CHEM1612 - Pharmacy Week 13: Surfactants

1. CHEM1612 - PharmacyWeek 13: Surfactants Dr. SiegbertSchmid School of Chemistry, Rm 223 Phone: 9351 4196 E-mail: siegbert.schmid@sydney.edu.au

2. Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille,Chemistry, John Wiley & Sons Australia, Ltd. 2008      ISBN: 9 78047081 0866

3. self-cleaning surfaces goretex, teflon... Surface Tension and Surfactants • Blackman, Bottle, Schmid, Mocerino & Wille: Chapters 7, 22 ‘lotus effect’

4. Hi-tech: printing Need to understand, and be able to control both the breakup of liquid jets into small droplets, and the subsequent interaction of the droplets with the target. inkjet

5. BASF Surface Tension Liquid/air interface • Any interface (liquid/liquid, liquid/solid, liquid/gas, etc.) has an associated surface energy. • To understand, think of a liquid droplet: the molecules in the interior of the droplet are surrounded by other molecules of the same kind. However, those at the liquid surface are subject to attractions only from the sides and from below. • The effect of this uneven pull on the surface molecules is to draw them into the body of the droplet. Therefore a droplet assumes a spherical shape. • The sphere is the geometry with minimum surface area. • The liquid appears to have a skin over the surface. Bulk liquid

6. Herminghaus, 2003. Surface Tension • Molecules on the surface would rather be in the bulk. • To increase the surface area, molecule within a phase must move from the bulk to the surface, by breaking some attractive interactions, which requires energy. The resistance of a medium to an increase in its surface area is called the surface tension of the medium (can be solid or liquid). The surface tension is the energy required to increase the surface area by a unit amount (units J/m2 = N/m).

7. Surface Tension of Liquids • The stronger the forces between the molecules, the higher the surface tension. • Polar solvents like water tend to have fairly high surface tension because they interact strongly by H bonding. hydrogen bond donor hydrogen bond acceptor H bond Blackman Figure 6.32

8. Hydrophobic tail hydrophilic head Surfactants • Surface-active agents: any molecule that is amphiphilic, i.e with portions that are • hydrophobic (= water hating, therefore lipophilic, fat loving) and • hydrophilic (= lipophobic) portions. Surfactants decrease the surface tension of water by adsorbing at the water/air interface and disrupting the H bonds. Figure from Silberberg, “Chemistry”, McGraw Hill, 2006. Soaps and detergents are surfactants, often salts of fatty acids.

9. Mineral oil Water Water Demo: Adding Surfactant to Water • A beaker full of water • We add fine sulphur powder, whose weight can be supported by the water/air interface due to water surface tension g0. • Then we add soap, which decreases the surface tension of water, and so the powder cannot be supported anymore. Water Surface tension g0 Olive oil

10. Soap Much of what we call dirt is non-polar. Grease for example consists of long chain hydrocarbons. • However water, the solvent most commonly available to us is very polar and will not dissolve ‘greasy dirt’ • Soap can be viewed as an emulsifying agent, since it acts to suspend the normally incompatible grease in the water. • Because of this ability to assist water in ‘wetting’ and suspending nonpolar materials, soap is called a wetting agent or surfactant.

11. Reduced Interaction of chains with water Hydrophobic interactions between chains Self-Assembly: Micelles • As a surfactant is added to water, the molecules adsorb at the air/liquid interface, but otherwise are free in solution. • Above a certain conc., they spontaneously aggregate into micelles. • This occurs at the CRITICAL MICELLE CONCENTRATION (c.m.c.) • A “soap” solution contains both individual surfactants dispersed in water and aggregates (micelles). Thus a soap-water mixture is a suspension of micelles in water. Because the relatively large micelles scatter light (colloidal), soapy water looks cloudy. at the c.m.c.

12. Detergents • Artificial soaps are known as detergents • Most widely used class of detergents used are the alkylbenzenesulfonates(with –SO3- group) • Fabric softener are often quaternary ammonium salts

13. “Hard” and “Soft” Water • “Hard water” contains high amounts of divalent ions such as Ca2+, Mg2+, Fe2+. • The disadvantage of soaps is that the anions form precipitates with the cations like Ca2+ and Mg2+. This forms a scum and reduces the soaps’ efficiency. • Ca2+ + 2 C17H35COONa(aq) → (C17H35COO)Ca (s) + 2 Na+

14. Revision Lecture - Thursday • If you have questions send them to me by e-mail and I will incorporate relevant material. • We will not cover any new material. • Everything we have covered in lectures (not lecture notes) is examinable, as is the chemistry relating to your laboratory work.

15. Soaps are produced by saponification: the hydrolysis of lipids to glycerol and salts of fatty acids (carboxylate salts = soaps) by KOH or NaOH. KOH carboxylate salt glycerol Polar-ionic head Lipids Figure from Silberberg, “Chemistry”, McGraw Hill, 2006. • Lipids are broadly defined as any amphiphilic, naturally-occurring molecules. The term is also used more specifically to refer to fatty-acids and their derivatives, as well as other fat-soluble sterol-containing metabolites such as cholesterol. • fats • phospholipids (with a organophosphate group) • waxes • steroids • Fats that are esters of glycerol are called triglycerides. Hydrophobic tail

16. Head group Tail group air water Self-Assembly in Lipids • In the self-assembly of surfactants and lipids, hydrophobic interactions are important A lipid is an amphiphilic molecule, but rarely exists as a monomer. Inverse micelle (in nonpolar solvent) Lipid bilayer monolayer Micelle

17. Phospholipids Phospatidylcholine hydrophilic • Phospholipids are similar in structure to fats in that they are esters of glycerol. However unlike fats they contain only two fatty acids. The third ester linkage involves a phosphate group, which gives phospholipids two distinct parts: • long non-polar tail • polar substituted phosphate “head” • Phospholipids tend to form bilayers in aqueous solution with the tails in the interior and the polar heads interfacing with the polar water molecules. Apolar hydrophobic Long hydrophobic tails a bilayer Polar head

18. Self-assembly in Phospholipids • Phospholipids prefer to form bilayer structures in aqueous solution because their two fatty acid chains do not pack well; • Phospholipids can form either unilamellar vesicles (liposomes) or multilamellar vesicles; multilamellar vesicles unilamellar vesicles (liposomes); highly stable, can be used as drug and enzyme delivery systems

19. Cell Membrane Figure from Silberberg, “Chemistry”, McGraw Hill, 2006. • A cell membrane is a bilayer of phospholipids, embedded with various proteins, and protects the cell from the extracellular fluid that surrounds it. • Allows nutrients and other necessary chemicals to enter the cell and waste products to leave, through the proteins that act as pumps, gates, and channels. • Biological membranes are sites of biochemical reactions that include photosynthesis, electron transfer, oxidative phosphorylation; • Facilitate cell motion; provide cell recognition and cell fusion. Ca. 8 nm thick

20. Fluid Mosaic Model • The most widely accepted model of this transfer of nutrients and waste is called the fluid mosaic mode, proposed by S. J. Singer and G. L. Nicolson, in 1972. • The phospholipid bilayer is a fluid matrix: the bilayer is a two-dimensional solvent, lipids and proteins can undergo rotational and lateral movement • Small uncharged molecules such as water, oxygen and carbon dioxide diffuse freely through the bilayer, while other substances pass through “gates and passages” provided by specific proteins embedded in the membrane. Figure from Silberberg, “Chemistry”, McGraw Hill, 2006.

21. You should now be able to • explain the molecular origin of surface tension • understand the chemical nature and action of surfactants and detergents, and the process of self-assembly • describe the function and chemical structure of biological membranes