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