How soap works: micelles
• Soap is an excellent cleanser
non-polar CH2 units; ionic COO-Na+ group
• soap acts as an emulsifying agent
• it can disperse a liquid into another
immiscible liquid.
Application: Fats and oils don't dissolve in
H2O.
• Soap can suspend oil/dirt in such a way
that it can be removed.
• Grease and oil are polar/non-polar and
soluble/insoluble in HOH.
(non-polar, insoluble)
• When soap added to oil-containing
solution, the nonpolar hydrocarbon portion
of the soap breaks up the nonpolar oil
molecules.
• A micelle then forms—with nonpolar
solutes in the centre.
• The outside of the micelle is ionic—water
soluble. Hence . . .
grease and oil washed away.
ie. Each soap molecule has a long
hydrocarbon chain, sometimes called its
'tail', with a carboxylate 'head'.
a soap micelle 
3-D diagram of a micelle . . .
“Hard” Water Reduces
Efficiency of Soap
• Hard water contains Ca2+ and Mg2+ ions
• These cations react with the anionic
portion of soap to form an insoluble “scum”
–aka calcium stearate.
Soap scum has given rise to special
cleaners . . .
What’s the problem with these cleaners . . .
Cleaners are . . .
expensive;
they need to be applied;
they get washed down the drain.
So what’s the solution?
1. Don’t wash.
2. Remove Ca2+ and Mg2+ ions from the
water.
water softening
unit
This is expensive and may not be possible.
3. Make a different kind of soap.
But how?
What to change?
We can make a detergent (soap) where the
“head” of the molecule a positive charge...
ie. a cationic synthetic detergent
Synthetic Detergents
• have similar molecular structures and
properties as soap;
• less likely—or unlikely—to form soap
“scum” with Ca2+ and Mg2+ ions (hard
water);
• many have a negatively-charged “head”—
they are anionic detergents.
Cationic Detergents
• good cleansing agents
• possess germicidal properties
• most cationic detergents are derivatives of
ammonia.
cationic detergents (con’t)
• often found in a shampoo
or clothes "rinse".
• Able to neutralize the static electrical
charges from residual anionic detergent
molecules.
Also: Non-ionic syn. dets:
Non-ionic syn detergents
• do not react with hard water ions;
• used for dish washing liquid;
• foam less than ionic detergents.
can form H-bonds with water
also polar
non-polar tail dissolves fats,
and oils, etc
Biology Application: Bile Salts
• major action of a bile salt is to emulsify
fats and oils into smaller droplets.
• enzymes can then break down the fats
and oils.
In what organ of the body is bile produced?
Look at structure of a bile salt on next slide.
How do you think bile salts emulsify fats?
In the same way that soaps and detergents
do . . .
with a polar “head” and non-polar “tail”.
Lab: Preparation and
Testing of Soap
General preparation of soap:
fat or oil + NaOH(aq)  soap + glycerol
what we do:
coconut oil + NaOH(aq)  soap + glycerol
20 g
20 mL
6.0 mol/L
Test our hardened soap by adding it to:
• tap water
• deionized water
• “hard” water
• “soft” water
. . . results . . .
Observations
In SOFT WATER
• good lather
• soap is in solution
In DE-IONIZED WATER
• some lather
• good solution of soap
Observations
In TAP WATER
• virtually no lather
• small pieces of soap
scum (ppt’ed soap)
floating
In HARD WATER
• no lather
• no soap in solution
• thick layer of soap
scum on top
Conclusion:
Simple soap, such as the one that we made,
works better in soft/hard water.
soft . . . why?
Soft water contains no Ca2+ or Mg2+ ions
that can precipitate the soap.