Surfactant (Surface Active Agent) Deep Dive
Definitions:
Surfactant: umbrella term for anything that has a two-part structure made of a hydrophilic (attracted to water) head and hydrophobic (attracted to oil) tail.
Soap: a natural surfactant made through saponification of fat or oil + alkaline solution
Detergent: a synthetic surfactant derived from petroleum or modified from plant-based oils
Why choose detergent over soap for laundry?
Because of hard water. Unless you live somewhere with very soft water, soaps will build up in your washing machine and your clothes. Soaps are found in the formula as a salt, usually bonded with sodium. However, the mineral ions in hard water, like calcium and magnesium, are more attractive to the soap molecule than sodium. So the calcium will bond with the soap molecule instead and form calcium stearate. aka soap scum. It’s a waxy, insoluble, grayish gunk of a nightmare that makes your clothes stiff and gray. The worst part is that it builds up in your washing machine.
Detergents were invented to solve the soap scum problem. They’re created with a polar head that’s not attracted to the minerals found in hard water. Because it ignores the calcium and magnesium in the water, it can focus on grabbing the dirt from your clothes.
Subtypes of detergents by electrical charge
Anionic surfactants: negatively charged molecules that are great at suspending dirt. Unfortunately, they create a lot of suds so they require an anti-foaming agent in the formulation for HE machines.
Non-ionic surfactants: they have no charge and thus less sudsy. And because of the lack of charge, they’re also a great choice for removing oil and grease stains.
Amphoteric surfactants: they have both a positive and a negative charge, depending on the pH. They’re usually a secondary surfactant.