Working Mechanism Of Soaps And Detergents

Working Mechanism Of Soaps And Detergents

The “like dissolves like” rule explain why hot soapy water easily removes oily and greasy mater from fabrics or skin but hot water alone does not. All detergents have a common feature whether they are ionic or molecular. Their structures include a long, nonpolar, hydrocarbon “tail” holding a very polar or ionic “head”. A typical system in ordinary soap, for example has the following anion. (The charge−ball−ancing cation is Na+ in most soaps).

C₁₅H₃₁COONa C₁₅H₃₁COO⁻ + Na+

Soap

ⁿC₁₅H₃₁COO (C15H₃₁COO−)ⁿ

Palmitate ions Ionic miscelle of colloidal dimension

The tail of this species is like all hydrocarbons−nonpolar. And it would have a strong tendency to dissolve in oil or grease but not in water. Its natural tendency is to avoid water, it is hydrophobic (water−gearing). It is dragged into solution in hot water only because of the ionic head, which can be hydrated, (And this rather dramaticlally speaks to the significance of hydration as a special kind of driving force.) The ionic head is hydrophilic (water−loving). To minimize contacts between the hydrophilic heads and water, the soap’s anions gather into colloidal−sized groups called soap micelles.

When soap micelles encounter a greasy film, their anions find something else that can accommodate hydrophobic tails. As illustrated in above reaction, the tails of innumerable anions work their way into the film. But they cannot drag the hydrophilic heads entirely in. The heads stay out in the water. With a little agitating help from the washing machine, the greasy film breaks up into countless tiny droplets each pincushioned by projecting negatively charged heads. Since the droplets, now colloidally dispersed, all have the same kind of charge, they repel each other and cannot again coalesce.

The micelle has fascinating structure. It allows a small volume with an interior milieu like that of lipid to exist suspended in an aqueous medium. The action of detergents is based on their ability to forms micelles, which can sequester fatty materials, rendering them soluble in a surfactant solution.