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#Post#: 454--------------------------------------------------
How soap cleans?
DIR By: Karwan
Date: July 29, 2012, 10:12 pm
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Soaps are sodium or potassium fatty acids salts, produced from
the hydrolysis of fats in a chemical reaction called
saponification. Each soap molecule has a long hydrocarbon chain,
sometimes called its 'tail', with a carboxylate 'head'. In
water, the sodium or potassium ions float free, leaving a
negatively-charged head.
Soap is an excellent cleanser because of its ability to act as
an emulsifying agent. An emulsifier is capable of dispersing one
liquid into another immiscible liquid. This means that while oil
(which attracts dirt) doesn't naturally mix with water, soap can
suspend oil/dirt in such a way that it can be removed.
The organic part of a natural soap is a negatively-charged,
polar molecule. Its hydrophilic (water-loving) carboxylate group
(-CO2) interacts with water molecules via ion-dipole
interactions and hydrogen bonding. The hydrophobic
(water-fearing) part of a soap molecule, its long, nonpolar
hydrocarbon chain, does not interact with water molecules. The
hydrocarbon chains are attracted to each other by dispersion
forces and cluster together, forming structures called micelles.
In these micelles, the carboxylate groups form a
negatively-charged spherical surface, with the hydrocarbon
chains inside the sphere. Because they are negatively charged,
soap micelles repel each other and remain dispersed in water.
Grease and oil are nonpolar and insoluble in water. When soap
and soiling oils are mixed, the nonpolar hydrocarbon portion of
the micelles break up the nonpolar oil molecules. A different
type of micelle then forms, with nonpolar soiling molecules in
the center. Thus, grease and oil and the 'dirt' attached to them
are caught inside the micelle and can be rinsed away.
Although soaps are excellent cleansers, they do have
disadvantages. As salts of weak acids, they are converted by
mineral acids into free fatty acids:
CH3(CH2)16CO2-Na+ + HCl → CH3(CH2)16CO2H + Na+ + Cl-
These fatty acids are less soluble than the sodium or potassium
salts and form a precipitate or soap scum. Because of this,
soaps are ineffective in acidic water. Also, soaps form
insoluble salts in hard water, such as water containing
magnesium, calcium, or iron.
2 CH3(CH2)16CO2-Na+ + Mg2+ → [CH3(CH2)16CO2-]2Mg2+ + 2 Na+
The insoluble salts form bathtub rings, leave films that reduce
hair luster, and gray/roughen textiles after repeated washings.
Synthetic detergents, however, may be soluble in both acidic and
alkaline solutions and don't form insoluble precipitates in hard
water.
from chemistry.about
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