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Amphiphiles: A Amphiphiles: A Tale of Heads and Tale of Heads and
TailsTailsAlokmay Datta
Saha Institute of Nuclear Physics
Saha Institute of Nuclear Physics, IndiaSaha Institute of Nuclear Physics, IndiaS. Kundu, M. K. Sanyal, S. Hazra
Northwestern University, USANorthwestern University, USAP. DuttaJ. Kmetko (Cornell University)C.-J. Yu (Pohang Light Source, Korea)A.G. Richter (University of Memphis)
Université Paris-Sud, FranceUniversité Paris-Sud, FranceJ. Daillant, D. Luzet, C. Blot
Co-workersCo-workers
Amphiphiles and Langmuir Monolayers Langmuir Monolayers with Metal Ions The Bi-molecular Layer with Ferric Stearate
I shall talk about …I shall talk about …
AmphiphilesAmphiphiles
(found ~ 2000 B.C.)(found ~ 2000 B.C.)
1. The Whalelephant
2. The Fatty Acid
Head
Tail
Hydrophobic tail Hydrophilic head
Simpler Picture
Amphiphile – One Half Loves Water, Other Part Does Not
Hydrophobic head Hydrophilic tail
Langmuir MonolayersLangmuir Monolayers
(found in 1910-11)(found in 1910-11)
If an amphiphile is dissolved in some liquid which has Surface Tension, Density and Boiling Point much less than Water and a Very Small Amount of this Dilute Solution is Spread on Water
Then after the Solvent Spreads Evenly over the Water Surface and Evaporates
The Amphiphiles form a Monomolecular Layer at the Air Water Interface with Heads in Water and Tails in Air
This is the LANGMUIR MONOLAYER - Perhaps the Most Easily Achievable Two-Dimensional System
Head
Tail
LANGMUIR MONOLAYER PHASESA Langmuir Monolayer can be compressed two-dimensionally by pushing a mechanical barrier across the water surface
The compressed monolayer becomes denser and its surface tension (measured generally by Wilhelmy Plate) gradually reduces from that of pure water to that of pure hydrocarbons (oils)
This drop in surface tension is called SURFACE PRESSURE and is the measured two-dimensional analogue of pressure
= water - monolayer
When surface pressure is plotted as a function of the area per amphiphile molecule in the monolayer (A), we get a -A isotherm which is again the two-dimensional analogue of the p-V Isotherm of a three-dimensional system
The -A isotherm shows discontinuities such as abrupt changes in slope (kinks) and straight sections which were thought to be and are now known to be Phase Transitions
20253035404550
0
10
20
30
40
50
S
L2/
L2
Isotherm for T=10oC
Surface Pressure(mN
/m)
Area/Molecule
Langmuir Monolayer Phases
Pressure-area isotherm
From the isotherms the SurfacePressure-Temperature PhaseDiagrams are constructed
Kaganer, V. M.; Möhwald, H.; Dutta, P. Rev. Mod.Phys. 1999, 71(3), 779
How do we know whatHow do we know whatis going on?is going on?
Diffraction Studies of Langmuir Monolayers
Schematic View of Set-upActual Set-up at NSLS and APS
The Different Monolayer Phases have different 2D Lattices
These can be studied by Grazing Incidence Diffraction
Grazing Incidence Grazing Incidence DiffractionDiffractionIn GID technique, a total external reflection occurs at the interface, < c
= incidence anglec = the critical angle (~ milliradians for X-rays)
Field decays to 1/e of its value at top within a depth of ll /[2 (- c )1/2]At these incident angles (~ milliradians) l is independent of and 25 Å to 100 Å, inversely as electron density of medium
Condition for GID: Incident X-rays are confined to ~ 100Å below interfacial plane, whereas they travel in the plane (x and y directions) with a wavelength close to the free-space wavelength
x
y
z
q
The Langmuir Monolayer has randomly oriented domains in the horizontalplane hence x and y directions are not resolved
Kxy = (kfxy
2 + kixy
2 - 2 kfxyki
xycos2)½
= 2/ (cos2i + cos2f - 2 cosi cosf cos2)½
Kz = 2/ (sinf + sini )
For Nearest Neighbour (NN) Tilts
Tilt Angle = = tan-1 (Kdz/[Kdxy2 – (Knxy/2) 2] ½)
For Next Nearest Neighbour Tilts
= tan-1 (Knz /Knxy)
GID of Langmuir Monolayers is Simpler
In-plane peaks Bragg rods
Langmuir Monolayers in Reciprocal Space
Very Briefly on Synchrotron RadiationVery Briefly on Synchrotron Radiation
Wherever the path of the electrons bends, the acceleration causes them to produce electromagnetic radiation
In the lab rest frame, this produces a horizontal fan of x-rays that is highly collimated (to ΔΘ≈ 1/γ) in the vertical direction and extends to high energies
Synchrotron x-rays are:1. Highly Collimated hence Very
Bright2. Linearly Polarized in the
Horizontal Plane3. Energy Tunable
Electrons moving near speed of light
X-ray Reflectivity
Rotating-Anode Generator, Cu k1 , =1.540562Å
•Angle of Incidence used: from 0° - 3.5° •Scan-step: 2 mdeg for films > 800 Å, 5 mdeg for films 800 Å •3.8 kW of X-ray Power used
Grazing Incidence X-ray Diffractometer, FR591, Enraf-Nonius
X-ray Reflectivity: Principles•In x-ray region, refractive index n < 1, i.e., phase velocity of x-rays in material > phase velocity in vacuum.
total external reflection (specular reflection)Incident and scattered wave-vectors in same plane normal to
surfaceIncident angle () = scattered angle ()
10-6, electron density, r0 classical electron radius ~ 2.810-5 Å•n = (1-) =1-(r0 2/2 ) •qz = normal momentum transfer = kf - ki= 4/(sin)c = critical angle for sample film = (2 )½
z
x
At > c, x-rays penetrate into sample, are scattered for each change in , and these scattered x-rays interfere interference (Kiessig) fringes in reflectivity profile with periodicity 2/d, d = thickness of a layer with a constant , while amplitude of fringes change in
kt
ki kf
n = 1-
n=1
qz = 2/d
d
Air
Film
Substrate
Interference (Kiessig) fringes with periodicity 2Interference (Kiessig) fringes with periodicity 2/d, d = /d, d = thickness of a layer with a constant thickness of a layer with a constant , while amplitude of , while amplitude of fringes fringes change in change in
Mirror
Laser Diode
Focusing Lens
Piezo Scanner
Sample Holder
Integrator
Divider / Multiplier
Differential amplifier
4-quadrant PSPD
X-Y Translator
X Y
TipSampleCantilever
Atomic Force MicroscopyAtomic Force Microscopy
Forc
e
attractive force
distance(tip-to-sample )
repulsive force
non-contact
contact
Intermittent-contact
Multimode Nanoscope IV (Digital Instruments)
Intermittent-Contact (tapping) modeEtched Si tipPhosphorus-doped Si cantileverForce constant 40N/mCharacteristic frequency 344kHz
Langmuir MonolayersLangmuir Monolayers+ Metal Ions+ Metal Ions
(found in 1994)(found in 1994)
From GID studies this is phase diagram ofFatty-acid Langmuir Monolayers
1.4 1.5 1.6 1.70.00
0.04
0.09
0.14
0.18
pH ~ 12
15oC, = 30dynes/cm
pH ~ 11.5
pH ~ 11
pH ~ 10.5
Norm
aliz
ed In
tens
ity
Qxy (Å-1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
8
10
12
14
16
18
20
22
24
Open Symbols - Ref 7Solid Symbols - Our Work
= 30 dynes/cm
Rotator-II
Rotator-I
S
T (o C)
pH
Changing the Subphase (Water) pH
System: Heneicosanoic Acid on water, pH changed by NaOH, noOther Metal IonsHigh pH behaves like High Temperature
Datta, A.; Kmetko, J.; Richter, A. G.; Yu, C. J.; Dutta, P. ; Chung, K. S.; Bai, J. M. Langmuir 2000, 16, 1239.
Putting Different Metal Ions in Water: Cadmium Ions
Organic MonolayerPeaksSuperlattice peaks
A superlattice of metal ions(?) is formed under the organic layerDatta, A.; Kmetko, J.; Yu, C. J.; Richter, A. G.; Chung, K. S.; Bai, J. M.; Dutta, P. J. Phys. Chem. B 2000, 104, 5797.
0.0
0.1
0.3
pH ~ 8.5
pH ~ 9.3
pH ~ 6.2
Qxy
Norm
aliz
ed In
tens
ity
(arb
.u.)
0.0
0.2
0.4
1.4 1.5 1.6 1.70.0
0.7
1.4
Cadmium Ions: the pH ‘Window’
Superlattice and asymmetrically (chiral)distorted organic lattice are formed withina range of pH
Putting Different Metal Ions in Water: Lead Ions
Superlattice and Largechiral distortion oforganic lattice
Are there Lead Ionsin the superlattice?
GID of monolayer just above andbelow L3 absorption edge of leadshows no change in either organicor superlattice peaks!
Lead forms less than 20% (atom%) of the superlattice
Kmetko, J.; Datta, A.; Evmenenko,G.; Durbin, M. K.; Richter, A. G.; Dutta, P. Langmuir 2001, 17, 4697.
Putting Different Metal Ions in Water
Do not formSuperlattices
Form Superlattices but no chiral distortion of organic lattice
Putting Different Metal Ions in Water: Summary
Kmetko, J.; Datta, A.; Evmenenko, G.; Dutta, P. J. Phys. Chem. B 2001, 105, 10818.
A Closer Look at Pb Ions with EXAFS
Pb with the HeadPb-OH ‘trimer’
The Lattice
Maxim I. Boyanov, Jan Kmetko, Tomohiro Shibata, Alokmay Datta, Pulak Dutta, andBruce A. Bunker, J. Phys. Chem. B 2003, 107, 9780.
Putting Different Metal Ions: Conclusions
Metal ions can do three things to a fatty acid Langmuir Monolayer. They can:
1. Take the monolayer to a high pressure phase at low pressure i.e., an Achiral structure. Cu, Ni, Ca, Ba, Zn
2. Take the monolayer to a high pressure phase and create a superlattice underneath i.e., Achiral + Superlattice. Mg, Mn
3. Take the monolayer to a new chiral phase and create a superlattice underneath i.e., Chiral + Superlattice. Cd, Pb
•Pinhole defects are observed for ‘normal pH’ LB films and increases from substrate surface upto top film surface•Defects (like ridges) are only within the upper bilayer for ‘high pH’ LB films
20×20 μm2 10×10 μm2
‘Normal pH’ and ‘High pH’ Langmuir-Blodgett multilayer films
Surface pressure = 30 mN/m, Temperature 13oCIon (Cd+2) concentration 10-4 M - 10-5 M
Subp
hase
pH
6
.5Su
bpha
se p
H
9.0
– 9
.5
Results from x-ray reflectivity analysis
0 2 4 6 8 10 1210-8
10-6
1x10-4
10-2
100
0 5 10 15 20 250
500
1000
1500
Ref
lect
ivity
qz(nm-1)
(e/n
m3 )
z (nm)
Normal pH LB films High pH LB films
18.6 19.2 19.80
500
1000
1500
0 2 4 610-7
1x10-5
10-3
10-1
101
(e/
nm3 )
z (nm)
Normal pH High pH
Ref
lect
ivity
qz(nm1)
• Reflectivity increases for ‘high pH’ film twice than that of ‘normal pH’ LB film• From EDP we get one more cadmium ion in the headgroup of ‘high pH’ LB film• Enhancement in the reflectivity by incorporating one extra cadmium ion in the headgroup
Conclusions
Normal pH LB films
High pH LB films
• Manipulating Headgroups in Langmuir-Blodgett Films through Subphase pH Variation S. Kundu, A. Datta and S. Hazra, Chem. Phys. Lett. 405, 282 (2005)
Bi-molecular LayerBi-molecular Layer
(found in 2004-05)(found in 2004-05)
Monolayer of a Preformed Amphiphilic Salt:Ferric Stearate
Isotherm for stearic acid monolayer on water Isotherm for ferric stearate
monolayer on waterDatta, A.; Sanyal, M. K.; Dhanabalan, A.; Major, S. S. J. Phys. Chem. B 1997, 101, 9280.
Isotherm suggests that ferric stearate molecules have configurations of Y and inverted Y with one-and-half tails per head
This, rather than this
Ferric Stearate Monolayer Configuration
Bimolecular layer on water: Dramatic reduction in surface tensionBimolecular layer on water: Dramatic reduction in surface tension
m m
OnWaterSurface
OnSiliconSurface
Out-of-plane x-ray scattering
In-p
lane
x-r
ay s
catt
erin
g
Ferric stearate on water
water
Out-of-plane x-ray scattering Atomic Force Microscopy Images
Electron Density Profile
Low coverage High coverage
Bi-molecular layer on water surface (Grazing Incidence Diffraction)
How does the structure change with surface pressure ?
1.38 1.44 1.50 1.56 1.62 1.68
0.0
0.2
0.4
0.6
0.8
1.0
10 mN/m
q||(Å-1)
0.0
0.2
0.4
0.6
0.8
1.0
15 mN/m
Inte
nsity
0.0
0.2
0.4
0.6
0.8
1.0
1 mN/m
Surface Pressure (mN/m)
Lattice type
Lattice Parameters of In-Plane Primitive Unit cell
a (Å) b (Å) Area/tail (Å2) Domain size (Å)
1 DH 4.73 8.72 20.60 107 (s) &150 (w)
10 DH 4.72 8.68 20.48 75 (s) & 60 (w)
15 DH 4.73 8.68 20.55 72 (s) & 71 (w)
Only one hydrocarbon tail length (22Å) is required to fit the Bragg rod profile
bi-molecular layer is formed on water surface from very low surface pressure
0.00 0.05 0.10 0.15
1x10-10
1x10-9
1x10-8
1x10-7
0 20 40 60 800.00.20.40.6(a)
Ab
solu
te In
tens
ity
sc(radians)
(eÅ
-3)
z (Å)
0.15 0.30 0.45 0.60
1x10-12
2x10-12
3x10-12
4x10-12
5x10-12
6x10-12
(b)
Inte
nsity
qz(Å-1)
Bragg rodRod at ψ = 0.2º
Clip of trough
Substrate holdersubstrate
MILS method
Clip of trough
Substrate
Gold particles
LB method
Deposition of layers horizontally from very low (1mN/m) to very high (55mN/m) surface pressure
:
0.0 0.3 0.6 0.91x10-27
1x10-23
1x10-19
1x10-15
1x10-11
1x10-7
1x10-3
1x101
55 mN/m
1 mN/m
(a)R
efle
ctiv
ity
qz (Å-1)
0 20 40 60 80 100 1200.0
0.5
1.0
1.5
2.0
2.555 mN/m
1 mN/m(b)
(eÅ
-3)
z (Å)
0 10 20 30 40 50
0.6
0.8
1.0
f 1 (mN/m)
0.2
0.4
0.6
0.8
1.0
(d)
(c)
f 2
Different growth behavior of upper and lower monomolecular layers
While the lower layer behaves solid-like the number of molecules in the upper layer increases continuously with surface pressure
ConclusionsConclusions
PreformedPreformed fatty acid salts behave like a fatty acid salts behave like a membrane on water surfacemembrane on water surface
• They form a bi-molecular layer on water They form a bi-molecular layer on water surfacesurface• They reduce surface tension of water from They reduce surface tension of water from 72mN/m to 1mN/m72mN/m to 1mN/m• The growth behaviour of each layer is The growth behaviour of each layer is differentdifferentA. Datta, S. Kundu, M. K. Sanyal, J. Daillant, D. Luzet, C. Blot and B. Struth Phys. Rev.
E 71, 041604 (2005). Selected for ESRF Highlights 2005
S. Kundu, A. Datta, M. K. Sanyal, J. Daillant, D. Luzet, C. Blot and B. Struth, submitted