Detergents and Detergency
Authors: Lynn Jesse L. 
Published: July 2005

The cleaning of a solid object, i.e., the removal of unwanted foreign matter from its surface, is done by methods ranging from simple mechanical separation such as blotting or abrasion to removal by solution or selective chemical action. The termdetergencyis limited to systems in which a liquid bath is present and is the main cleaning component of the system. The action of the bath involves more than simple solution or simple hydraulic dislodging of soil, although both will occur and contribute to the cleaning. The cleaning is enhanced primarily by the presence in the bath of a special solute, the surfactant, that alters interfacial effects at the various phase boundaries within the system. Thus, a typical detersive system consists of a solid object to be cleaned, called the substrate, soil or dirt attached to it that is to be removed in the washing process, and a liquid bath that is applied to the soiled substrate. In turn, each of these elements can vary widely in properties and composition. The final cleaning benefit results from interaction of these elements and the conditions used, i.e., temperature, time, mechanical energy input (agitation), and, in the case of aqueous baths, the presence of hardness ions in the water.

In the cleaning or washing process in a typical detersive system, the soiled substrate is immersed in or brought into contact with a large excess of the bath liquor. Enough bath is used to provide a thick layer over the whole surface of the substrate. During this stage, air is displaced from soil and substrate surfaces; i.e., they are wetted by the bath. The system is subjected to mechanical agitation, either rubbing or shaking, which provides the necessary shearing action to separate the soil from substrate and disperse it in the bath. Agitation also promotes mass transfer in the system, just as in a heterogeneous chemical reaction. The bath carrying the removed soil is drained, wiped, squeezed, or otherwise removed from the substrate. The substrate is rinsed free of the remaining soiled bath. This rinsing step determines the final cleanliness of the substrate. The cleaned substrate is dried or otherwise finished.

A meaningful discussion of detergency requires a definition of clean. In the physiochemical sense, a surface is clean if it contains no molecular species other than those in the interior of the two adjoining phases. It is difficult to achieve such a state even under the most exacting laboratory conditions. Practically, a surface is clean if it has been brought to a desired state with regard to foreign matter present upon it, as judged by agreed‐upon criteria. Household linen, for example, is considered clean when it is free of visible soil even though it may carry a starch and a softening finish. In the dyehouse of a textile mill, a piece of goods such as this would be rejected as dirty and returned for scouring because these finishes interfere with dyeing. Most standards for cleanness involve a visual or optical judgment for the presence of foreign matter. In some systems, for example, the desizing of cotton, the degree of cleanness may be specified by weight percentage of soil on the substrate. In other systems, such as the degreasing of metal, it is the weight of soil per unit area of substrate surface that specifies cleanness. In washing dishes or glassware, cleanness is often specified by complete water wettability or freedom from water break, as well as by appearance.

Although it is impossible to list all the practical detersive systems that might be encountered, a large proportion fall in a small number of classes. This classification disregards surfactant structure and type of substrate (fibrous or hard surface) and is restricted to a consideration of the soil present on the substrate, the mechanical action employed, the bath ratio, and the detergent used. Some of the more commonly encountered detersive systems are classified on this basis here.