The Search for Surfactants: What Is the Best Soap?
Although this paper clip is denser than water, it floats on the surface due to surface tension.Copyright 2009 Alvesgaspar, Wikimedia Commons https://commons.wikimedia.org/wiki/File:Surface_tension_March_2009-3.jpg
Student teams are challenged to evaluate the design of several liquid soaps to answer the question, “Which soap is the best?” Through two simple teacher class demonstrations and the activity investigation, students learn about surface tension and how it is measured, the properties of surfactants (soaps), and how surfactants change the surface properties of liquids. As they evaluate the engineering design of real-world products (different liquid dish washing soap brands), students see the range of design constraints such as cost, reliability, effectiveness and environmental impact. By investigating the critical micelle concentration of various soaps, students determine which requires less volume to be an effective cleaning agent, factors related to both the cost and environmental impact of the surfactant. By investigating the minimum surface tension of the soap, students determine which dissolves dirt and oil most effectively and thus cleans with the least effort. Students evaluate these competing criteria and make their own determination as to which of five liquid soaps make the “best” soap, giving their own evidence and scientific reasoning. They make the connection between gathered data and the real-world experience in using these liquid soaps.
The study of surfactants, surface tension and the critical micelle concentration has many engineering applications. In the search for more efficient extraction of oil from underground reservoirs, primary and secondary techniques (pumping and washing with water) only remove ~30% of the total oil present. Using enhanced oil recovery techniques, chemical and petroleum engineers design surfactants that are low-cost, safe and effective at greatly reducing the surface tension because when the surface tension is lowered enough, trapped underground oil can be more easily washed out of the small pores of rock structures.
As another example, chemical engineers design soaps and cleaners to lower the surface tension of the water, which lowers the force between molecules, enabling water to more effectively bond with dirt and oil particles during washing, and thus achieve cleaner dishes and hands. Engineers in this field design soaps to be cost effective, good cleaning agents, non-toxic and efficient.
As an example of electrical applications, chemical and electrical engineers manipulate the surface tension of printer ink used in inkjet printers to specifically control the droplet size sprayed onto paper. Larger droplets require much larger surface tension to hold the droplets together. So engineers design ink that has low surface tension so that only small droplets can form, therefore enabling the creation of high-resolution images (high dots per inch, or dpi).
After this activity, students should be able to:
- Define surface tension and describe how it can be measured.
- Describe the effects of surfactants like soap on surface tension.
- Relate the surface tension of an aqueous solution to its ability to clean.
- Name some real-world applications in which the control of surface tension is important.