After cooking I fill my frying pan with water to let it soak. The result is lots of globules of grease/fat floating on the waters surface. When I put a drop (even the tiniest drop) of washing up liquid in the water filled pan all the globules are quickly repelled from the point of impact, they usually retreat to the edge of the pan.
Is this due to surface tension? Is this due to some repellent property of washing up liquid? Any answers greatly appreciated!
Dish soap contains surfactants, which reduce the surface tension of the water. So you toss in a drop of dish soap at the center of a pan full of grease-laden water, the surface tension in that central area drops, and the surface tension on the rest of the water pulls that greasy layer of water toward the edge.
The grease doesn’t really have anything (chemically) to do with that particular phenomenon; it’s just along for the ride. When I was a kid, we sometimes did the same trick with a bunch of ground pepper sprinkled on top of the water, and got the same effect.
we used to do something similar to estimate the cross section of soap (SDS) molecules.
Divide area by number of molecules to determine average cross sectional area of a single molecule. Calculate the average diameter of the SDS molecules.
This assumes the surfactant forms a monolayer film where the molecules are standing more or less normal to the water surface.
I just came in to ask your location because I have never heard the expression “washing up liquid.” In the U.S. I’ve only heard “dishwashing liquid.” Or “detergent.”
It’s a UK thing.
Thanks a lot for the answers guys, however I now have an additional question: Why does the grease get repelled only once?
When I put another drop of washing up liquid (dishwashing liquid;)) in the pan nothing happens. After all the grease gets repelled to the sides of the pan I can’t make them get repelled again, even if I put another drop of washing up liquid right on top of the globules. How can one drop of detergent effect the whole pan?