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Hydrophobic vs. Hydrophilic, Polar vs. Non-polar

Tom Kuntzleman's picture
Mon, 05/13/2013 - 20:47 -- Tom Kuntzleman

Wow! Talk about an interesting idea! A very neat experiment, called “Hydroglyphics”, has been published by Philseok Kim, Jack Alvarenga, Joanna Aizenberg and Raymond Sleeper in the Journal of Chemical Education. This experiment allows you to transform a common plastic Petri dish into a unique teaching tool that can be used to demonstrate the difference between hydrophobic and hydrophilic surfaces. Check it out in the video below.

Corona discharge from a Tesla coil is used to promote a reaction between the hydrophobic, polystyrene Petri dish with oxygen in the air. The reaction causes oxygen to be added the polystyrene, forming carbonyl and hydroxide groups, rendering it hydrophilic. Regions on the surface of the Petri dish protected by a sticker do not react with oxygen and remain hydrophobic. Water added to the corona-treated Petri dish “sticks” to the hydrophilic portions, but avoids the hydrophobic portions of the dish.

As long as you have a Tesla coil (which costs about $200), the experiment is remarkably simple to conduct. The video below outlines in detail how to make these interesting Petri dishes. See the article for more information on this experiment, including a discussion of the chemistry involved in the chemical conversion of polystyrene to oxygenated polystyrene.

I hope you get a chance to try this experiment. If you do try it, I’d love to hear about any interesting modifications you discover. I’ve tried this experiment myself, and have shared it with many of my college students (they enjoy it, too). Having conducted this experiment several times now, I have two questions I’m curious about. Maybe some of you can help me understand what is going on here; please comment if you have some possible answers. First, when you breathe on the special Petri dish, water from your breath appears to condense on the untreated, hydrophobic portion of the dish, but not on the hydrophilic, corona-treated portion of the dish. This effect seems backwards to me. Second, because oxygen atoms are inserted into the polystyrene chain during corona treatment, it would seem that the mass of the dish should increase upon corona treatment. However, (I’ve only done this experiment once), I have noticed that a corona-treated Petri dish actually showed significantly lower mass upon corona treatment. Why might this be so? I look forward to hearing from you.