Post on 27-Jan-2022
1
ISM07
Surfactants
2
Topics
• Common surfactants• Micellization• Solubilization• Emulsification• Adsorption• Detergency
3
Types of surfactant
Fatty acid synthesis by saponification: heat fat (triglyceride ester) in alkaline solution (zeep zieden, je krijgt groene zeep)
Very old surfactants
Why is the melting point of unsaturated fatty acids lower?
4
het kan ook anders: zeepkruid
www.botanical.com
A Modern Herbal,by Mrs. M. Grieve
(Saponaria officinalis LINN.)soapwort
Saponins: sugars attached to steroid(phytochemistry)
Exercise: find molecular structure, and explain why it is a surfactant
Traditional non-alkaline soap
5
Nog andersHet 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 rosette-vorm). Norbertijneabdij Tongerlo (B)http://www.tongerlo.org/iconen/iconen_index.htm
Ossegal = bile salt
6
Modern surfactants
Shampoo!
SDS is one of the largest-produced synthetic surfactants
7
Modern surfactants II
Toxic in aquatic environment. Why?
8
Modern surfactants III
(very) mild
Washing powder
9
Biological lipidsfat
Notice the doubletailsMost lipidscan be found inbiomembranes(ISM08)…In general, they areinsoluble
10
Micellization
The micelle is in equilibrium withfree surfactant (unimers)
More extended shapes arepossible (ellipsoids etc.),depending on the Packing parameter
The hydrophilic ‘heads’ shieldthe hydrophobic ‘tails’ fromthe aqueous environment.
11
Critical micelle concentration
Below the CMC, there are no micellesFor c > CMC, theunimer concentration isapproximately 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 hasthe lower CMC?
12
Models for micellizationIn general, the number p, of surfactants per micelle varies slightlydepending on conditions. In the Closed Association Model, p is a constant: monodisperse micelles
13
Closed Association Model properties
0.80.60.40.2
3
2.5
2
1.5
1
0.5
0
x
y
x
y
K=1, p=20
Unimers only
Unimers + micellesExercise:draw micelle concentrationversus total concentration
14
Closed Association Model properties
(operational definition)
15
Phase separation model
• When p is large, the micelles can be regarded as a separate phase
• In equilibrium
16
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
17
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
18
CMC: effect of tail length• The hydrophobic effect is a surface effect:
isapproximately-3 kJ/mol per methyleneunit
19
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…
20
Solubilization
In the unimer regime, solubilization is not possible.Notice that the micelles (will be) can be distorted becauseof the apolar molecule sitting in the core
21
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)
22
Remember saponin?Remember ossegal?
23
Emulsification
+
Oil dropletunstable Protected oil
droplet
Emulsification is further discussed in ISM08
24
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 adsorbedsurfactant molecules
25
Gibbs adsorption isothermIn ISM06 we have discussed the Gibbs adsorption equation
26
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 constantExercise: from the given γ(c) curve, sketch theadsorption versus c
27
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
28
Langmuir modelKinetic derivation
Questionable!diffusion to surface neglected
29
Langmuir modelThermodynamic derivation
surface
solution:
30
• 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) curveExercise: can you see the CMC? (no)
is the Langmuir model valid above the CMC?do you expect micelles to adsorb?
Langmuir-Gibbs
31
DetergencyDetergent:Agent to remove dirtMany 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 isenormous!
32
Detergency
When the dirt is liquid,the detergent acts by changingincreasing the contact angle,emulsification and solubilization
33
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 solid-water or liquid(oil)-water interface, not the water-air interface…
• But consumers think a good foam is necessary…• Hence the industry adds special surfactants for
making lots of foam…
34
This ends file ISM08
Please explain why the monks add ossegal