This document summarizes research on amphiphiles and Langmuir monolayers. It discusses how amphiphiles are composed of a hydrophilic head and hydrophobic tail. When spread on water, amphiphiles form Langmuir monolayers where the heads interact with water and tails with air. Pressure-area isotherms of these monolayers show phase transitions as pressure increases. Adding metal ions to the water subphase can induce superlattice formation underneath the monolayer. Studies using x-ray diffraction and other techniques characterized the structures of various Langmuir monolayers and how they change with conditions like subphase pH and metal ion type.

1. Amphiphiles: A TaleAmphiphiles: A Tale
of Heads and Tailsof Heads and Tails
Alokmay Datta
Saha Institute of Nuclear
Physics

2. Saha Institute of Nuclear Physics, IndiaSaha Institute of Nuclear Physics, India
S. Kundu, M. K. Sanyal, S. Hazra
Northwestern University, USANorthwestern University, USA
P. Dutta
J. Kmetko (Cornell University)
C.-J. Yu (Pohang Light Source, Korea)
A.G. Richter (University of Memphis)
Université Paris-Sud, FranceUniversité Paris-Sud, France
J. Daillant, D. Luzet, C. Blot
Co-workersCo-workers

3. Amphiphiles and Langmuir Monolayers
Langmuir Monolayers with Metal Ions
The Bi-molecular Layer with Ferric Stearate
I shall talk about …I shall talk about …

4. AmphiphilesAmphiphiles
(found ~ 2000 B.C.)(found ~ 2000 B.C.)

5. 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

6. Langmuir MonolayersLangmuir Monolayers
(found in 1910-11)(found in 1910-11)

7. 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

8. LANGMUIR MONOLAYER PHASES
A 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
π = γwater - γmonolayer
When surface pressure is plotted as a function of the area per amphiphile
molecule in the monolayer (A), we get a π-A isotherm which is again the
two-dimensional analogue of the p-V Isotherm of a three-dimensional system
The π-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

9. 20253035404550
0
10
20
30
40
50
S
L2
/
L2
IsothermforT=10
o
C
SurfacePressure(mN/m)
Area/Molecule
Langmuir Monolayer Phases
• Pressure-area isotherm
From the isotherms the Surface
Pressure-Temperature Phase
Diagrams are constructed
Kaganer, V. M.; Möhwald, H.; Dutta, P. Rev. Mod.Phys. 1999, 71(3), 779

10. How do we know whatHow do we know what
is going on?is going on?

11. Diffraction Studies of Langmuir Monolayers
Schematic View of Set-up
Actual Set-up at NSLS and APS
The Different Monolayer Phases have different 2D Lattices
These can be studied by Grazing Incidence Diffraction

12. Grazing IncidenceGrazing Incidence
DiffractionDiffraction
In GID technique, a total external reflection
occurs at the interface,
α< αc
α = incidence angle
αc = the critical angle (~ milliradians for X-
rays)
Field decays to 1/e of its value at top within a depth of l
l ≈ λ/[2π (α- αc )1/2
]
At these incident angles (~ milliradians)
l is independent of λ 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

13. The Langmuir Monolayer has randomly oriented domains in the horizontal
plane hence x and y directions are not resolved
Kxy = (kf
xy
2
+ ki
xy
2
- 2 kf
xyki
xycos2θ)½
= 2π/λ(cos2
αi + cos2
αf - 2 cosαi cosαf cos2θ)½
Kz = 2π/λ(sinαf + sinαi )
For Nearest Neighbour (NN) Tilts
Tilt Angle = τ= tan-1
(Kdz/[Kdxy
2
– (Knxy/2) 2
] ½
)
For Next Nearest Neighbour Tilts
τ= tan-1
(Knz /Knxy)
GID of Langmuir Monolayers is Simpler
In-plane peaks Bragg rods

14. Langmuir Monolayers in Reciprocal Space

15. 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 Bright
2. Linearly Polarized in the Horizontal
Plane
3. Energy Tunable
Electrons moving near speed of light

16. X-ray
Reflectivity
Rotating-Anode
Generator,
Cu kα1 , λ=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

17. 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 surface
Incident angle (α) = scattered angle (β)
δ∼10-6
, ρ → electron density, r0 → classical electron radius ~ 2.8×10-5
Å
•n = (1-δ) =1-(ρr0 λ2
/2π )
•qz = normal momentum transfer = kf - ki= 4π/λ(sinα)
∀αc = critical angle for sample film = (2 δ)½
z
x
At α > αc, x-rays penetrate into sample, are scattered for each change in
ρ, and these scattered x-rays interfere  interference (Kiessig) fringes in
reflectivity profile with periodicity 2π/d, d = thickness of a layer with a
constant ρ, while amplitude of fringes ∝ change in ρ
kt
ki kf
α α
β
n = 1-δ
n=1

18. ∆qz = 2π/d
d
Air
Film
Substrate
Interference (Kiessig) fringes with periodicity 2Interference (Kiessig) fringes with periodicity 2ππ/d, d = thickness of/d, d = thickness of
a layer with a constanta layer with a constant ρρ, while amplitude of fringes, while amplitude of fringes ∝∝ change inchange in ρρ

19. Mirror
Laser
Diode
Focusing Lens
Piezo Scanner
Sample
Holder
Integrator
Divider /
Multiplier
Differential
amplifier
4-quadrant PSPD
X-Y Translator
X Y
Tip
SampleCantilever
Atomic Force MicroscopyAtomic Force Microscopy
Force
attractive force
distance
(tip-to-sample
)
repulsive force
non-contact
contact
Intermittent-
contact
Multimode Nanoscope IV
(Digital Instruments)
Intermittent-Contact (tapping) mode
Etched Si tip
Phosphorus-doped Si cantilever
Force constant 40N/m
Characteristic frequency 344kHz

20. Langmuir MonolayersLangmuir Monolayers
+ Metal Ions+ Metal Ions
(found in 1994)(found in 1994)

21. From GID studies this is phase diagram of
Fatty-acid Langmuir Monolayers

22. 1.4 1.5 1.6 1.7
0.00
0.04
0.09
0.14
0.18
pH ~ 12
15
o
C, π = 30dynes/cm
pH ~ 11.5
pH ~ 11
pH ~ 10.5
NormalizedIntensity
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 7
Solid Symbols - Our Work
π = 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, no
Other Metal Ions
High 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.

23. Putting Different Metal Ions in Water:
Cadmium Ions
Organic
Monolayer
Peaks
Superlattice peaks
A superlattice of metal ions(?) is formed under the organic layer
Datta, A.; Kmetko, J.; Yu, C. J.; Richter, A. G.; Chung, K. S.; Bai, J. M.; Dutta, P. J. Phys. Chem. B 2000, 104, 5797.

24. 0.0
0.1
0.3
pH ~ 8.5
pH ~ 9.3
pH ~ 6.2
Qxy
NormalizedIntensity(arb.u.)
0.0
0.2
0.4
1.4 1.5 1.6 1.7
0.0
0.7
1.4
Cadmium Ions: the pH ‘Window’
Superlattice and asymmetrically (chiral)
distorted organic lattice are formed within
a range of pH

25. Putting Different Metal Ions in Water:
Lead Ions
Superlattice and Large
chiral distortion of
organic lattice

26. Are there Lead Ions
in the superlattice?
GID of monolayer just above and
below L3 absorption edge of lead
shows no change in either organic
or 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.

27. Putting Different Metal Ions in Water
Do not form
Superlattices
Form Superlattices but no chiral
distortion of organic lattice

28. Putting Different Metal Ions in Water: Summary
Kmetko, J.; Datta, A.; Evmenenko, G.; Dutta, P. J. Phys. Chem. B 2001, 105, 10818.

29. A Closer Look at Pb Ions with EXAFS
Pb with the Head
Pb-OH ‘trimer’
The Lattice
Maxim I. Boyanov, Jan Kmetko, Tomohiro Shibata, Alokmay Datta, Pulak Dutta, and
Bruce A. Bunker, J. Phys. Chem. B 2003, 107, 9780.

30. 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

31. •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 ≅ 13o
C
Ion (Cd+2
) concentration ≅ 10-4
M - 10-5
M
SubphasepH≅6.5SubphasepH≅9.0–9.5

32. Results from x-ray reflectivity analysis
0 2 4 6 8 10 12
10
-8
10
-6
1x10
-4
10
-2
10
0
0 5 10 15 20 25
0
500
1000
1500
Reflectivity
qz
(nm
-1
)
ρ(e/nm
3
)
z (nm)
Normal pH LB films High pH LB films
18.6 19.2 19.8
0
500
1000
1500
0 2 4 6
10
-7
1x10
-5
10
-3
10
-1
10
1
ρ(e/nm
3
)
z (nm)
Normal pH
High pH
Reflectivity
qz
(nm
−1
)
• 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

33. 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)

34. Bi-molecular LayerBi-molecular Layer
(found in 2004-05)(found in 2004-05)

35. Monolayer of a Preformed Amphiphilic Salt:
Ferric Stearate
Isotherm for stearic acid
monolayer on water Isotherm for ferric stearate
monolayer on water
Datta, A.; Sanyal, M. K.; Dhanabalan, A.; Major, S. S. J. Phys. Chem. B 1997, 101, 9280.

36. 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

37. Bimolecular layer on water: Dramatic reduction in surface tensionBimolecular layer on water: Dramatic reduction in surface tension
µm µm
On
Water
Surface
On
Silicon
Surface
Out-of-plane x-ray scattering
In-planex-rayscattering
Ferric stearate on water
water
Out-of-plane x-ray scattering
Atomic Force Microscopy Images
Electron
Density
Profile
Low coverage High coverage

38. 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
Intensity
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)

39. 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 80
0.0
0.2
0.4
0.6
(a)
AbsoluteIntensity
θsc
(radians)
ρ(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)
Intensity
qz
(Å
-1
)
Bragg rod
Rod at ψ = 0.2º

40. Clip of trough
Substrate holder
substrate
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 :

41. 0.0 0.3 0.6 0.9
1x10
-27
1x10
-23
1x10
-19
1x10
-15
1x10
-11
1x10
-7
1x10
-3
1x10
1
55 mN/m
1 mN/m
(a)Reflectivity
qz
(Å
-1
)
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
55 mN/m
1 mN/m (b)
ρ(eÅ
-3
)
z (Å)
0 10 20 30 40 50
0.6
0.8
1.0
f1
π (mN/m)
0.2
0.4
0.6
0.8
1.0
(d)
(c)
f2
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

42. ConclusionsConclusions
PreformedPreformed fatty acid salts behave like a membranefatty acid salts behave like a membrane
on water surfaceon water surface
• They form a bi-molecular layer on water surfaceThey form a bi-molecular layer on water surface
• They reduce surface tension of water fromThey reduce surface tension of water from
72mN/m to 1mN/m72mN/m to 1mN/m
• The growth behaviour of each layer is differentThe growth behaviour of each layer is different
A. 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