Colloid chemistry Lecture 10: Surfactants
Applications of surfactants: cleaning/detergents (40%); textiles; cosmetics; pharmacy; paint; food; etc.
Etymology
Surfactant micelles surfactant molecule spherical micelle hydrophobic alkyl chain self-assembling (association) hydrophilic head group 30-100 molecules d-3-5 nm hydrophilic shell hydrophobic core cationic surfactant anionic surfactant nonionic surfactant OSO 3- Na + surfactant = surface active agent orientation energy minimum Hardy-Harkins principle
Soap molecules/micelles Hydrophilic ( lyophobic, waterloving) head containing a charged functional group Hydrophobic ( lyophilic, water-fearing ) tail containing a hydrocarbon chain If enough soap is added to water the molecules arrange themselves into a structure called a micelle
How are soap produced? Soaps are produced from complex esters: animal fat The saponification of a triglyceride
Classification of Surfactants
Classification of Surfactants Anionic Sodium dodecylsulfate (SDS) O O S - + O O Na Cationic Cetylpyridinium bromide N + Br - Zwitterionic O O O O Dipalmitoylphosphatidylcholine (lecithin) + OCH 2 CH 2 N(CH 3 ) 3 P O - O Nonionic O Tetraoxyethylene lauryl ether O O O OH
Micellar shapes
Packing parameter affects micellar shape
Surfactant phases
Surfactants as biocolloids
Surfactants as biocolloids plasma membranes are primarily lipid bilayers with associated proteins and glycolipids (cholesterol is also a major component of plasma membranes)
Surfactants as biocolloids
Peptide micelles Small, viral sized (10-50 nm) particles Similar to lipid micelles Composed of peptide core (hydrophobic part) and PEG shell (hydrophilic part) Peptide core composition allows peptide/protein solubilization Also good for small molecules
Diblock copolymer micelles PB 202 -PEO 360 molecule micelle
Solution Properties γ 10 8 6 4 2 0 cmc Surface tension CMC 0log (surfactant concentration) 1 14 12 10 8 6 4 2 0 Concentration π 14 12 10 8 6 4 2 0 CMC unimers micelles 0 Surfactant concentration1 Osmotic pressure CMC 0 Surfactant concentration1 14 Λ 12 10 8 6 4 2 0 Molar conductivity CMC 0 (Surfactant concentration) 1 1/2 τ 14 12 10 8 6 4 2 0 CMC Light scattering 0 Surfactant concentration1
Krafft point; cloud point; cmt I. Above the Krafft point, the solubility is sharply increases due to the formation of micelles. II. Some surfactants (e.g. poly(ethylene)-poly(ethylene-oxide) = PE PEO = C m E n ) - based surfactants) dehydrate and phase separate upon temperature elevation ( cloud point ). III. For pluronics; poly(propylene-oxide)-poly(ethylene-oxide) = (PPO-PEO) diblock copolymer surfactants, the micelles form as a result of the dehydration of the PEO segments ( critical micelle temperature or CMT ).
Factors affecting the cmc: molecular structure of the surface active agent
Factors affecting the cmc: effect of additives
HLB of surfactants
HLB and use of surfactants Amphiphilic surfactants are characterized by the hydrophiliclipophilic balance (HLB): a relative ratio of polar and nonpolar groups in the surfactant HLB ca. 1 to 3: Antifoams / inverse micelles HLB ca. 3 to 8: Water-in-Oil Emulsifiers HLB ca. 7 to 9: Wetting and spreading agents HLB ca. 8 to 16: Oil-in-Water Emulsifiers HLB ca. 13 to 16: Detergents HLB ca. >15: Solubilizers
Griffin s scale: it is an arbitrary (group additivity) scale of values serving as a measure of the HLB of surfactants (polysorbates) (sorbitan monooleates)
HLB of surfactant blends Surfactant blends are commonly used to obtain desired emulsifying properties. What is the HLB of the mixture of 40 % Span 60 (HLB = 4.7) and 60 % Tween 60 (HLB = 14.9)? HLB of mixture: 4.7 x 0.4 + 14.9 x 0.6 = 10.8 In what proportion should Span 80 (HLB = 4.3) and Tween 80 (HLB = 15.0) be mixed to obtain required HLB of 12.0? 4.3. (1-x) + 15. x = 12 x = 0.72 72 % Tween 80 and 28 % Span 80
Solubilization Spontaneous transfer of a compound insoluble in the bulk solvent into solution due to incorporation into the surfactant micelles: normal micelles reverse micelles non-polar compound polar compound in polar solvents amphiphilic compound in non-polar solvents
Detergency (contact angle increases)
amount of substance solubilized K-soap concentration, mol/l elementary steps of washing solubilization efficiency of K-soaps (solubilization capacity: tg α)