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...
1
Introduction to surfactants andself-assembly
2
Self-assembled nanostructures from amphiphilic molecules
Micelle Bilayer Cubic phase
1 nmSurfactant science is nano-science!
3
Amphiphilic (cf. amphi (gr.) = both)
Hydrophobic hydrophilic+
OIL WATER SOLUBLE STUFF
Surfactants, polymers, proteins,……
4
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
5
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
6
Also, other water soluble groups can be used:
O
OHO
OHOH
CH2
OH
OO
OHOH
CH2
OH
O
OHO
OHOH
CH2
OH
Sugars
7
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)
8
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!
9
Proteinfolding
Immiscibility
Wetting
J. Phys. Chem., 2002,106,521.
10
Let us put SDS in water!
+
11
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
12
An SDS – micelle! Approx. 60 SDS moleculesper micelle.
13
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.
14
Micellization is a start-stop process:Start: hydrophobic effect (phase separation!).Stop: electrostatic repulsion among headgroups.
- -- --
Repulsion!
Micelle has finite size!
Size distribution
15
The micellization is a co-operative process!
Unfavorable!
16 14
12 8
16
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.
17
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!
18
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≈
19
What happens if[SDS] is increasedfurther?
First, aggregate growth, then crystallization intolyotropic liquid crystals.
cpp = critical packing parameter.
20
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:
21
Reversed
cpp ≤ 0.5
cpp > 1
cpp ≈ 1
Normal
Bilayer
Reversed
22
cpp can be changed by:
Changing concentration, adding a secondsurfactant, changing temperature, addingsalt etc.
- -- --
+ + ++
Screening
++
- -- -
++
Adding a co-surfactant, non-charged
- T -
23
Microemulsions:
“Are stable solutions containing “oil”, water anda surfactant”
Fish-cut:
Sample with equal volumesof oil and water, addsurfactant!
24
Two examples:
1. Washing (practical)
2. Fat digestion (biological)
25
1. Washing
Dirt on fibers
Surfactants adsorb and replace dirt
Dirt is solubilized in aggregates
Aggregates and dirtrinsed off
26
Shampoo, a complex product:
27
2: Fat digestion.
Bile salt
28
Fat digestive enzyme, lipas
Bile salt emulsifiesthe fat, so the enzymecan brake it down!
29
Schematic illustration of steps in solubilizationand digestion of fats!
30
Read more: