How Soap Works

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The best soaps leave your skin clean while adding and maintaining natural oils to keep your skin from drying out.  But have you ever wondered how soap works?  If you think about it, would you ever consider washing your hands only with oil?  Sure, your hands would be soft and smooth, but they wouldn’t be very clean.  How about washing your hands with baking soda?   They would be cleaner than with the oil, but the basic pH of the baking soda would leave your hands dry and cracking.  Amazing things happen when you combine oil with a base.  You wind up with a soap that cleans while it moisturizes.  Lets find out how soap is made, and how soap works!

Oils

Oils are around us all the time.  Our bodies produce natural oils all day long, we use oil to cook in, and we use oil to help keep the world around us protected from over drying.

Not oils are created equal.  How fun would that be if they were?  For instance, peanut oil is liquid at room temperature and can withstand the high temperature of frying, while coconut oil is solid at room temperature and is best suited for lower heat cooking.  Each oil is made a of unique combination of fatty acids, but they also have a glycerol molecule attached.  This is the chemical make up that will make it possible for oil to become soap.

Any fat molecule that you come across is actually a salt, at least chemically speaking that is.  The definition of a salt is a positively charged ion bonded to a negatively charged ion.  In oil, the fatty acid groups are the positively charged ions, while a glycerol group has a negative charge.

The Base

The most common base used in soap making is caustic lye, or Sodium Hydroxide.  Occasionally people will use caustic soda, potash, or Potassium Hydroxide.  The resulting compound using the caustic soda is a softer soap, but the chemical reaction remains the same.

The Reaction

The fat molecules and the sodium hydroxide have what are called ionic bonds.  Relatively speaking, these bonds are weak and can be broken apart easily.  It is similar to having a seam sewn versus a seam that has velcro.  More stable, stronger bonds are formed in a way that makes the connection strong like a sewn seam.  Ionic bonds are more like velcro, they can be taken apart and reattached, or even put together in different combinations.

When to lye mixes with fats you will get a slow transformation into soap.  In order to speed this process we use water to break apart (hydrolysis – using water to cut something apart) the sodium ion from the hydroxide ion, and the glycerol molecules from the fatty acids.  The process produces heat on its own, but a soaper can add additional heat to speed the process.

After the fat and lye break down into their separate components the sodium attaches to the fatty acids and the hydrogen and oxygen molecules join with the glycerol to make glycerine.  This process is called saponification.  There are other ways that soap can be made, but not by the home soaper.

How Soap Works

We’ve seen how soap is made, now lets look at how soap works.  Remember the little bit about the ionic bonds?  Well, a soap molecule will have one end (sometimes called the head) that has a small magnetic charge (or is considered polar), and one end that does not.  The end that does not have a charge attracts oil and grease particles.  As the grease is gathered up by the soap molecules the charged heads are attracted to the water molecules.  They then gather together and trap the grease suspended in the water (called a micelle) so it can be washed down the drain.

how soap works

That is why water gets that dingy gray color as you wash up.  Oil, grease and grime are suspended inside these micelles where they are waiting to rinse away.  The dirt will stay suspended provided there is enough soap in the water to keep the grease separated from the water.  If there is more grease than the soap can handle these micelles will break apart allowing the grime to be redeposited on what ever you are trying to wash.

Cool, that is the clean part.  But what about the moisture?  Remember, I said glycerin is produced in the saponification process?  That is the magic moisture source.

Glycerin stays near the surface of the skin and draws moisture.  The moisture can come from deeper layers of the skin (that is then replenished by staying well hydrated), and it can be absorbed from the air.  As the moisture in the air or deeper in the skin is attracted to the glycerin, the cells then drink in that water and the glycerin goes back to work attracting more.

Pretty neat, right?  Yeah…  I’m a nerd.  But that is okay!  I’ll be the nerd and study all this sciency-stuff so you can either read and learn along with me, or just benefit from me digging into how soap works.

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