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How Detergents Work

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how-detergents-work

Detergent is any of a large group of substances that have a cleansing action like that of soap. Synthetic detergents, which are described in this article, differ from soap in that they are not made from fats and oils.

The chief advantage of synthetic detergents over soap is that they clean better in hard water, which contains salts of calcium and magnesium. Soap reacts with these salts to produce insoluble compounds that hinder the cleaning process. Synthetic detergents do not form such compounds.

Synthetic detergents also have the advantage that they can be given a wide range of properties. For example, because most people mistakenly believe that suds are needed for cleansing, many household detergents are designed to produce high suds. Special low-sudsing detergents are made for use in washing machines that may be clogged by high suds. In addition, synthetic detergents, like soap, can be made in either liquid or powdered form with no loss of cleansing power.

Structure

A molecule of any detergent consists of two parts, a hydrophilic ("water-loving") part and a hydrophobic ("water-hating") part. The hydrophilic end of a detergent molecule is soluble in water. The hydrophobic part tries to remove itself as far as possible from water, and projects into air or oil that may be in contact with the water. As a result of this unusual behavior, detergent molecules in a water solution tend to become concentrated at the surface of the water. If the detergent solution is in contact with oil, the molecules also become concentrated where the water and oil meet. Because of this surface-seeking characteristic, detergents are said to have high "surface activity".

Cleansing Action

Detergents are good wetting agents. For this reason, they enable water to make close contact with dirt particles. The wetting ability of detergent solutions is due to the surface activity of detergents, which lowers the surface tension of water. In addition to increasing the wetting ability of water, detergents emulsify oil and grease; that is, they hold oil and grease particles in suspension in water.

When dirty clothes are soaked in water containing a detergent, the wetting action of the detergent brings the water into close contact with the dirt particles on the clothes. Detergent molecules group themselves around the oil droplets contained in much hard-to-clean dirt. The hydrophobic parts of the molecules project into the oil droplets, and the hydrophilic parts of the molecules remain in the water. The oil and solid particles of dirt are then washed off into the water, where the oil is held in suspension by the emulsifying action of the detergent. Because the oil is held in suspension, it is not deposited on the clothes, and it is carried away with the dirty water.

Kinds of Detergents

Detergents are divided into two general types, according to the electrical charge on the hydrophilic parts of their molecules. Anionic detergents carry a negative charge on the hydrophilic parts of their molecules. Most household detergents are of this type. Cationic detergents are positively charged, and are used mostly for industrial purposes.

Additives

Most packaged detergents contain other substances in addition to the synthetic detergent itself. They usually contain large amounts of substances that prevent hard-water salts from reacting with the dirt on articles being washed. Such substances tend to corrode metals, so corrosion-inhibiting chemicals are also added. Since the long-lasting suds produced by many synthetic detergents are a nuisance and may be a source of water pollution, foam-suppressing agents are usually added also. Other substances that are often added in small amounts are bleaches, disinfectants, coloring agents, and perfumes.

Structure

A molecule of any detergent consists of two parts, a hydrophilic ("water-loving") part and a hydrophobic ("water-hating") part. The hydrophilic end of a detergent molecule is soluble in water. The hydrophobic part tries to remove itself as far as possible from water, and projects into air or oil that may be in contact with the water. As a result of this unusual behavior, detergent molecules in a water solution tend to become concentrated at the surface of the water. If the detergent solution is in contact with oil, the molecules also become concentrated where the water and oil meet. Because of this surface-seeking characteristic, detergents are said to have high "surface activity".

Cleansing Action

Detergents are good wetting agents. For this reason, they enable water to make close contact with dirt particles. The wetting ability of detergent solutions is due to the surface activity of detergents, which lowers the surface tension of water. In addition to increasing the wetting ability of water, detergents emulsify oil and grease; that is, they hold oil and grease particles in suspension in water.

When dirty clothes are soaked in water containing a detergent, the wetting action of the detergent brings the water into close contact with the dirt particles on the clothes. Detergent molecules group themselves around the oil droplets contained in much hard-to-clean dirt. The hydrophobic parts of the molecules project into the oil droplets, and the hydrophilic parts of the molecules remain in the water. The oil and solid particles of dirt are then washed off into the water, where the oil is held in suspension by the emulsifying action of the detergent. Because the oil is held in suspension, it is not deposited on the clothes, and it is carried away with the dirty water.

Kinds of Detergents

Detergents are divided into two general types, according to the electrical charge on the hydrophilic parts of their molecules. Anionic detergents carry a negative charge on the hydrophilic parts of their molecules. Most household detergents are of this type. Cationic detergents are positively charged, and are used mostly for industrial purposes.

Additives

Most packaged detergents contain other substances in addition to the synthetic detergent itself. They usually contain large amounts of substances that prevent hard-water salts from reacting with the dirt on articles being washed. Such substances tend to corrode metals, so corrosion-inhibiting chemicals are also added. Since the long-lasting suds produced by many synthetic detergents are a nuisance and may be a source of water pollution, foam-suppressing agents are usually added also. Other substances that are often added in small amounts are bleaches, disinfectants, coloring agents, and perfumes.

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