People often use the terms soap and detergent interchangeably. They both have the same basic function but chemically they are different compounds. Soaps are usually made by processing a fat with an alkali, like sodium or potassium hydroxide, and results in salts of the acids contained in the fat. Detergents are typically made from synthetic substances such as petroleum by-products.
Both soap and detergent act as a surfactant in the cleaning process. The term surfactant is a portmanteau of the words surface acting agent. Surfactants accomplish two things. First, they lower the surface tension of a liquid which allows easier spreading. Used with water in a cleaning application then they essentially “make water wetter”. The second thing a surfactant does is lower the interfacial tension between two liquids. Surfactants have a hydrophilic end (water-loving) and hydrophobic (water-fearing) end. Surfactants work by interacting with water molecules through their water-loving end and other liquids, solids, and gasses through the water-fearing end by forming spherical structures called micelles.
In the case of soap, the water-fearing end is a long hydrocarbon chain called its tail and the water-loving end is a carboxylate head. Oil and grease, which attract dirt, are non-polar molecules insoluble in water. The micelles that form when soap and oil are combined have their tails facing inward surrounding an oil molecule with the heads facing outward held in suspension in the water. Therefore, the grease and oil and the dirt they attract are trapped inside the micelle and can be rinsed away.
Soap is an excellent cleanser but has one attribute that should be understood and counter-acted when used in a cleaning formula. Mineral acids convert the salts of fatty acids in soap into free fatty acids. Free fatty acids are less soluble than the salts of fatty acids – this is what causes soap film – and can be counter-acted with a rinsing agent. Hard water â€“ water rich in magnesium, calcium or iron for example, will cause soap to form insoluble salts. These insoluble salts are what cause “water spots”, bathtub rings and leave our clothes dingy and rough after repeated washings. This can be alleviated by including a water softening agent in cleaning solutions. While synthetic detergents don’t form insoluble precipitates in hard water the use of petroleum by-products in their production makes them a non-choice in my personal cleaning activities.