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Colloid chemistry
Lecture 10: Surfactants
Applications of surfactants: cleaning/detergents (40%);textiles; cosmetics; pharmacy; paint; food; etc.
Etymology
Surfactant micellessurfactant molecule
hydrophobicalkyl chain
hydrophilichead group
self-assembling
spherical micelle
hydrophilic shellhydrophobic core
cationic surfactantanionic surfactantnonionic surfactant
orientation → energy minimumHardy-Harkins principle
30-100 moleculesd-3-5 nm(association)
OSOOSO33--NaNa++
surfactant = surface active agent
If enough soap is added to water the molecules arrange themselves into a structure called a micelle
Hydrophilic ( lyophobic, water-loving) head containing a charged functional group
Hydrophobic ( lyophilic,water-fearing ) tail containing a hydrocarbon chain
Soap molecules/micelles
Soaps are produced from complex esters:
animal fat
How are soap produced?
The saponification of a triglyceride
Classification of Surfactants
Anionic
Cationic
Zwitterionic
Nonionic
Sodium dodecylsulfate (SDS)
Cetylpyridinium bromide
Dipalmitoylphosphatidylcholine (lecithin)
O O O O OHTetraoxyethylene lauryl ether
SO-Na+
O
OO
OO
P
O
OO
OCH2CH2N(CH3)3+
O-
Classification of Surfactants
N+
Br-
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
Small, viral sized (10-50 nm) particles
Similar to lipid micelles
Composed of peptide core (hydrophobicpart) and PEG shell (hydrophilic part)
Peptide core composition allows peptide/protein solubilization
Also good for small molecules
Peptide micelles
Diblock copolymer micelles
molecule micelle
PB202-PEO360
02468
101214
0 1Surfactant concentration
CMC
Con
cent
ratio
n unimers
micelles
02468
101214
0 1Surfactant concentration
CMC
Osmotic pressureπ
02468
101214
0 1(Surfactant concentration)1/2
CMC
Molar conductivityΛ
02468
101214
0 1Surfactant concentration
CMC
Light scattering
Solution Properties
02468
10
0 1log (surfactant concentration)
CMC
Surface tensionγ
τ
cmc
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 = CmEn) - 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”).
Krafft point; cloud point; cmT
Factors affecting the cmc: molecular structure of the surface active agent
Factors affecting the cmc: effect of additives
HLB of surfactants
• 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
Amphiphilic surfactants are characterized by the hydrophilic-lipophilic balance (HLB): a relative ratio of polar and non-polar groups in the surfactant
HLB and use of surfactants
Griffin’s scale: it is an arbitrary (group additivity) scale of values serving as a measure of the HLB of surfactants
(polysorbates)
(sorbitan monooleates)
• 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
Surfactant blends are commonly used to obtain desired emulsifying properties.
HLB of surfactant blends
polar compound
reverse micelles
non-polar compound
normal micelles
amphiphilic compound
Solubilization
Spontaneous transfer of a compound insoluble in the bulk solventinto solution due to incorporation into the surfactant micelles:
in polarsolvents
in non-polarsolvents
Detergency
(contact angle increases)(contact angle increases)
elementary steps of washing solubilization efficiency of K-soaps(solubilization capacity: tg α)
amou
nt o
f sub
stan
ce so
lubi
lized
K-soap concentration, mol/L