5.2 - Cleaning Agents

Soap and Detergent

• Soap is a sodium or potassium salt of a fatty acid. Its general formula is $\mathrm{RCO}{\mathrm{O}}^{-}\mathrm{N}{\mathrm{a}}^{+}$ or $\mathrm{RCO}{\mathrm{O}}^{-}{\mathrm{K}}^{+}$.
• $R$ is a saturated or unsaturated alkyl group with at least 8 carbon atoms, usually 12–20.
• Fatty acids from animal fats or vegetable oils can also be neutralised with alkalis to produce soap.
• Examples include sodium laurate, $\mathrm{C}{\mathrm{H}}_3(\mathrm{C}{\mathrm{H}}_2{)}_{10}\mathrm{COONa}$, from coconut oil and sodium palmitate, $\mathrm{C}{\mathrm{H}}_3(\mathrm{C}{\mathrm{H}}_2{)}_{14}\mathrm{COONa}$, from palm oil.
• Detergent is a non-soap cleaning agent. Detergents are sodium salts of sulphonic acids and are usually made from synthetic sources such as petroleum fractions.
• The two main groups are sodium alkyl sulphates, $\mathrm{ROS}{\mathrm{O}}_3^{-}\mathrm{N}{\mathrm{a}}^{+}$, and sodium alkylbenzene sulphonates.

Preparation of Soap and Detergent

• Soap is prepared by saponification, the alkaline hydrolysis of oils or fats. Natural triglycerides react with concentrated sodium hydroxide or potassium hydroxide to form glycerol and fatty-acid salts.

• Detergents are prepared from petroleum fractions in two stages:
  1. Sulphonation or sulphation: concentrated ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ reacts with alkylbenzene to form alkylbenzene sulphonic acid, or with a long-chain alcohol to form alkyl hydrogen sulphate. Water is also produced.
  2. Neutralisation: $\mathrm{NaOH}$ neutralises these products to form sodium alkylbenzene sulphonate or sodium alkyl sulphate, respectively, and water.

• Colour and fragrance can make handmade soap more attractive.

Cleansing Action

• Soap and detergent anions have a hydrophilic part, which dissolves in water, and a hydrophobic part, which dissolves in oil or grease. They therefore act as emulsifying agents.
• Their cleansing action follows these steps:
  1. Soap or detergent lowers the surface tension of water, increasing its ability to wet cloth.
  2. It ionises in water, producing mobile cleaning-agent anions and sodium or potassium ions.
  3. Hydrophobic tails dissolve in grease while hydrophilic heads remain in water.
  4. Scrubbing and agitation pull the grease away from the cloth.
  5. Anions surround the grease, which floats to the water surface, and divide it into small droplets.
  6. Repulsion between negatively charged hydrophilic heads prevents the droplets from reattaching; an emulsion forms and rinsing removes the suspended grease.

• Dry cleaning uses solvents such as 1,1,1-trichloroethane instead of water.

Soap and Detergent in Different Types of Water

• Hard water contains $\mathrm{C}{\mathrm{a}}^{2+}$ and $\mathrm{M}{\mathrm{g}}^{2+}$ ions.
• Soap anions form insoluble calcium or magnesium salts called scum, wasting soap and reducing cleansing effectiveness:

• Detergent anions form soluble salts with $\mathrm{C}{\mathrm{a}}^{2+}$ and $\mathrm{M}{\mathrm{g}}^{2+}$, so detergents remain effective in hard water.

Water	Cleansing effectiveness
Soft	Soap and detergent are effective.
Hard	Soap is less effective because scum forms; detergent remains effective because no scum forms.
Acidic	Soap is ineffective because insoluble organic acid forms; detergent remains effective because its organic acid is soluble.

Experiment 5A - Effectiveness in Hard Water

• Aim: Compare the cleansing effectiveness of soap and detergent in hard water.
• Problem statement: Is soap more effective than detergent in hard water?
• Materials: Hard water, soap, detergent and cloth with a greasy stain.
• Apparatus: Beaker, measuring cylinder, spatula, glass rod and electronic balance.

Additives in Detergents

Additive	Function and example
Biological enzyme	Removes stains such as blood, milk and sugar; amylase, protease, cellulase, lipase
Whitening agent	Converts dirt to colourless substances; sodium perborate, sodium hypochlorite
Optical whitener	Makes fabric appear whiter and brighter; fluorescent dyes
Water softener and builder	Softens water and improves cleansing; sodium tripolyphosphate
Foam-control agent	Controls foam; alkyl monoethanolamide
Anti-suspension agent	Prevents removed dirt from redepositing; sodium carboxymethylcellulose
Fragrance	Gives detergent and fabric a pleasant smell; jasmine, lavender
Drying agent	Keeps powdered detergent dry; sodium sulphate, sodium silicate

• Detergent selection depends on its intended use, physical form, additives, side effects, suitability for household water and environmental impact.