Research Techniques in Detergency

For the purposes of this discussion, “detergency” will be defined as the removal of an unwanted soil from a substrate by the physicochemical action of an aqueous bath. The bath could consist for reference purposes of pure water or of water of specified hardness, salt content, pH, etc. In the practical situation, however, the bath will contain one or more solutes that greatly increase both the rate and the degree of soil removal over that achievable by water alone. These solutes include surfactants, builders (materials that by various mechanisms promote the soil-removing action of surfactants), and antiredeposition agents (materials that inhibit the recombination of soil with previously separated substrate). Substrates can vary quite widely in both chemical composition and physical form. In general, they must be solid, with a melting or softening point well above the washing temperature, and they must be insoluble and resistant to attack by the bath; in short, the type of material that is normally cleaned with “soap and water.” Substrates are usually considered, for practical purposes, in two categories:—fibrous, comprising textile fabrics, and hard surface, comprising tile, painted surfaces, metal, glass, ceramic, and similar materials. Soil has broadly and jokingly been defined as “matter out of place.” In the study of detergency, however, certain types of “matter out of place” are usually excluded from consideration. Among these are substances easily removed by dissolution or rinsing in water alone, and at the other extreme, stains of dyelike materials that are molecularly dispersed below the surface of the substrate and can be removed in reasonably short time only by chemical action. Soils are generally grouped as solid, liquid (or “oily”), and mixed. They adhere to the substrate but do not exhibit strong chemical interaction with it. The detersive process consists essentially of a breaking or weakening of the soil-substrate adhesive bond so that the soil can be separated and carried away easily by the hydraulic action of the bath. The literature on surfactants and other detergent components and their behavior and properties is exceedingly extensive and will not be considered here. Theoretical discussions of the physical chemistry of detergency are also available elsewhere.(1)The present discussion will be concerned primarily with the methodology and techniques used in studying the detersive process.