How soap works: micelles

1. How soap works: micelles

2. 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.

3. Application: Fats and oils don't dissolve in H2O. • Soap can suspend oil/dirt in such a way that it can be removed.

4. 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.

5. 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.

6. ie. Each soap molecule has a long hydrocarbon chain, sometimes called its 'tail', with a carboxylate 'head'. a soap micelle

7. 3-D diagram of a micelle . . .

8. “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.

9. Soap scum has given rise to special cleaners . . . What’s the problem with these cleaners . . .

10. Cleaners are . . . expensive; they need to be applied; they get washed down the drain. So what’s the solution?

11. 1. Don’t wash. 2. Remove Ca2+ and Mg2+ ions from the water. water softening unit This is expensive and may not be possible.

12. 3. Make a different kind of soap. But how? What to change?

13. We can make a detergent (soap) where the “head” of the molecule a positive charge... ie. a cationic synthetic detergent

14. 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.

16. Cationic Detergents • good cleansing agents • possess germicidal properties • most cationic detergents are derivatives of ammonia.

17. cationic detergents (con’t) • often found in a shampoo or clothes "rinse". • Able to neutralize the static electrical charges from residual anionic detergent molecules.

18. Also: Non-ionic syn. dets:

19. Non-ionic syn detergents • do not react with hard water ions; • used for dish washing liquid; • foam less than ionic detergents.

20. can form H-bonds with water also polar non-polar tail dissolves fats, and oils, etc

21. 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.

22. In what organ of the body is bile produced?

23. 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”.

25. 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

26. Test our hardened soap by adding it to: • tap water • deionized water • “hard” water • “soft” water . . . results . . .

27. Observations In SOFT WATER • good lather • soap is in solution • In DE-IONIZED WATER • some lather • good solution of soap

28. 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

29. 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.