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Name: Sara
Status: student
Grade: 9-12
Country: Austria
Date: Fall 2011


Question:
In my textbook, it is written: Teflon is poly(tetrafluoroethene) PTFE. The instantaneous induced dipole forces between oil or grease and PTFE are much weaker than those present in the oil or grease itself. This gives rise to the polymer's non-stick properties. My question is: So what if the forces in oil were stronger? There are still forces between oil and PTFE; so why does it not stick?



Replies:
When you pour oil into the pan the oil molecules are only interacting with other oil molecules or air. When the oil molecules hit the PTFE it will start to interact with the PTFE but when you then swab the oil out the oil there is a "tug-of-war" situation with the oil molecules on the PTFE surface being pulled toward the PTFE by weak interactions with the PTFE and pulled towards the bulk of the oil by strong interactions with oil molecules. The weak interactions with the PTFE surface lose.

Greg Bradburn


It is essentially a matter of priorities. If a molecule of grease is adjacent to a PTFE surface, the intermolecular forces holding it there are not very stabilizing. It is more favorable for the molecule to be surrounded by other grease molecules. If it were more stabilizing for the grease molecule to stay on the PTFE surface, then the grease molecules would spread out over the surface. But instead it is more stabilizing for the molecules to be with each other.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


Sara, let me introduce an analogy that might help. Think of yourself and how you relate to other people. You want to hang out with your friends, and if you have a group of friends you really like, you might all hang out together, forming a clique. If there are some other people around that you do not like, you would want to avoid them. You and your friends might form a very tight group, and avoid contact with the folks you do not like. However, if you are with a bunch of people you like equally, you might be willing to interact more broadly, and not form a small, tight group at all.

The oil molecule behaves in the same way. In the analogy, the other "people" are other molecules -- either oil, or PTFE, or anything else. Because oil molecules are much more strongly attracted to other oil molecules (their "friends"), they tend to congregate together, forming droplets of oil. If the oil molecules were strongly attracted to a pan's surface, they would try to maximize their contact with the surface of the pan -- forming a thin layer across the pan (this is known as 'wetting').

I would also point out that non-stick pans are targeted not so much toward oil, but more for materials that tend to stick -- especially proteins. If you were to drop an egg onto a steel pan, with no oil, it would stick quite strongly. The uncooked egg proteins bond well to the steel. The oil wets the steel pan and prevents the egg proteins from chemically bonding to the pan. Once the egg proteins are cooked ("denatured"), they no longer will stick to the pan. Hence, the cooked egg will slide right out. However, with a non-stick pan, you may not need oil at all. The liquid egg proteins don't bond well to the PTFE, so no (or less) oil would be needed.

The attraction between molecules, whether it is egg, oil, steel, or PTFE is not like an on/off switch; there is a full continuum of attraction between molecules from high to low. If you drop a liquid droplet on a surface, the level of attraction between the surface and the oil will govern the shape of the droplet it forms. If a liquid is strongly repelled by a surface, it will form a nearly spherical droplet on the surface (maximizing contact with other liquid molecules, and minimizing contact with the surface). If it is only a little attracted, it might form a low, flat droplet. If it is highly attracted, it will spread out across the surface and not form a droplet at all. Viscosity also plays a role in the droplet shape -- it is intuitive to see that thicker liquids may not wet as fully as thinner liquids.

For more information, I suggest you read more about "hydrophobicity" and "droplet contact angle". These are both related to a concept known as "surface energy", which basically means "how unhappy is a molecule to be with the molecule next to it".

Hope this helps, Burr Zimmerman





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