Soaps and detergents

Fats and oils

  • Fats and oils are triglycerides (triesters of glycerol)
  • Fats are solid at RT, oils are liquids
  • Degree of saturation, as well as cis/trans isomers, partially determines this as well

triglyceride

Saponification

Fat and oils (esters) are reacted with sodium hydroxide (alkaline hydrolysis) to form glycerol and a sodium fatty acid/carboxylic acid.

Detergents

  • Usually, sodium sulphonate salts.
  • Usually synthetic – reaction of sodium trioxide with a benzene etc.

A typical reaction:

Phenol alkyl ( or any alcohol) + Sulphuric acid –> sulfonic acid + water

detergent reaction1

Sulfonic acid + NaOH –> detergent + water

detergent reaction2

How soaps (and detergents i.e. surfactants) work

  • The long carbon chain buries itself in the oil droplet, since dispersion forces between them are of similar strength, forming a micelle.
  • The anionic part of the chain is attracted to water, which then washes away the micelle.

Problems with soap

  • Precipitation in hard water (Ca2+ and Mg2+) as well as in acidic solution.
  • They form insoluble calcium carboxylate.
  • Detergents do not have this problem.

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