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Introduction to surfactants andself-assembly

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Self-assembled nanostructures from amphiphilic molecules

Micelle Bilayer Cubic phase

1 nmSurfactant science is nano-science!

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Amphiphilic (cf. amphi (gr.) = both)

Hydrophobic hydrophilic+

OIL WATER SOLUBLE STUFF

Surfactants, polymers, proteins,……

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SDS: CHSDS: CH33(CH(CH22))1111SOSO44-- NaNa++

CTAB: CHCTAB: CH33(CH(CH22))1515NN++(CH(CH33))33 ClCl--

CC1212EE88: CH: CH33(CH(CH22))1111(OCH(OCH22CHCH22))88OHOH

Examples

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In general:

CH3-N+-CH3 Cl-CH3

CH3

NaNa22SOSO44

H(OCH2CH2)8OH

CH3(CH2)10CH3

CH3(CH2)14CH3

CH3(CH2)10CH3

Water solubleOil insoluble

Water insolubleOil soluble

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Also, other water soluble groups can be used:

O

OHO

OHOH

CH2

OH

OO

OHOH

CH2

OH

O

OHO

OHOH

CH2

OH

Sugars

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When amphihilic molecules are put in a system with an interface: water/oil or water/air:

oil

water

they enrich at the surface: they are surface activewe call them surfactants (in biology often lipids)

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oil

water

The water – waterinteraction is more favorable thenthe water – oil interaction!In other words, it is the strong cohesion between water moleculesthat imparts low solubility of hydrocarbons in water.

Hydrophobic effect!

Like dissolves like!

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Proteinfolding

Immiscibility

Wetting

J. Phys. Chem., 2002,106,521.

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Let us put SDS in water!

+

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Equivalent conductivity

At low concentrations, SDS behaves as a “simple” salt. At a well-defined concentration

many physical parameters change. Here micellesare formed, hence theconcentration is calledcritical micelle concentrationcmc.

(micelle = small bit)

cmc

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An SDS – micelle! Approx. 60 SDS moleculesper micelle.

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Interior is liquid-like, similar to hydrocarbon droplet.The micelle can solubilize non-polar substances.

Surface is highly charged with a high concentration of counter-ions.

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Micellization is a start-stop process:Start: hydrophobic effect (phase separation!).Stop: electrostatic repulsion among headgroups.

- -- --

Repulsion!

Micelle has finite size!

Size distribution

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The micellization is a co-operative process!

Unfavorable!

16 14

12 8

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Consider the aggregation of N surfactants into a micelle of aggregation number N

( )lnof f fkT Cμ μ= +

lno micmic mic

CkTN N

μ μ ⎛ ⎞= + ⎜ ⎟⎝ ⎠

( ) ( )1/

o omic f

Nmic

fCC eN

μ μ−⎛ ⎞= ⎜ ⎟⎝ ⎠

NS SN

Chemical potential monomer

Chemical potential micelle

-

Solve for Cf and Cmic as functions of Ctot

If N large, Cf constant.

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N = 60

N→∞

For N→∞, theconcentration of free SDSconstant above cmc:phase separation model.For finite N,concentration of freeSDS decreases abovecmc(experimental result)

This is due to electrostatic effects, not capturedin a description based on an equilibrium constant!

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Kinetics:The micelle is held together by physical forces;it is a “living” entity!We can identify two kinetic time-scales:

1. Life-time of one surfactant in a micelle:

2. Life time of one micelle, considerably longer: for SDSat 25 C around 1 ms.

1 1

off onk k cmcτ= =1N NA A A ++

kon

koff

1 25s for SDS at Cτ μ≈

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What happens if[SDS] is increasedfurther?

First, aggregate growth, then crystallization intolyotropic liquid crystals.

cpp = critical packing parameter.

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The critical packing parameter, cpp(surfactant number).

A

v

- cpp = v

A x

For sphere: 1/3For cylinder: 1/2For bilayer 1

From volume to area ratios:

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Reversed

cpp ≤ 0.5

cpp > 1

cpp ≈ 1

Normal

Bilayer

Reversed

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cpp can be changed by:

Changing concentration, adding a secondsurfactant, changing temperature, addingsalt etc.

- -- --

+ + ++

Screening

++

- -- -

++

Adding a co-surfactant, non-charged

- T -

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Microemulsions:

“Are stable solutions containing “oil”, water anda surfactant”

Fish-cut:

Sample with equal volumesof oil and water, addsurfactant!

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Two examples:

1. Washing (practical)

2. Fat digestion (biological)

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1. Washing

Dirt on fibers

Surfactants adsorb and replace dirt

Dirt is solubilized in aggregates

Aggregates and dirtrinsed off

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Shampoo, a complex product:

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2: Fat digestion.

Bile salt

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Fat digestive enzyme, lipas

Bile salt emulsifiesthe fat, so the enzymecan brake it down!

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Schematic illustration of steps in solubilizationand digestion of fats!

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