Ask: Scienceman

Dear Liz,
The terms soap and detergent are used synonymously, but in actuality they are very different ingredients. In the most basic terms, soaps are made by mixing a fat (vegetable oil or animal tallow) with caustic soda (lye or potassium hydroxide), a process that has been used for hundreds of years. Detergents, also called surfactants, are created by various chemical reactions between natural or synthetic ingredients to create a new chemical.

The advantage of detergents over soaps for laundry is that soaps tend to react with minerals in the water resulting in an insoluble film that can give clothes a grayish tint. Detergents tend to react less to minerals in water and laundry formulations that use detergents typically include water softening agents such as sodium carbonate or sodium citrate to prevent these types of reactions from occurring. It is important to note that not all detergents are created equal from an environmental perspective. Synthetic detergents are typically petroleum derived and some are also toxic to aquatic life[1-2].

Although many naturally-derived surfactants have petroleum-derived components, choosing natural laundry liquids or powders is an environmentally responsible approach to doing your laundry which will provide better results than using soap.

References:

1. Lee GSJ, McCain JH, Bhasin MM. Synthetic Organic Chemicals. In: Kent and Riegel's Handbook of Industrial Chemistry and Biotechnology, Raymond Riegel, James A. Kent, Eds. 11th Edition, Springer, 2007.
2. Ying GG. Fate, behavior and effects of surfactants and their degradation products in the environment. Environment International. 2006;32(3):417-431.