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ISM07

Surfactants

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Topics

• Common surfactants• Micellization• Solubilization• Emulsification• Adsorption• Detergency

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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?

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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

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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

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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

(very) mild

Washing powder

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Biological lipidsfat

Notice the doubletailsMost lipidscan be found inbiomembranes(ISM08)…In general, they areinsoluble

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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.

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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?

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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

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Closed Association Model properties

0.80.60.40.2

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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

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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:

isapproximately-3 kJ/mol per methyleneunit

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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 becauseof 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 dropletunstable 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 adsorbedsurfactant molecules

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Gibbs adsorption isothermIn 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 constantExercise: from the given γ(c) curve, sketch theadsorption 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 modelKinetic derivation

Questionable!diffusion to surface neglected

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Langmuir modelThermodynamic derivation

surface

solution:

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• 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

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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!

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Detergency

When the dirt is liquid,the detergent acts by changingincreasing 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 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…

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This ends file ISM08

Please explain why the monks add ossegal