The Standard Free Energy of Surfactant Adsorption at Air ...
Structure of Surfactants Surfactant Surface Active Agent...
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Surfactant Surface Active Agent polar headgroup hydrophobic tail Examples: SDS: C 10 H 21 OSO 3 - Na + DDAB: (C 12 H 25 ) 2 N(CH 3 ) 2 + Br - C 12 E 5 : C 12 H 25 O(C 2 H 4 O) 5 H DMPC: C 13 H 27 COOCH 2 C 13 H 27 COOCH CH 2 OPO 3 - CH 2 CH 2 N(CH 3 ) 3 + Structure of Surfactants
Transcript of Structure of Surfactants Surfactant Surface Active Agent...
Surfactant Surface Active Agent
polar headgrouphydrophobic tail
Examples: SDS: C10H21OSO3- Na+
DDAB: (C12H25 )2N(CH3)2+ Br -
C12E5: C12H25O(C2H4O)5H
DMPC: C13H27COOCH2
C13H27COOCH CH2OPO3
- CH2CH2N(CH3)3+
Structure of Surfactants
OR Na+
R = OSO3- / OCH2COO- / OPO3
2-
O
O
O
O
SO3- Na+
OO
OO
OR Na+
NH3+ X-
X = Cl, Br
N+
CH3
CH3
CH3X
N+
CH3
CH3X
Typical Anionic Surfactants
Typical Cationic Surfactants
Sodium alkyl sulfate / carboxylate / phosphate
Sodium bis(2-ethylhexyl)sulfosucinate (AOT)
Sodium alkylether sulfate / carboxylate / phosphate
Fatty amine halide Alkyl trimethylammonium halide
Dialkyl dimethylammonium halide
OO
OO
OOH
O
OHOH
O
OH
OO
OO
OO
OH
Typical Nonionic Surfactants
Fatty alcohol ethoxylate (CmEn)
Sorbitan alkanoate (´Span´)
Alkylphenol ethoxylate
O
O
CH2
CH
CH2
O
O
O
O
O
O
CH2
CH
CH2
O
O
O P OR
O
O
R = CH2CH2N(CH3)3+ Phosphatidylcholine
R = CH2CH2NH3+ Phosphatidylethanolamine
COO-
Typical Lipids
Fatty acid salt
triglyceride
phospholipid
log xs
g
cmc
Surfactants in Water
Gibbs Adsorption Isotherm: ˜¯
ˆÁË
Ê∂
g∂-=G
clnRT
1s
Surface tensionw = gDA
cmc _ exp(-De/kT)
De : transfer energy associated with moving a surfactant molecule from its monomeric form into its micellized state
cmc: typical values 10-2 M (e.g. ionic single chained soaps)
10-16 M (e.g.double chained phospholipids)to
Residence time of surfactant molecules within aggregates: tR _ t0 /cmc
t0 : motional correlation time (_10-9 - 10-7 s)
tR _ 10-6 s (micelles) - 10 8 s (biological membranes)
Dynamics of Self-Assembly
Self-Assembly of Surfactants
sphere rod disc
I NL NC
H L
amphiphile
The close packing volume fractions increase as the curvature of the colloidal particles decreases:
fmax =
p
12= 0.91
fmax =1
fmax =
p
18= 0. 74solid spheres:
cylinders:
planar bilayers:
decreasing curvature less excluded volume
lower interaction free energy
“self-energy” (surfactant parameter) + entropy of dispersion
I1 H1 V1 La V2 H2 I2
T
0 1f
Schematic Surfactant-Water Phase Diagram
no change in the sequence of mesophases
Cone:
V = al/3
a
l
Association number of a spherical micelle
V
Rp
3mic3
4 p=
a
R4p
2micp
=or
3
1
aR
V
mic
=
composed of surfactant molecules with volume V and area per molecule a:
Molecular Shape
v/al < 1 v/al = 1 v/al > 1
Surfactant Parameter: v/al
v: volume of the hydrophobic region
l: preferred length of the hydrophobic chain
a: polar headgroup cross sectional area
Local curvature of surfactant aggregates:
Surfactant packing parameter range for various surfactant aggregates
spherical micelles V/al < 1/3
cylindrcal micelles 1/3 < V/al < 1/2
vesicles, flexible bilayers 1/2 < V/al < 1
lamellae, planar bilayers V/al inverse micelles V/al > 1
ª 1
Cubic-I Phases
I1: direct micellesI2: reversed micelles
• “ringing gels”• non birefringent
NL NC
Nematic Phases
• no spatial periodicity• low viscosity• alignment in external fields
‘Unit Cell’ of the Cp - Phase
Phase Symmetry: Im3m
two interwoven polar labyrinths divided by a surfactant-bilayer
Minimal Surfaces
D G P
Structure of the Lamellar Phase
1D periodicity
basic bilayer structure of biological membranes
variations of interlamellar spacings
The Structure of the Hexagonal Phase
2-D Periodicityinfinitely long rods
