Before soap labels can make sense, the distinction between soap and detergent must be made. Soap and detergent are the two main category of cleaners we use, both in home cleaning products and on our bodies. I have a natural “soap good, detergent bad!” prejudice when it comes to my skin. I decided to do some research and see if this is my inner crunchy-mama going on a blind rampage or if this is a legit prejudice. What, exactly, is a detergent? Why I have heard that they’re bad for my skin? And why did we invent them to begin with?

Soap and detergent are both surfactants which technically means they break the surface tension of water, i.e. they stop water from beading up. Remember that oil and water don’t mix? That’s why you can’t just stand under your shower and come out feeling clean. Water by itself will carry away vinegar and dissolve any salt and sugar off of you, but it’ll glide right over mayonnaise, olive oil as well as all that dirt that your natural skin oil has collected over the day. Surfactants, for some molecular reason I don’t exactly follow, give water the ability to carry away oil and all the dirt it has attracted. Hence we use surfactants to clean!

What is soap?

Soap is the chemical result of combining fat with alkali (alkali is a base, as in the opposite of acid, that dissolves in water). Usually sodium hydroxide (caustic soda) is the alkali used to make hard soap and potassium hydroxide (potash) is used to make liquid soap. Either alkali is commonly referred to as lye. When you mix water with lye and then with fat in correct proportions, a reaction occurs (called saponification) which results in soap, water and glycerin (a moisturizer that’s safe enough to eat, also labeled as glycerol). Traditional soap is made with animal fats, typically lard or tallow (pig or cow fat), but it can also be made with plant fats, such as coconut oil, olive oil and shea butter. Different types of fat produce different properties in the soap. For example, lard adds longevity to a soap. Olive oil adds moisturizing properties. Coconut oil adds lather. Most soaps intended for use on skin are made with a combination of fats with different properties to achieve a well-balanced bar of soap. Soaps for use in laundry are typically made with a single fat, such as tallow or coconut oil, that cleans without moisturizing.

Soap has a history dating back thousands of years. The earliest known use of it was in 2800 B.C. in ancient Sumeria (modern Iraq). In the first century AD, Pliny the Elder reported that this new-fangled soap thing was created by the Gauls and Germanic tribes. (Apparently ancient Sumeria didn’t share.) By 300 AD, the Romans were using it, too, with claims that the Germans made the best soap. Since then, soap has been in pretty regular use throughout Western history. (Eastern civilizations used different cleaning methods that I don’t know much about!) The first World War, as we all know, changed life as we know it by forcing innovation when materials were needed for the war effort and were scarce for home use. The fats needed to make soap were one these scarcities. What to do? Enter science!

What is detergent?

In 1916 Germany invented detergents as a cleaning substitute, specifically for use in laundry and dishwashing (although now detergents are used in personal cleaning products, often called “beauty bars” or “body/hand wash”). Detergents combine lye with the hydrocarbons found in either petrochemicals (petroleum derivatives) or oleochemicals (fat derivatives). Back in the early 1900s, petroleum was considered a plentiful resource, so they thought it was pretty awesome to find more uses for it.

Detergents, due to being entirely synthetic, can be engineered to work in a variety of circumstances. The one huge drawback of real soap is that it reacts with mineral salts–like calcium, magnesium, iron and manganese, the components in hard water–to form insoluble deposits, i.e. soap scum. If you have hard water, real soap doesn’t work as well and dissolves extra fast due to how much of it is used up reacting with mineral salts. I’ve noticed this with my own soap when trying to wash my hair in hard water. It leaves a grody film all over my roots. Not clean! Detergent doesn’t necessarily do this. (At least, it doesn’t do this when phosphates, a water softener, are included in the formula. I’m unclear whether detergents need phosphates or a phosphate alternative to prevent the same hard water problems as soap.)

Detergents, however, have an environmental downside as they tend to be persistent (i.e. don’t biodegrade) and those aforementioned phosphates encourage algae growth that consumes all the oxygen in water, killing aquatic life. Detergents can also contain a variety of thickening agents (like cocamide diethanolamine, i.e. cocamide DEA), fragrances (soap frequently also contains this practically meaningless “ingredient” that allows companies to add an unlisted chemical cornucopia), foaming agents (like sodium lauryl sulfate) and preservatives (like methylchloroisothiazolinone). Unlike detergent, true soap without fragrances or colorants is biodegradable with negligible to no environmental impact.

So what are you supposed to use?

It’s up to you. Because of my dry, easily irritated skin and the environmental questionability of detergents, I’m personally moving away from detergents and toward soap in my personal cleaning products. This has required me to get water filters for my showers because central Texas has terribly hard water and I don’t like grody hair. I haven’t used up the detergents yet in my laundry and dish cleaners, so I’ll make decisions about them after they’re gone and I’ve had time to experiment with homemade cleaners.

*phew* That took longer than I thought! Next week I’ll start listing ingredients and what they actually mean. What do you think about soap vs. detergent? Do you have a preference and why?

~ Featured Image: Five Soaps by 朱亦洁