ISM07
Surfactants
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Topics
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Common surfactants
Micellization
Solubilization
Emulsification
Adsorption
Detergency
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Types of surfactant
Very old surfactants
Fatty acid synthesis by saponification:
heat fat (triglyceride ester) in alkaline solution
(zeep zieden, je krijgt groene zeep)
Why is the melting point of unsaturated fatty acids lower?
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het kan ook anders: zeepkruid
Saponins: sugars attached to steroid
(phytochemistry)
Traditional non-alkaline soap
(Saponaria officinalis LINN.)
soapwort
A Modern Herbal,
by Mrs. M. Grieve
www.botanical.com
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Exercise: find molecular structure, and explain why it is a surfactant
Nog anders
Het specifieke materiaal dat men nog heeft om
iconen te schilderen is soms moeilijk te verkrijgen
is in de meeste speciaalzaken. Wie wil kan daarom
terecht bij onze cursus. De volgende materialen
zijn in de abdij te bekomen: iconenplanken (met of
zonder kalklaag, 31x26 cm), bladgoud,
goudborstel, alcoholvernis, ossegal, lijnolie
(olifa), aanlegmelk, schellak, miction (droogtijd 12
u), penselen Da Vinci (Kolinsky marder 1510, nrs.
1,2,3,4,6,8), penselen Kat-ton (vulkanisiert, nrs.
2,4,6,8,10,12), pigmenten in poedervorm
(chroomgroen, hemelsblauw, engels rood,
cadmiumrood, ivoorzwart, lichte oker, sienna
natuur, zinkwit, titaanwit, kraplak, umbra natuur,
umbra groen), menpaneeltjes (porselein in rosettevorm).
Norbertijneabdij Tongerlo (B)
http://www.tongerlo.org/iconen/iconen_index.htm
Ossegal = bile salt
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Modern surfactants
Shampoo!
SDS is one of the largest-produced synthetic surfactants
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Modern surfactants II
Toxic in aquatic environment. Why?
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Modern surfactants III
Washing powder
(very) mild
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Biological lipids
fat
Notice the double
tails
Most lipids
can be found in
biomembranes
(ISM08)…
In general, they are
insoluble
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Micellization
The micelle is
in equilibrium with
free surfactant (unimers)
More extended shapes are
possible (ellipsoids etc.),
depending on the
Packing parameter
The hydrophilic ‘heads’ shield
the hydrophobic ‘tails’ from
the aqueous environment.
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Critical micelle concentration
Below the CMC,
there are no micelles
For c > CMC, the
unimer concentration is
approximately constant
Exercise: compare surfactants A en B, with the same ‘heads’,
but the tail of A is larger than the tail of B. Which surfactant has
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the lower CMC?
Models for micellization
In general, the number p, of surfactants per micelle varies slightly
depending on conditions. In the Closed Association Model,
p is a constant: monodisperse micelles
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Closed Association Model properties
y
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K=1, p=20
2.5
Unimers + micelles
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Exercise:
draw micelle concentration
versus total concentration
1.5
1
Unimers only
0.5
0
0.2
0.4
0.6
0.8
x
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Closed Association Model properties
(operational definition)
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Phase separation model
• When p is large, the micelles can be
regarded as a separate phase
• In equilibrium
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Variation of CMC and p
• For most anionic surfactants: CMC is
independent from temperature (enthalpy of
micellization is small)
• Nonionics: CMC decreases with
temperature (hydrophilic poly ethylene
oxide becomes insoluble at higher T)
• p is more or less constant
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CMC: effect of surfactant
• The longer the tail, the lower the CMC
• The stronger the head-head repulsion, the
higher the CMC
• The stronger the hydrophobic nature of the
tail (fluorination), the lower the CMC
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CMC: effect of tail length
• The hydrophobic effect is a surface effect:
is
approximately
-3 kJ/mol per methylene
unit
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Solubilization
• When small apolar molecules are added they can
accumulate in the hydrophobic core of the
micelles (or more general: amphiphile aggregates)
• This process of solubilization is very important in
industrial and biological processes (emulsion
polymerization, detergency, drug delivery etc.)
• It is the only way we have of dispersing water
insoluble molecules into an aqueous
environment…
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Solubilization
In the unimer regime, solubilization is not possible.
Notice that the micelles (will be) can be distorted because
of the apolar molecule sitting in the core
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Solubilization example
• Digestion of fats by animals
• Bile salts are effective surfactants in the stomach
and intestine
• Ingested fats are first emulsified mechanically in
upper intestine
• Then hydrolyzed by pancreatic enzymes
• The fatty acids (insoluble at the intestine’s low
pH) are solubilized by the bile salts (mixed
micelle formation)
• And transported down the intestine, absorbed and
processed
(see next slide for figure)
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Remember saponin?
Remember ossegal?
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Emulsification
+
Oil droplet
unstable
Protected oil
droplet
Emulsification is further discussed in ISM08
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Adsorption
• Surfactants readily adsorb on apolar surfaces
• For example: air-water interface, surfaces of
apolar colloids (grease or dirt particles)
• In this way, the apolar surface AND apolar tails
are both protected from the aqueous environment
by the hydrophilic heads
• Adsorption on such apolar surfaces readily occurs
at c < CMC (why?)
Exercise: draw a picture showing a layer of adsorbed
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surfactant molecules
Gibbs adsorption isotherm
In ISM06 we have discussed the Gibbs adsorption equation
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Gibbs adsorption isotherm
• For c < CMC, we can calculate the
adsorption from the γ(c) curve
• For c > CMC this is not possible, since the
chemical potential of the surfactant is
constant
Exercise: from the given γ(c) curve, sketch the
adsorption versus c
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Langmuir model
• Langmuir developed a simple model to explain the
adsorption quantitatively
• Assume the solution is ideal and below the CMC
• Assume the surface is covered with adsorption
‘sites’ (like a 2D lattice), where each surfactant
molecule fits exactly into one surface cell, and
does not interfere with adsorption in neighboring
cells
• Assume chemical equilibrium between surface
and solution
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Langmuir model
Kinetic derivation
Questionable!
diffusion to surface neglected
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Langmuir model
Thermodynamic derivation
surface
solution:
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Langmuir-Gibbs
• Integration of the Langmuir model, using the
Gibbs isotherm gives, the Langmuir model
(ISM06) for the surface tension
Exercise: sketch both the adsorption curve, and the γ(c) curve
Exercise: can you see the CMC? (no)
is the Langmuir model valid above the CMC?
do you expect micelles to adsorb?
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Detergency
Detergent:
Agent to remove dirt
Many different detergents
(mild, strong, ionic, non-ionic)
depending on type of dirt and surface
Three properties of the detergent
are essential:
-it must wet the surface
-It must displace dirt from the surface
-It must sollubilize the dirt in solution
World-wide production is
enormous!
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Detergency
When the dirt is liquid,
the detergent acts by changing
increasing the contact angle,
emulsification and solubilization
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Detergency
• Some surfactants can also create a foam, this
means: stable air-water interface (further
discussed in next lecture)
• These surfactants are not necessarily good
detergents!, since detergents must act at the solidwater or liquid(oil)-water interface, not the waterair interface…
• But consumers think a good foam is necessary…
• Hence the industry adds special surfactants for
making lots of foam…
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Please explain why the monks add ossegal
This ends file ISM08
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