The oldest chemical reactions known to man are the manufacture of soap and the production of alcohol. Since everyone fancies himself a self-styled authority on alcohol, the theme of this article will deal with soaps, detergents, and other surface-active agents and their mechanism of cleaning, surface activity, and other physical properties.
It is difficult to select one individual who could be called a pioneer in this field, because since Pliny first wrote about soap-making in the first century A. D., until Dalton theorized about surface activity and therefore explained the principle of wetting, the production of soap, or more properly, surfactants, has gradually proceeded from an art to a science without the great investigation that has been enjoyed by other chemical processes.
Soaps play a vital role in our everyday life in promoting health and cleanliness. It is not generally known that these products also play an equally important role in many industries.
Disinfectants, waxes, polishes, cosmetics, and lubricants and greases are analogues of soap, but since the topic of this paper is detergency and surface activity or wetting, the discussion will be limited to the water-soluble soaps or detergents.
Water-soluble soaps are detergents and wetting agents. Recently certain new synthetic organic compounds have been exploited and are being used for detergency purposes, wetting purposes or both. These new products have proved advantageous in numerous applications in industry and are also popular in household products because their effectiveness is not materially affected by hard water.
Ordinary soaps react with the calcium and magnesium in hard water to form the often heard “bath tub ring”, that is common in hard water areas of the country, especially in rural areas.
It may be said that detergents and wetting agents consists of two parts; first, the water-soluble or hydrophilic portion of the molecule; and second, the oil-soluble (water-repelling) or hydrophobic portion. The combination of these two groups gives a molecule, which because of its dual tendencies, is extremely active at oil and water interfaces in reducing surface tension. For example, water and oil do not mix with each other because water cannot wet an oily surface.
The forces of attraction between water and oil molecules are much smaller than the attraction of water for other water molecule, and not large enough to overcome the surface tension of the water. In the presence of a third substance that contains a hydrophilic (water-loving) and a hydrophobic (oil-loving) group in the same molecule, the surface tension is reduced so that the droplets of oil and water can become commingled to form an emulsion.
Globules of grease or oil are emulsified by being coated completely with a film of soap molecules. The hydrophobic (oil-loving) portion of the soap molecule dissolves in the grease or oil while the polar or hydrophilic portions place themselves outside in contact with the water.
The removal of oily soils from hard surfaces, such as metallic particles, etc. involves changes in both surface tension i.e. – the ability of water to wet a hard surface, and interfacial tension i.e. – the free energy between the water and the oily soil.
The first sign of detersive action is an irregular swelling on the surface due to the wetting action of the detergent which is creeping beneath the oily soil and lifting it from the solid surface.
As stated above, surfactants act as cleaners through emulsification or wetting or, a combination of both these properties. Current technology can vary these characteristics to meet specific requirements. The charge of the molecule can also be varied to produce negatively charged surfactants, i.e., anionics, or positively charged, i.e., cationics, no charge, i.e., non-ionics, or both a positive and negative charge on the same molecule, i. e., amphoteric surfactants. The physical characteristics can be differed to promote foam or create an anti-foaming compound; to disperse a powder into a suspension or to flocculate and precipitate it. The molecular structure of surface active agents can also be adjusted to obtain optimum effects as germicides, bactericides, etc.
The widespread use of synthetic detergents in household products is not without its problems. The chemical and biological stability of these materials has plagued municipal sewage treatment and disposal operations, often resulting in large volumes of unmanageable foam not only in the treatment plants, but also in canals, rivers, and other waterways. Newer developments in surfactants are possessing less resistance to bacteria, i.e., biodegradable, are already successfully being used.