(B) Soaps, detergents and emulsions How Soaps and Detergents work HC CHEMISTRY (B) Soaps, detergents and emulsions HC CHEMISTRY How Soaps and Detergents work

(B) Soaps, detergents and emulsions How soaps and detergents work HC CHEMISTRY HC CHEMISTRY (B) Soaps, detergents and emulsions How soaps and detergents work After completing this lesson you should be able to : During cleaning using soaps the hydrophobic tails dissolve in a droplet of oil or grease, whilst the hydrophilic heads face out into the surrounding water. Agitation of the mixture results in ball-like structure forming with the hydrophobic tails on the inside and the negative hydrophilic head on the outside. Repulsion between these negative charges results in an emulsion being formed and the dirt released. During cleaning using detergents the hydrophobic tails dissolve in a droplet of oil or grease, whilst the hydrophilic heads face out into the surrounding water. Agitation of the mixture results in ball-like structure forming with the hydrophobic tails on the inside and the hydrophilic head on the outside. Detergents are particularly useful in hard water areas because they do not form scum.

COPY Detergents

Example of Detergent Structure COPY Example of Detergent Structure

COPY Cleansing action of soaps The following ball (blue for hydrophilic head group) and stick (yellow for hydrophobic tail group) diagram represents the initial interaction of soap on addition to water and material with a grease stain:

COPY When the solution containing soap and water is agitated (stirred vigorously) the interactions of hydrophobicity and hydrophilicity become apparent. The hydrophobic, non-polar, tails burrow into the greasy, non-polar molecule – like attracting like. In the same way the polar hydrophilic head groups are attracted to polar water molecules. The head groups all point up into the water at the top of the grease stain.

COPY The attraction of the head group to the surrounding water, via polar-to-polar interactions, is so strong that it causes mechanical lift of the grease molecule away from the material on which it was deposited. The hydrophobic tails are anchored into the grease due to non-polar to non-polar attraction. In combination, these effects allow for the removal of the grease stain.

COPY Cleansing action of soap animation – click here

Experiment Collect 3 measuring cylinders Measure 50cm3 of distilled water 50 cm3 of soap solution Make up a 50% soap solution Add a spatula of MnO2 to each Record observations Explain the chemistry of what you see