar results or something different.
3. (3 min) Think-Pair-Share: Brainstorm possible reasons for why the liquid that showed the highest wettability had these properties. What do we know about the chemical structure of the liquid that might explain this?
4. (3 min) Draw names and share out answers. Write down all the possible factors that students came up with that could have affected wettability.
5. (2 min) If students haven't already identified it, point out that polarity is one factor that can affect wetting. Solutions that are highly polar will tend to have lower adhesion to highly nonpolar substances (e.g., water on wax paper) than nonpolar liquids will (e.g., oil on wax paper).
Direct Instruction & Modeling (17 min):
1. (4 min) Tell the students that their job for today is to design an experiment to test how one of the possible factors affected wettability. For example, in my IISME research, I had to start with a solution of methanol (CH3OH) and add different amounts of ammonium hydroxide (NH4OH), which breaks up in solution into positively charged NH4+ ions and negatively charged OH- ions. Ask the students: how do they think adding NH4OH might change the solution's wettability? Give them two minutes to discuss the possibilities with a partner and then write down their ideas.
2. (2 min) Solicit answers from students and write them on the board. Ask other students whether they agree with the answers that are offered. At this stage the most important thing is not whether the students' ideas are right or wrong, but whether they are testable.
3. (6 min) Write out a table with columns labeled Sample # (column 1), Volume of CH3OH (column 2), Volume of NH4OH) (column 3), Test Surface (column 4), and Measure (column 5). Remind the students that a scientific experiment may include many variables, but a good experimental design changes only ONE variable (the independent variable) and measures the resulting differences in one or more other variables (the dependent variables). Every other variable that can be CONTROLLED must be kept the same (controlled variables). For example:
|Sample||CH3OH (mL)||NH4OH (drops)||Test Surface||Measure|
|1||10||0||glass||contact angle & drop diameter|
|2||10||3||glass||contact angle & drop diameter|
|3||10||6||glass||contact angle & drop diameter|
|4||10||9||glass||contact angle & drop diameter|
Students must complete an experimental design table like this one for homework before Session 3. They do not need to choose methanol; they can use water or mineral oil as their liquid of interest.
4. (5 min) Explain that since the changes we are making may only have small effects on wettability, we need a more precise way to measure the contact angle of the drops than just observing them with our eyes. We will do this by examining the digital photos of the drops that we took in the previous lesson.
The method students can use to measure the contact angle will vary depending on the technology you have available. On a Mac, you can use PixelStick (http://plumamazing.com/mac/pixelstick/); on a PC, you can use Screen Protractor (http://www.iconico.com/protractor/). Both programs are shareware. If you print out the photos, students can measure the contact angles with protractors. Demonstrate the appropriate method of measuring contact angle for your classroom's available technology.
Guided Practice (10 min):
Have students measure the angles for all of the drops they photographed, add their data to the class's data table, and complete the contact angles data sheet. They will need the data from all of the groups in order to complete their own data tables, so it is important for all groups to participate fully. Circulate around the room and help students who are having trouble making their measurements.
Once students copy down all of the data, have them calculate the average contact angle measurements and drop diameters for each of the three liquids (water, methanol, mineral oil).
Water should have fairly large contact angle measurements: it is a light molecule, so dispersion forces are very small, but the hydrogen bonding force is very strong, so water is very cohesive. Water is also adhesive to the polar surface of the glass, however, so contact angles on glass will not be as high as on a nonpolar surface like wax paper.
Mineral oil does not have hydrogen bonding, but it is made of large, heavy molecules, so its dispersion forces are strong. This gives mineral oil great cohesion. It does not show high adhesion to glass because glass is a polar surface. Its contact angles should be larger on glass than on a nonpolar surface.
Methanol has some hydrogen bonding, but it is greatly weakened by the presence of the methyl group, so each methanol molecule only has one hydrogen that can engage in H-bonding. It is also a small, light molecule, so the dispersion forces are very weak. Because of these factors, methanol has very low cohesion compared to other two liquids. However, it is still a polar molecule, and thus it adheres well to a polar surface like glass. This combination of low cohesion and high adhesion allows methanol to spread very easily over glass. Contact angles will be close to zero. Methanol's low cohesion also means it will evaporate quickly, especially if your room has good ventilation.
Debrief (7 min):
Explain that the data from our first experiment gives us a range of expected values that we might see for contact angle measurements. This sort of discovery experiment helps us to make better predictions about the effects of changing our independent variable than if we were just guessing. For example, the mineral oil shows us how a dense, nonpolar liquid behaves on glass, the water shows how a light and highly polar liquid behaves, and the methanol shows how light and not-very-polar liquid behaves. If we add something to one of these liquids that will affect its cohesive or adhesive forces, we can now predict how its contact angle would change.
For homework, students must design their own experiment testing the effects of an additive on the wettability of one of these liquids. Each student must write a prelab for his/her proposed experiment according to previously-established class procedures. Briefly, this requires the student to state the purpose of the lab, describe the procedure, identify the measurements that will be made, and predict the expected outcome for the experiment. Students must have their prelabs checked and approved at the beginning of Session 3.
Possible additives you can make available to the students for the experiment include: acid (e.g., HCl), base (e.g., NaOH), salt (e.g., NaCl), water, heavier alcohols (e.g., isopropanol), and mineral oil. Remind students to choose ONE liquid to test and ONE additive for their experiment, with 4 samples testing different amounts of the additive.
An alternative experiment might be to test the liquids on different surfaces, such as plastic clingwrap, aluminum foil, ceramic and wax paper. I recommend being open to ideas from the students that will engage their curiosity, so long as their proposed experiment has a testable idea behind it.
In this session students will present their prelabs and then perform their experiments. Each group should have time to perform the experiments developed by each of the team members. By now most students should be well-versed in photographing the drops and making the measurements, and thus will be able to operate more independently. Circulate and provide help as needed. Make sure that all students complete their measurements before the end of the session, and leave at least 10 minutes at the end for cleanup and debrief.
Debrief the results of the students' experiments. Ask each group to present the questions they were investigating, the data they found, and conclusions that they drew from the data. On the board, keep track of these results (e.g., Add acid --> increase contact angle, Add mineral oil --> decrease contact angle). If two groups get opposite results, flag these two experiments for further discussion. Try to identify differences in technique or procedure that might have led to these differing results.
Based on the data, come to a consensus as a group about whether the glass slides show more adhesion to polar or nonpolar solutions. Work with the students to identify possible reasons why this might be so. If the data are not clear, brainstorm possible ways that the class could research the matter further. This could lead to an online research project if you have the time and the students have the inclination.
Go over the guidelines for the lab report that students must write about the experiment. They only need to describe the Methods and Results for their own experiment, but in the Discussion section they should include the insights gained from the other students' results. Give the students at least 3 days to write the lab report and turn it in.
For grading the lab report, use the lab report rubric. An "A" in a section is worth the full listed number of points (5 or 10). A "B" is worth 60% of the listed number (3 or 6 points), and a "C" is worth 40% of the listed number (2 or 4 points). A "D" is worth 1 point.
1.) Data Sheet & Analysis Questions
2.) Contact Angles Data Sheet
3.) Prelab & Lab Report Guidelines
4.) Drop photographs from Session 1 (shared digital access or hard copies)
small bottles of water, methanol, and mineral oil (<5 mL of each are needed per group)
droppers or pipets for each bottle (3 per group)
clean glass slides (3 per group)
centimeter rulers (1 per group)
dark sheets of paper (1 per group)
magnifying glass (1 per group)
digital cameras, or students' cell phones with built-in cameras
overhead projector or document camera
Additional Supplies for Session 4:
These are variable depending on your supplies and the creativity of your students. They might include acids, bases, salts, or organic solvents, depending on what you have at your disposal and what your students decide to try. Students may also bring supplies from home if they need something unique, but encourage them to only test pure substances because it will make the results easier to interpret.
Bibliographic or other resources you used in creating this curriculum:
Clark, Jim. 2012a. "The strengths of Van Der Waals dispersion forces." Chemguide. Web. 8 Jul 2013.
Clark, Jim. 2012b. "Intermolecular bonding - hydrogen bonds." Chemguide. Web. 8 Jul 2013.
Goodman, Jeff. 2001. "Water drops: cohesion and adhesion." Appalachian State University. Web. 8 Jul 2013.
Wikimedia Commons. "Wettability." Wikipedia. Web. 8 Jul 2013.
Keywords:adhesion, cohesion, wetting, wettability, polarity, experimental design, scientific method, inquiry based, intermolecular forces