The most important nonionic detergents are obtained by condensing compounds having a hydrophobic molecular group, usually a hydroxyl (OH) group, with ethylene oxide or propylene oxide. The most usual compounds are either alkylphenol or long-chain alcohol having a hydroxyl group at the end of the molecule. During the condensation reaction, the ethylene oxide molecules form a chain that links to the hydroxyl group. The length of this chain and the structure of the alkylphenol or alcohol determine the properties of the detergent.
The reaction may take place continuously or in batches. It is strongly exothermic (heat-producing), and both ethylene and propylene oxide are toxic and dangerously explosive. They are liquid only when under pressure. Hence, the synthesis of these detergents requires specialized, explosion-proof equipment and careful, skilled supervision and control.
Other nonionic detergents are condensed from fatty acids and organic amines. They are important as foam stabilizers in liquid detergent preparations and shampoos.
Some nonionic synthetic detergents may cause problems with unwanted foam in wastewater systems; the problem is not as serious as with anionic synthetic detergents, however.
Cationic detergents contain a long-chain cation that is responsible for their surface-active properties. Marketed in powder form, as paste, or in aqueous solution, they possess important wetting, foaming, and emulsifying properties but are not good detergents. Most applications are in areas in which anionic detergents cannot be used. Cationic-active agents are used as emulsifying agents for asphalt in the surfacing of roads; these emulsions are expected to “break” soon after being applied and to deposit an adhering coat of asphalt on the surface of the stone aggregate. These agents absorb strongly on minerals, particularly on silicates, and therefore make a strong bond between the asphalt and the aggregate. Cationic detergents also possess excellent germicidal properties and are utilized in surgery in dilute form.
Ampholytic detergents are used for special purposes in shampoos, cosmetics, and in the electroplating industry. They are not consumed in large quantities at present.
Finishing synthetic detergents
The largest quantities of synthetic detergents are consumed in the household in the form of spray-dried powders. They are produced from an aqueous slurry, which is prepared continuously or in batches and which contains all the builder components. Builders, consisting of certain alkaline materials, are almost universally present in laundry soaps. These materials give increased detergent action. The most important are sodium silicate (water glass), sodium carbonate (soda ash), and various phosphates; the latter has contributed to the problem of wastewater pollution by contributing nutrients which sustain undesirable algae and bacteria growth, and much work is being done to find acceptable builders which may replace, at least partially, phosphates. The slurry is atomized in heat to remove practically all the water. The powder thus obtained consists of hollow particles, called beads, that dissolve quickly in water and are practically dust-free. Another portion of the syndets is transformed into liquid detergent products and used primarily for hand dishwashing. Although syndet pastes are seldom produced, solid products, manufactured in the same way as the toilet or laundry soap, have been sold in increasingly greater quantity. Sodium perborate is sometimes added to the spray-dried beads to increase cleaning power by oxidation. Enzymes may be added as well. Many modern washing powders combine synthetic detergents, anionic and nonionic, with soap to give maximum efficiency and controlled foam for use in household washing machines.