why is it so much more difficult to wash oil off plastic containers than glass or metal ones
In: Chemistry
Oil and plastic are both made from *non-polar* molecules. *Broadly speaking*, molecules will either be polar, like water, or non-polar, like oils. Water is polar because the oxygen and hydrogen don’t share their electrons equally. Oxygen holds them a bit harder, making the oxygen side slightly negatively charged and the hydrogen side slightly positively charged.
Polar molecules stick to other polar molecules very well, like sticking magnets together. They do *not* stick to non-polar molecules very well at all, which is why oil and water don’t mix. Non-polar molecules stick to other non-polar molecules very well – which is why oil and fat will mix, or why gasoline will dissolve grease.
Plastics are made from non-polar molecules that come from processing crude oil. They are stuck together mechanically: the molecules are long chains of atoms that are tangled up like Christmas lights. Being made of non-polar molecules is one of the things that makes plastics so useful, because it means water *won’t* stick to them well, which means water can’t dissolve the plastic. However, that does mean that oil and grease will stick to some plastics.
Glass is made of polar molecules (silicate, mostly). Metals are neither polar nor nonpolar because there’s no covalent bond sharing (or failing to share) electrons. All of the atoms in the metal are sharing their outer electrons. Teflon is polar, kind of: the fluorine atoms in it are *super* electronegative (meaning that, like oxygen, they hold onto the electrons very hard; fluorine holds them harder than oxygen). Those fluorine atoms are bonded to carbon atoms, which are surrounded by the fluorine atoms. Rather than having a negative side (fluorine) and positive side (carbon), the positive side is completely surrounded so the surface is nothing but negative, making it impossible for polar *or* non-polar molecules to stick to it easily.
Plastic is porous in a way that oils like to slip in to it. Which is why greasy and oily foods can colour the container, especially when the surface is worn and scratched.
Meanwhile metal* and glass isn’t. (Steel and aluminium are, but not so much that oil or greasy can soak in to the structure in a practical sense, but give it a long time and you’ll never get that smell out of it).
Latest Answers