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To understand how soap interacts with hard and soft waters, as well as how it cleans effectively, it helps to have a basic knowledge of soap and detergent chemistry.
Each molecule of water is usually surrounded and attracted by other water molecules. However, at the water’s surface, those molecules are surrounded by other water molecules only on the water side. Surface tension is created as the water molecules at the surface are pulled into the body of the water.
This causes water to bead up on surfaces, such as glass and fabric, which slows wetting of the surface and inhibits the cleaning process.
Chemicals that reduce surface tension so that water can spread and wet surfaces, making water “wetter,” are called surface active agents, or surfactants.
Surfactants perform other important functions in cleaning, such as loosening, emulsifying (dispersing in water) and holding soil in suspension until it can be rinsed away. Surfactants can also provide alkalinity, which is useful in removing acidic soils.
Surfactants are classified by their ionic properties in water: anionic (negative charge), nonionic (no charge), cationic (positive charge), and amphoteric (either positive or negative charge).
Soap is an anionic surfactant. Other anionic as well as nonionic surfactants are the main ingredients in detergents.
Composition of soap
Soaps are water-soluble sodium or potassium salts of fatty acids. They are made from fats and oils, or their fatty acids, by treating them chemically with a strong alkali.
Fatty acids are weak acids composed of two parts: a carboxylic acid group consisting of one hydrogen (H) atom, two oxygen (O) atoms, and one carbon (C) atom; plus a hydrocarbon chain attached to the carboxylic acid group. Generally, the hydrocarbon chain is made up of a long, straight chain of carbon atoms each carrying two hydrogen atoms.
Saponification, the most widely used soapmaking process, involves heating fats and oils and reacting them with a liquid alkali to produce soap and water (“neat soap”) plus glycerine.
Another major soapmaking process is the neutralization of fatty acids with an alkali. Fats and oils are hydrolyzed (split) with high-pressure steam to yield crude fatty acids and glycerine. The fatty acids are then purified by distillation and neutralized with an alkali to produce soap and water (neat soap).
When the alkali used in soapmaking is sodium hydroxide, the result is a sodium or “hard” soap. When the alkali is potassium hydroxide, softer potassium soaps are formed, often found in some liquid hand soaps and shaving creams.
The carboxylate end of the soap molecule is attracted to water. It is called the hydrophilic (water-loving) end. The hydrocarbon chain is attracted to oil and grease and repelled by water, and is the hydrophobic (water-fearing) end.
Water alone will not remove oily, greasy soil on clothing or a surface, because the oil and grease repel the water molecules.
After adding soap (or detergent), the surfactant’s hydrophobic end is repelled by water but attracted to the oil in the soil. At the same time, the hydrophilic end is attracted to the water molecules. These opposing forces loosen the soil and suspend it in the water for removal in cleaning.
Water hardness and cleaning
Although soap is a good cleaning agent, its effectiveness is reduced when used in hard water. Hardness in water is caused by the presence of mineral salts — mostly those of calcium (Ca) and magnesium (Mg), but sometimes also of iron (Fe) and manganese (Mn). These react with soap to form an insoluble precipitate, a soap film or scum.
Soap film does not rinse away easily, produces visible deposits on clothing and makes fabrics feel stiff. It also attaches to bathtubs, sinks and washing machines.
When soap is used with hard water, some of the soap is used up by reacting with hard water minerals to form the film. This reduces the amount of soap available for cleaning.
Detergent surfactants are made from a variety of petroleum-based chemicals and/or oleochemicals (derived from fats and oils), are more adaptable to a variety of water conditions, and are even less sensitive than soap to the hardness minerals in water.
Like the fatty acids used in soapmaking, both petroleum and fats and oils contain hydrocarbon chains that are repelled by water but attracted to oil and grease in soils. These hydrocarbon chain sources are used to make the hydrophobic end of the surfactant molecule.
Chemicals such as sulfur trioxide, sulfuric acid and ethylene oxide are used to produce the hydrophilic end of the detergent surfactant molecule. As in soapmaking, an alkali is used to make detergent surfactants. Sodium and potassium hydroxides are the most common alkalis.