Welcome back to my channel. Today we're going to be looking at soaps. Soaps are made by alkaline hydrolysis of fats and oils.
So here's the structure of a fat or an oil where R is the rest of the chain, so it's a long chain fatty acid. When we hydrolyse the fat or oil, we split the bond here to break off the fatty acids and at the same time we're neutralising to produce a salt. So when we split this up we end up having glycerol. which can be separated and used for other purposes and this salt here of the fatty acid which is the soap molecule let's have a look at soap's structure soap has a structure which is in two parts it has this end here which is hydrophilic hydrophilic means water loving And this is the polar head part of the soap molecule.
So that's up to about here. And then this part here is hydrophobic. That means that it repels water and it's a long chain, non-polar tail.
Having both of these parts is essential to how soap works. This part is soluble in non-polar things like grease and stains. This part is soluble in water which we use to wash stains away.
Here is the polar head and then we will draw out the chain as a kind of zigzag line where each corner is one of the carbons. Sometimes this is simplified even further just to be a head and a tail if you're doing the sketch to try and explain how soaps work. Let's now have a look at their mode of action.
So stains themselves are usually greasy and non-polar, so they don't wash out with water. So if we draw a stain here, so I'll draw that in green. So here is our stain. It's a greasy, non-polar part.
When we try and wash this away, water can't wash it away because like dissolves like rule means that these two are not miscible. So when we introduce some soap, we have the tails of the soap, which are non-polar, will dissolve into the stain. The heads of the soap. which are polar and negative. We've now dissolved a salt into water so they've dissociated so we're only showing the negative head parts here.
They stay on the outside of the stain. We then need to agitate that so you would do that by rubbing it or if it was in a washing machine. Agitation lifts the stain from the clothing or from whatever you're trying to wash to form small balls. So here we have these little balls of grease which have the non-polar tails stuck inside them.
and on the outside they have the polar heads. Now these polar heads are all negative, so they repel away from each other, so that the little balls of grease can no longer stick back together. We call these things that we've formed here my-cells. We can now wash these away with water. because the negative charges on the outside are soluble in water so we can just wash away these little parts of grease soaps are formed with the carboxylate head and if they're used in places where you have hard water so you've got calcium and magnesium ions present then you can get a scum forming where that is the case detergents tend to be used and they have sulfate heads instead which stop the production of the scum forming.
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