Introduction to surfactants and self-assemblywpage.unina.it/lpaduano/PhD Lessons/INTRODUCTION...
Transcript of Introduction to surfactants and self-assemblywpage.unina.it/lpaduano/PhD Lessons/INTRODUCTION...
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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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