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What
is Soap?
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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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| Note
about 3D molecules -- Our files on this page
now use Jsmol instead of Jmol. These
files make use of Javascript which permits
viewing of molecules on tablets, phones and
easier use on Macs. - Jsmol is best viewed
with the Chrome browser. |
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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
oxygen and sodium.
Soap
molecules have both properties of non-polar and polar at
opposite ends of the molecule.
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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.
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[Micelle
shown using the Jsmol
Molecular dynamics simulations of dodecylphosphocholine
D. P. Tieleman, D. van der Spoel, H.J.C. Berendsen] --File
shown without water molecules
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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
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Micelle
(above) shown with water molecules. |
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