Surfactants – Detergent vs. Soap

Detergents are not soaps. Both detergents and soaps are cleaning surfactants. They both have a hydrophilic (water loving) end and a hydrophobic (water-fearing/fat loving) end. However, the hydrophilic end of soaps is made of a carboxyl group. It has a single negative charge distributed over two oxygens. It can form a salt with a positively charged ion like sodium or potassium, or it can form a salt between more than one soap molecule and magnesium or calcium, the minerals that make water “hard.” The hydrophilic end of soap is not water-loving enough to make both the soil and soap soluble in a way that the combo mixes evenly with water. Soaps form balls around soil and basically float to the surface of the water. This is fine if you’re applying a stream of water and friction to wash away something, but not ok in a washing machine where the water drains and whatever’s floating has the potential to settle back onto clothes. It also results in soap scum when it combines with hard water.


Detergents can have various hydrophilic ends, but one of the most common are sulfate groups. Regardless of what makes up the hydrophilic end, it’s more water-loving than the carboxyl group of soaps. Even when it’s holding onto hydrophobic soil through chemical interactions, the whole thing is generally water soluble and will rinse away cleanly. Detergents are also much less likely to interact with magnesium and calcium, making them more effective than soaps in hard water and also less likely to contribute to soap scum. Detergents can be either plant based or synthetic.


In a laundry setting, soaps are great stain fighters as long as they’re used sparingly and in the presence of a strong detergent to wash them away after their job is done. Examples of such laundry soaps are Fels Naphtha and Zote Soap.


Superfat refers to the amount of oil that remains after saponification (the chemical reaction between hydroxide (lye) and an oil). Obviously any oil added to the mixture will behave like soil, so you wouldn’t want superfat in a laundry soap used to treat stains, but the soap itself is the problem, not the superfat.


Sodium cocoate is a SOAP found in many laundry detergents. It’s not inherently bad, but if it’s paired with a weak detergent or included at too high a concentration, it can build up just like any other soap. That’s why ALL (brand) Free & Clear liquid and other detergents containing this ingredient are a no go for absorbent items, like towels.


By Amanda Perez