Through the years, I've gotten some odd looks and confused responses when I tell people the soaps I sell are detergent free. The word "detergent" has become almost interchangeable with "soap" in our culture, so many wonder how the soap can clean if there's no detergent in it. You have to look at the process of making each and their chemistry to see the difference in the two, so it's easy to see why there's some confusion.
The biggest difference between soap and detergent is the sources used to make them. Soaps are made from oils or fats treated with an alkaline such as lye in a process called saponification. Detergents are made from synthetic sources.
Both soap and detergent act as surfactants. What that means is they break the surface tension responsible for causing water to "bead up" on surfaces, and make it spread out. It's also described as "making water wetter."
They are also both "two ended" molecules. One end of the molecule loves water and hates oil. The other end hates water and loves oil, so to speak. So in addition to helping water wash over whatever you're cleaning more effectively, it basically grabs onto the oil and pulls it along when it flows away with the water.
So why should I worry about detergents being in soap?
If your skin isn't sensitive, you might not need to concern yourself.
The biggest problem with detergents, the most common of which are sodium laurel sulfate (SLS) and sodium laureth sulfate (SLES), is the fact they are irritants. That's one of the reasons they have to be "watered down" in shampoos, soaps, and other detergents. It's also the reason many with skin sensitivities may begin itching within minutes of using a product containing SLS or SLES.
Then there's the fact detergents are stronger than soap.
On the surface, that might seem like a clear win for detergents. It certainly is when it comes to washing clothes and dishes, especially for those with hard water. However, our skin and hair doesn't need to be "squeaky clean." In fact, that squeaking sound usually means the surface has absolutely no oil left to lubricate it. While this might be good for your dishes, it's not when you're talking about skin or hair. Excessive oil needs to be washed away, sure, but your skin and hair also produce that oil for a reason! It's a protectant meant to keep the skin and hair from breaking.
This is why it can seem like your skin and hair gets oilier and oilier the more often you shower. Quite often "oily" skin is actually dry skin that's trying desperately to heal itself by ramping up oil production to replace what's being washed away.
It's a fact I wish I'd known during my teenage years, when I'd wash my face two, three, sometimes even four or five times a day because I felt "greasy."
Last, there's the process of saponification itself. The chemical reaction between the oil and alkaline solution produces water, glycerin, and a mineral salt. The mineral salt is the component that makes soap work. Water's a byproduct, and it tends to evaporate away over time, especially if the soap is unwrapped. (That's what causes soaps to shrink and harden a little over time if they're not used.) Glycerin is a humectant, which means it attracts water to itself. It's the reason soap left unwrapped in a humid environment can sometimes collect a dew, and it's also what makes some soap bars moisturizing.
Some commercial soaps leave the glycerin in the bars, or add it, depending on their manufacturing process. Others separate it out to sell to skin cream manufacturers.
What it comes down to is what kind of cleaning agent works best for the job, and what your body is telling you.