Synthetic Detergents and Soaps

Synthetic detergents were developed in order to replace soaps for cleaning, to overcome some of soap's shortcomings. This included soap's inability to lather in hard or acidic environments, it is reliant on fats and oils. Synthetic detergents are derived from the petrochemical industry. There are three main classes of detergents: anionic, cationic and non-ionic.

Examples of Detergents:
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Soap

Structure and Chemical Composition

Soaps consist of a long hydrocarbon tail with a short negatively charged carboxylate head. Their chemical composition is usually CH3(CH2)nCOO-Na+

Effect in Hard Water

Soap anions precipitate with calcium and magnesium ions, forming a grey scum which persists on clothing. This reduces the number of available soap molecules and reduces its cleaning ability

Uses and Relation to Properties

These are mainly used in personal hygiene.

Anionic detergent

Structure and Chemical Composition

Anionic detergents consist of a long hydrocarbon tail and a negatively charged head group. However they differ from soaps in that the head is a different ion to carboxylate. Common anionic detergents include alkyl phosphates, where heads are benzensulfonate or phosphate ions respectively.
A possible (typical) chemical composition is:
CH3(CH2)n(C6H6)O(SO2)O-Na+

Effect in Hard Water

These lather in hard water well except alkyl sulfates and alkyl phosphates. However, they do not form scum in low pH water. Instead, they form soluble complexes, so their efficiency is slightly reduced as some of the surfactant is 'tied up'.

Uses and Relation to Properties

These are used for general cleaning, particularly in laundry and dishwashing detergents. They are effective in removing grease and stains from natural fibres, and excellent for cleaning glass. They are not used in personal hygiene as they are too powerful and remove much oil from skin and hair.

Cationic Detergent

Structure and Chemical Composition

These detergents consist of a long hydrocarbon tail with a positively charged head, which is usually a quaternary ammonium group (alkyl ammonium group). There can be one or two long alkyl chains connected to the nitrogen, with two or three methyl groups. The negative ion is often a halide (e.g. Br- or Cl-). Sometimes the positive head is a pyridium group. A possible (typical) chemical composition is:
CH3(CH2)nN+(CH3)3Cl-

Effect in Hard Water

These detergents are not affects by hard water and do not form complexes or precipitates with Ca2+ or Mg2+.

Uses and Relation to Properties

These detergents bond very strongly to negatively charged surfaces unless the pH is above 10. Thus, they find use in fabric softeners and hair conditioners, as they reduce the static friction and tangling as they persist (hair becomes negatively charges after washing). They can also be used to create antistatic surfaces on glass, but aren't used in dishwashers due to the greasy feel they give glass (adsorbed detergent). They can also be used to clean plastic effectively (better than anionic detergents) and as flocculating agents.
Monoalkyl cationic detergents are biocides, and their antiseptic properties are useful in household disinfectors and sanitisers (e.g. mouthwashers, antiseptic soaps).

Non-ionic detergent

Structure and Chemical Composition

These are characterised by the presence of hydrophilic groups rather than charged heads. They consist of a long hydrocarbon tail, and a polar alcohol ethoxylate (polyoxyetheylene ethers) group. They are molecules, not ions.
CH3(CH2)nO(CH2CH2O)n(CH2)2OH The 2nd n varies from 5 to 50.

Effect in Hard Water

These detergents are not affects by hard water and do not form complexes or precipitates with Ca2+ or Mg2+.

Uses and Relation to Properties

They produce less foam than other detergents. As such, they find use in applications where low foam is desired, such as dishwashing liquids. They can be added to other detergents to provide greater stability. They are used in applications such as paints, adhesives, pesticides and cosmetics, as improved contact between polar and non-polar substances is required.

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