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We have used the experiment entitled "The Spectrophotometric Analysis and Modeling of Sunscreens" (J. Chem. Educ. 1997, 74, 99-102) with great success in our undergraduate labs. The students respond well to its illustration of fundamental principles of spectrophotometry with an exercise that provides a real-world application familiar to them all. The "real world" has, however, thrown a spin in recent years that needs to be accounted for when running this experiment.
The method as reported requires the initial formation of a colloidal emulsion--a mixture of 0.050 g of sunscreen in 50 mL of water. This is problematic when modern waterproof sunscreens are used. Common sunscreens such as Hawaiian Blend, Coppertone, and Iguana do not form effective suspensions with water because of their hydrophobic nature. Only traces of these sunscreens are emulsified, and their resultant absorbance data are skewed. By replacing the water from the initial step with 2-propanol, the problems with suspension formation are overcome. All the sunscreen formulations tested in this fashion satisfactorily demonstrated the correlation between absorbance and sun protection factor.
We have found that this protocol modification lends itself well to a directed inquiry laboratory module. It is immediately apparent to students that some sunscreens behave anomalously when water is used, but it takes more thought for them to deduce why and to move toward a solution. Students in our labs tested hexane, acetonitrile, toluene, and dichloromethane as other possible extraction solvents, but 2-propanol was found both more effective and less toxic than the other choices.
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