Soaps have been prepared and used since ancient times to this day with the main purpose of their ability to remove greasy dirt. In the laboratory, their detergent action will be examined as well as its inhibition in the case of hard water (that is, water with a significant presence of calcium and magnesium salts). Finding solutions to this inhibition is also the reason why industrial detergents have been introduced: in the presence of hard water, the detergent action is not significantly affected.
Chemically, soaps are mineral salts of fatty acids. This means that you aqueous environment undergo salt hydrolysis resulting in the formation of ions hydroxide (OH-). As a result, aqueous solutions of soaps have alkaline pH. The well-known detergent action of soaps and surfactants in general molecules, is due to a complex physicochemical process. To understand this, we must first look at the structure of soaps as materials in an aqueous environment. The particles of fatty acid salts consist on the one hand of a relatively long carbon chain in length (11-17 carbons) which is very hydrophobic, and on the other hand by the carboxylate anion which, because it is charged, is excellent hydrophilic. The coexistence of the two extreme tendencies of hydrophobicity and hydrophilicity in the same molecule give it the property of amphiphilic. When these amphiphilic molecules are in water, they tend to organize into spherical structures called bisexual aggregates, the micelles (micelles).
In micellar structures all the molecules are organized to crowd the hydrophobic ones carbon chains inside the beads (and therefore away from the water) and the polar groups (carboxyl anions) to the outside so that, as hydrophilic, they are in contact with the water (see figure 2). The formation of micelles is also due to detergent action of soaps and other materials. One of the most important physical properties of water is the large surface tension of. Amphibious molecules (and therefore soaps) are called surfactants because they change her surface tension of water.
Indicative list of analyzes for detergents and soaps: