how soap works: micelles

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 + glycerol20 g 20 mL

6.0 mol/L

Test our hardened soap by adding it to:

• tap water

• deionized water

• “hard” water

• “soft” water . . . results . . .

ObservationsIn SOFT WATER• good lather• soap is in solution

In DE-IONIZED WATER• some lather• good solution of soap

ObservationsIn 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.