More About
Soaps, Insecticidal Soaps, and Detergents

Soaps are salts of fatty acids. In nature, fatty acids are components of fats and oils in plants and animals. Traditionally, soaps were made by reacting a lye (e.g., potash or potassium hydroxide made from wood ashes) with animal fat. Common hard soaps are sodium salts of fatty acids, while liquid soaps are potassium salts. Vegetable fats and oils were used as substitutes for animal fats when shortages of the latter developed during the first world war, and the vegetable based soaps proved to have better qualities.

Soaps have surfactant properties, meaning that the individual molecules have a hydrophilic (water-attracting) end, which dissolves in water, and a lipophilic (fat loving) end, which dissolves in fatty substances. These surfactant properties make soaps effective cleaning agents: the hydrophilic end keeps the molecules dispersed in water, while the hydrophobic, grease loving end seeks out greasy materials and pulls them into the water.

Calcium and magnesium in 'hard' water react with the soap molecules to produce calcium and magnesium salts of the fatty acids, which are insoluble in water and hence lack the surfactant properties. Thus in hard water, more soap has to be used to get the same cleaning power as in 'soft' water, or a conditioning agent such as Calgon (sodium metahexaphosphate) has to be added to tie up the calcium and magnesium.

Most soaps made from natural fats and oils are fairly innocuous to humans at least in diluted form. Soaps have been in use for thousands of years. It is not complicated to make soaps and they are common cottage industry products.

Detergents are commonly defined as synthetic surfactants meaning that they are made from petrochemicals rather than from natural fats and oils (which are called 'oleochemicals' to distinguish them from 'petrochemicals') although detergents may include compounds made from oleochemicals. Detergents may be 'anionic' surfactants similar to soaps, 'cationic' surfactants made from organic molecules other than organic acids or they may be 'nonionic' surfactants (e.g., made by converting a hydrocarbon to an alcohol and then reacting the fatty alcohol with ethylene oxide) .

In addition to the detergents used in the home environment, the term detergent includes a wide variety of surfactants used in industry. Some have quite harsh properties and are significant health and environmental hazards. Those used as dish and laundry detergents are selected to be less hazardous, but depending on the product, they can still cause adverse reactions in humans and/or have undesirable environmental effects. The main advantages of detergents over traditional soaps are (i) they are not affected by calcium and magnesium in the water; (ii) greater cleaning power; (iii) near neutral pH (soaps are alkaline).[V17]

Pressure-washing machines are environmentally friendly alternatives to harsh detergents used to clean cars, houses etc. There are also a variety of 'natural' detergents on the market. The natural detergent are generally mixtures of natural materials that have cleaning properties similar to detergents, but have very low toxicity and low skin-irritating qualities to humans and low toxicity to organisms in the environment; they are usually free of phosphate (which causes nutrient overloading of streams and lakes) and free of chlorine (which may be allergenic, and is environmentally unfriendly).

Pesticidal soaps
Pesticidal soaps are soaps that are specially selected to increase certain pesticidal or pest-deterring properties, while remaining relatively benign otherwise. The lipophyllic ends of the soap molecules penetrate insect cell membranes.[V18] Soaps based on short-chain fatty acids are used as moss-killers and in general use (non discriminatory) contact herbicide products. Longer chain fatty acids have insecticidal properties. The insecticidal soaps are based on natural products, e.g., coconut oil, that are processed to concentrate certain chain lengths of the fatty acids. Fatty acid toxicity to insects and fungi increases with chain length to a length of 10 carbons and then declines. Unsaturated fatty acids with 18 carbons also have insecticidal properties.[V18] Safer's soap is reported to be derived from oleic acid, a 10 carbon, saturated fatty acid.[V17] Ammonium soaps are used as wildlife deterrents (e.g. for deer), sulfur soaps as fungicides.

The same properties that give soaps pesticidal properties can also cause them to be irritating to handle and to be damaging to plants. Pesticidal soaps for plants are selected to be minimally damaging to plants; some plants are more susceptible to soap damage than others, e.g., those with hairy leaves that retain the soap are more likely to be damaged by soap. It is important to use soaps at the concentrations specified. In general, natural soaps are less irritating to humans than (synthetic) detergents, but some individuals are particularly sensitive to some soaps.

Dish and clothing soaps or detergents also have pesticidal activity.
Depending on the product and possibly the batch, pesticidal activity of a household soap could be greater or less than that of a registered pesticidal soap. Registered pesticidal soaps have several benefits over household soaps:

Hence the word 'pesticidal' does not mean that a soap is a more hazardous material than household soaps and detergents, in fact the reverse may be more generally true.

Household soaps should be used cautiously as pesticides in order to avoid damage to plants, e.g., by using them at a concentration of 2-3% (or lower), not applying the soap at mid-day under direct sun and conducting a test on some small part of the plants that will be treated to confirm they won't be damaged.


Page posted 4 May 2004
Modified 25 Jul. 2005

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