Laundry detergents may contain builders (50% by weight, approximately), surfactants (15%), bleach (7%), enzymes (2%), soil antiredeposition agents, foam regulators, corrosion inhibitors, optical brighteners, dye transfer inhibitors, fragrances, dyes, fillers and formulation aids.
Builders (also called chelating or sequestering agents) are water softeners. Hard water contains calcium, magnesium, and metallic cations (iron, copper, and manganese). These cations react with surfactant anions to form insoluble compounds (metallic or lime soaps) which precipitate onto fabrics and washing machines and which are difficult to remove. Builders remove the hard water ions through precipitation, chelation, or ion exchange. In addition, they help remove soil by dispersion. In most European regions, water is hard. In North America, Brazil, and Japan, the water is comparatively soft.
The earliest builders were sodium carbonate (washing soda) and sodium silicate (water glass). Since the 1930s, phosphates (sodium phosphates) and polyphosphates (sodium hexametaphosphate) were introduced, continuing with the introduction of phosphonates (HEDP, ATMP, TMP). These agents are now known to have serious environmental consequences leading to a drive towards more environmentally benign phosphorus-free agents, such as polycarboxylates (EDTA, NTA), citrates(trisodium citrate), silicates (sodium silicate), gluconic acid and polyacrylic acid; or ion exchange agents like zeolites.
Alkalis like soda ash precipitate hard water ions and are commonly used as builders. Additionally, they enhance washing performance. Hydrophilic fibers like cotton have a negative surface charge in water, whereas synthetic fibers are comparatively neutral. The negative charge is further increased by the adsorption of anionic surfactants. With increasing pH, soil and fibers become more negatively charged, resulting in increased mutual repulsion. This is one of the reasons why alkalis enhance wash performance, apart from effects like the saponification of fats. However, repulsive forces between soil and fibers alone do not produce satisfactory washing results even at high pH. The optimum pH range for good detergency is 9–10.5.
Builder and surfactant work synergistically to achieve soil removal, and the washing effect of the builder may exceed that of the surfactant. With hydrophilic fibers like cotton, wool, polyamide, and polyacrylonitrile, sodium triphosphate removes soil more effectively than a surfactant alone. With hydrophobic fibers like polyesters and polyolefins, the effectiveness of the surfactant surpasses that of the builder.
Main article: Surfactant
Anionic surfactants: branched alkylbenzenesulfonate, linear alkylbenzenesulfonate, and a soap.
Surfactants are responsible for most of the cleaning performance in laundry detergent. They provide this by absorption and emulsification of soil into the water and also by reducing the water’s surface tension to improve wetting.
Laundry detergents contain mostly anionic and non-ionic surfactants. Cationic surfactants are normally incompatible with anionic detergents and have poor cleaning efficiency; they are employed only for certain special effects, like fabric softeners, antistatic agents, and biocides. Zwitterionic surfactants are rarely employed in laundry detergents mainly for cost reasons. Most detergents use a combination of various surfactants to balance their performance.
Until the 1950s, soap was the predominant surfactant in laundry detergents. By the end of the 1950s, so-called “synthetic detergents” (syndets) like tetra propylene benzenesulfonate (TPS) had largely replaced soap in developed countries. Due to their poor biodegradability, these branched alkyl benzene sulfonates were replaced with linear alkylbenzene sulfonates (LAS) in the mid-1960s. Since the 1980s, alkyl sulfates such as SDS have found the increasing applications at the expense of LAS.
Since the 1970s, nonionic surfactants like alcohol ethoxylates have acquired a higher share in laundry detergents. In the 1990s, glucosides appeared as co-surfactants, and alkyl polyglycosides have been used in specialty detergents for fine fabrics.