What is Soap?
Soaps
are mixtures of sodium or potassium salts of fatty acids which can be derived
from oils or fats by reacting them with an alkali (such as sodium or potassium
hydroxide) at 80°–100 °C in a process known as saponification. fat
+ NaOH ---> glycerol + sodium salt of fatty acid - CH2-OOC-R
- CH-OOC-R - CH2-OOC-R (fat) + 3 NaOH ( or KOH)
both
heated ---> - CH2-OH
-CH-OH - CH2-OH (glycerol) + 3 R-CO2-Na (soap) R=(CH2)14CH3
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| [Soap
structure shown using the Jmol Applet] |
Try this!!
Click the
right mouse button with the cursor over the image--> Labels
--> Element Symbols
Click on the left mouse button and rotate the soap structure. Notice
that one end of the molecules is made up of a hydrocarbon chain -- the other end
is a very polar structure containing of oxygen and sodium.
Soap molecules have both properties of non-polar and polar at opposite ends
of the molecule. | | |
How does Soap Work? Nearly
all compounds fall into one of two categories: hydrophilic ('water-loving') and
hydrophobic ('water-hating'). Water and anything that
will mix with water are hydrophilic. Oil and anything that will mix with oil are
hydrophobic. When water and oil are mixed they separate. Hydrophilic and hydrophobic
compounds just don't mix. The
cleansing action of soap is determined by its polar and non-polar structures in
conjunction with an application of solubility principles. The
long hydrocarbon chain is non-polar and hydrophobic (repelled by water). The
"salt" end of the soap molecule is ionic and hydrophilic (water soluble). When
grease or oil (non-polar hydrocarbons) are mixed with a soap- water solution,
the soap molecules work as a bridge between polar water molecules and non-polar
oil molecules. Since soap molecules have both properties of non-polar and polar
molecules the soap can act
as an emulsifier. 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 soap will form micelles (see below) and trap the fats within the micelle.
Since the micelle is soluble in water, it can easily be washed away.
| [Micelle
shown using the Jmol Applet
Molecular dynamics simulations of dodecylphosphocholine
D.
P. Tieleman, D. van der Spoel, H.J.C. Berendsen] --File shown without water molecules | When
you mix soap into the water the soap molecules arrange themselves into tiny clusters
(called 'micelles'). The water-loving (hydrophilic) part of the soap molecules
points outwards, forming the outer surface of the micelle. The oil-loving (hydrophobic)
parts group together on the inside, where they don't come into contact with the
water at all. Micelles can trap fats in the center. Try
this ---> click right mouse button over image --> spin --> on |
 | | Micelle
(above) shown with water molecules. |
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