Let's Make Some Soap!

Most of us (or at least I hope) have used soap since birth, but we haven't really wondered how it works and what it is made of. We will delve deeper into the formulation of soap in this blog post:

Hydrolysis of these fats and oils yields glycerol and crude soap.

Saponification is the fancy schmancy term for making soap. It comes from the Greek root sapo- which means soap. "Basic"ally, it is the chemical reaction between an acid and a base to form a salt. You need to mix an oil, or a fat (which is your acid) with Lye, which is the soapmaker's term for sodium hydroxide, a base. This forms soap, which is a salt.

So how does the mixing of these two different things combine to form something like soap. In order to understand this, you must consider the chemical makeup of the acid and base being used in the reaction.

The base can vary, but it has to have one hydroxide ion. Lye, which as stated above is sodium hydroxide, is often used, but other bases such as potassium hydroxide can be used as well. Potassium hydroxide is more commonly used in liquid soaps, because it is more soluble than sodium hydroxide.

There are also many different types of acids that will react with your base to "saponify". It could be olive oil, coconut oil, or tallow, which is animal fat. These may not seem like the type of acids you might be used to (HCl, HNO3, etc.), but these are all "fatty acids", which is a carboxylic acid group with a long hydrocarbon tail. The carbon tail gives the soap its hydrophobic qualities.

Arachidic acid, with a carboxylic acid group on the right and a 20 carbon tail

Each acid has a unique combination of triglycerides (a compound made of three fatty acid tails attached to a single molecule of glycerol), which combines with the base differently. The fatty acid tails join with the base to make soap, and glycerol is leftover. This glycerol molecule keeps the soap moist.

The OH group from the base makes the soap hydrophilic, and the fatty acid tail makes the soap hydrophobic, thus overall, the soap molecule is amphiphilic. This amphiphilic property is essential in soaps, because the hydrocarbon tail will mix with the oil and grease on your hands, while the ionic end mixes with the water. This allows all the gross stuff on your hand to be pulled away by water, where normally, it would have just repelled the water.

bar soap is typically made from NaOH

Now you know about the soap making process, time to get your hands dirty and make your own!

Here is a recipe taken from Nuffield Foundation's Practical Chemistry

MAKING SOAP:

Making soap
a Place about 2 cm³ of castor oil in a 100 cm³ beaker using a dropping pipette, followed by 5 cm³ of ethanol. Stir with a glass rod to mix.

b Add 10 cm³ of sodium hydroxide solution..

c Prepare a waterbath containing near-boiling water from an electric kettle so that you can safely lower the small beaker into it without spillage. A 250 cm³ beaker may be used as the waterbath. Do not use too much water, as the small beaker needs to be supported without risk of the water coming over the top.

d Stir the mixture in the beaker with a glass rod for 5 minutes. If the water bath cools too much, you may need to renew with fresh boiling water.

e Meanwhile in a boiling tube make a saturated solution of sodium chloride by shaking solid sodium chloride with 10 cm³ of water until no more will dissolve. Allow to settle.

f After 5 minutes, add the saturated sodium chloride solution to the small beaker and stir.

g Cool the mixture by changing to a cold water bath (or an ice bath if available).

h Soft, white lumps of the soap will gradually form in the mixture. Leave for a few minutes to improve the yield. During this time the soap may rise to the surface and form a soft crust on cooling.

i Using a pump, with a fresh filter paper damped down in the funnel, filter off the soap, breaking up the crust with a glass rod if necessary

j Allow the soap to drain on a paper towel – do not touch it with your fingers, as it may still contain sodium hydroxide.

k Use a spatula to transfer a little of the soap to a test-tube, and add a few cm³ of purified water. Shake well! What happens? You have made a soap!

cups of homemade soap! The color comes from melted crayons

jennymu