What is the simple chemistry behind soaps, how is it polar and non polar at the same time?


Why does the formation of soap allow it to pick up different items?

In: Chemistry

soap molecules are relatively long molecules. The arrangement of the atoms (and their electrons) leads to one end being somewhat charged (polar) and the other end non-polar. Water is polar, so the polar end of the soap tends to “stick” to water, while the other end tends to stick non-polar things like grease. When you rinse with water, the water pulls the soap with the attached greasy thing down the drain

Think of it like an adapter. Soaps have a small polar “head” (e.g. a sulfate group or carboxylate group) followed by a tail of nonpolar stuff (i.e. plain hydrocarbon chain). One end goes on the grease, one end goes on the water.


Imagine a soap molecule is a long chain with two special velcro hooks on each end.

One end of the chain likes to hook to water, the other end of the chain likes to hook to oils.

When you cover your hands in soap, you’re moving these chains around your hands, and they find all the oils and nasty stuff and hook onto it.

When you run the water, the water hooks onto the other end of the chain and carries the soap away, taking the nasty stuff with it.

It looks like you’re thinking binary: something has to be either polar or non-polar, it can’t be both one thing and the opposite. But think of it like a shoestring. Can a shoe-string be both frayed and non-frayed? Yes: frayed at one end, not frayed at the other end. Pokey and pointed at one end and the opposite on the other, soft and spread out.

To get just a teensy more complicated, more ELI6, “polar” is just a label, and it’s not being applied to the whole molecule all together, but to the two ends: one can be polar and the other not. The Q is: what kind of ***local*** electrical field is there at one end or the other of the molecule? Does the *local* (confined to that end) field have a neat attractive charge in its *immediate* surroundings? If I glue a one-inch magnet to one end of a foot-long wooden stick, one end of the end result is magnetic and the other not. Soap is like that.

When a bunch of soap molecules ( o==== where o is “head”, === is tail) get together, they line up with all their “heads” and “tails” pointing the same way …. like a raft. If the raft gets big enough, it can curl back on itself and form a ball…. usually with all the “heads” on the surface, and all the “tails” pointing inside. Things like grease will get stuck to the raft and carried away with the soap — it’s like velcro for gunk