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Monolayers and Langmuir-Blodgett Films
Amélia M. P. S. Gonçalves da Silva
CENTRO DE QUÍMICA ESTRUTURAL, IST, LISBOA
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I – Introduction
II – Formation and Characterization of LM and LB films
III – Experimental studies
Monolayers and Langmuir-Blodgett Films
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Monolayers and Langmuir-Blodgett Films
Agnes Pockels1862 – 1935
carried out the first experiences in the
kitchen sink
Irving Langmuir1881 – 1957
Nobel Prize in Chemistry1932
developed the theory and the experimental method that are being
used until nowadays
Katherine Blodgett1898 – 1979
In collaboration with Langmuir developed (1935)
the technique to transfer monolayers onto solid substrates (LB films)
Initiators:
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Relevance:
Fundamental studies
Potential applications
Langmuir Monolayers (LM)
Distinctive characteristics:
• Simple system
• Low thickness
• High molecular order
Out coming information:
• Orientation and packing of molecules
• Molecular interactions
• Two-dimensional (2D) states
• Phase behavior
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Reactions in Monolayers
The monolayer components can react with some species in the subphase
Models in fundamental studies
Biomembranes and liposomes
Lung Surfactants
Development of synthetic lung surfactants
Evaporation Control
Dense monolayers spread on water reservoirs reduce the water evaporation
LB films
Building up of multilayered structures
Applications of Langmuir Monolayers
Langmuir-Blodgett Films
Langmuir monolayer
LB films characteristics
• High molecular order
• Large regions nearly free of defects
• Precise control of thickness
• Active or passive films
Building up of multilayered structures:
Composition and structure
components,
orientation of molecules,
substrate,
number of layers , …
can be adjusted to the expected properties and
function of LB film
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LB films
Solid substrates
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Chemistry and BiochemistryCatalyst; Biocompatible surfaces; biosensors(glucose sensor, for example)
MicroelectronicsFabrication of ultra-miniaturized devices, composed of conductor, semiconductor or insulator layers
Antireflection coatingApplied to glass make an antireflective action, allowing 99% of visible light to pass through.
Protection against corrosion and lubricationCondensed layers of amphiphilic molecules can act as surface coating, preventing erosion and corrosion
Applications of LB Films
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� Materials, method and equipment
� Analysis of π –A isotherms of single and mixed monolayers
� Brewster Angle Microscopy (BAM)
� LB films – dipping method
� General characterization
II – Formation and Characterization of LM and LB films
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Materials
The substances that form stable monolayers at the a ir-water interface are amphiphilic molecules insoluble in the subphase :
� Long-chain fatty substances (classic molecules)
N
N
N
N H H
R
RR
R
� Polycyclic aromatic compounds
loop
tailtail
train
� Amphiphilic polymers and proteins
Non-amphiphilic molecules can also be incorporated in monolayers anchored by the amphiphilic molecules
R = long alkyl chain
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Surface pressure: π = γπ = γπ = γπ = γ0000 – γγγγγ0− surface tension of the air-water interface (pure water ).γ − surface tension of the air-water interface with the film (monolayer )
Wilhelmy Method
π π π π −−−−Α Α Α Α isotherm: measurement of the surface pressure (ππππ) as a function of the surface area (A) at constant temperature during compression
balance
Wilhelmy plate
mobile barrier
water
mobile barrier
trough
� Before the spreading (for pure water): π = γπ = γπ = γπ = γ0000 – γγγγ0000 = = = = 0000
� After spreading: π = γπ = γπ = γπ = γ0000 – γγγγ
Formation of Langmuir monolayers: spreading - evaporation - compression
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System KSV 5000 (Helsinki)
Equipment: Langmuir balance
Trough
LC LE
Monolayer States and Phase Transformations
npT
s A
AK
,,
1
∂∂−=π
Compressibility :
G
Area
Sur
face
pre
ssur
e
LC
LE
S
C
Generalized ππππ –A Isotherm
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Termodynamic analysis
Miscibility
Mixed Monolayers
Phase separation
B
A
A2
x20 1
1(id.)
A1
A12
(mma)
2(exp.)
3(exp)
2121221112 )( xAAAAxAxAideal −+=+=
- mma of the ideal mixture
- molecular area of pure component (1,2)
- molar fraction of component (1,2)i
i
ideal
x
A
A12
(1): Ideal behaviour ( or phase separation )
Deviations from the ideal behaviour (1) indicate miscibility:
Positive deviations, expanding effect: (3) – (1) > 0
Negative deviations, condensing effect: (2) – (1) < 0
Mean molecular area, A12 (mma ) vs x2:(at ππππ = constant)
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• Surface potential ∆∆∆∆VThe orientation of dipoles at the interface gives information on molecular packing.
• Surface Viscosity ηηηηs
Density and molecular structure.
• Spectroscopic analysis ( in situ )Composition, interactions and orientation of hydrophilic and hydrophobic groups.
• Brewster Angle Microscopy (BAM)Film heterogeneity, coexistence of phases, morphology of domains, and collapse.
Characterization of Monolayers
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AnalyzerPolarizer
CCD camera
Objectivenair
Laser
n water
p-polarized incident ray
θB
Brewster Angle Microscope
Brewster angle or Polarizing angle ( θθθθB ):
Snell and Fresnel laws:
ni, refractive index of material i
air
waterB n
ntg =θ
At the Brewster angle the p- component of the incident light is completely absent in the reflected ray
nmonol ≠≠≠≠ nwater
Air - monolayer: θB ≠ 53º
Reflectivity > 0 bright image
Air-water Interface: θB = 53º
Reflectivity = 0 dark image
Incident ray(p-polarized)
refracted ray
No reflected ray
θθθθB
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BAM2 (NFT Göttingen, Germany)
BAM- Brewster Angle Microscope
LASER
Camera
Balance
water
AnalyzerPolarizer
Objective
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π
Liquid - Gas
BAM images of stearic acid
630×470 µm2
SA: C17H35COOH
LC –bright domains
Anisotropic LC phase:(the reflectivity varies with the orientation and tilting of chains)
SA
A
Collapse
S
LC
Gas (water) - dark domains
ϕ
Isotropic S phase:(homogeneous image)
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Formation of LB films - dipping technique
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LB Films architectures
Y- type
Z - type
X - type
hydrophilic hydrophobicSolid substrates:
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Characterization of LB Films
The common techniques for surface characterization:
• Spectroscopic analysisFTIR, UV/visible, Fluorescence, etc.Identification of interfacial groups; orientational and conformational order; aggregation
• X-ray and neutron diffractionLateral order in crystalline regions; crystallite sizes; tilt angles; electron density profiles
• Atomic Force Microscopy (AFM)High resolution topological (nm-scale)
• Optical microscopyTopology (µm-scale) and heterogeneity of films
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III - Experimental Studies
� Phospholipid (DPPC)
� Binary mixture (DPPC + Stearonitrile)
� Thermoresponsive diblock copolymer
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DPPC
O-
O
O
POO
O
O
O
CH3
CH3
N+
CH3
CH3CH3
Dipalmitoyl phosphatidyl choline (DPPC):
_+
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Monolayer as a Simple Model of Biomembranes
Schematic diagram of a typical biological membrane.The phospholipid bilayer is the basic structure of all cellular membranes.
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DPPC: temperature effect
A.M. Gonçalves da Silva, R.I.S. Romão, Chemistry and Physics of Lipids, 2005, 137, 62-67.
DPPC
0
10
20
30
40
50
60
70
30 40 50 60 70 80 90 100Area per segment (Å2)
Sur
face
Pre
ssur
e, π
(m
N/m
)
T = 10 ºCT = 20 ºCT = 25 ºCT = 30 ºCT = 35 ºC
T
LC LE
γ ≈ 25 mN/m
π = γ0 – γ ≈ 45 mN/m
Monolayer state at the end of expiration:
Area per molecule
LC ↔↔↔↔LE transition
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The lung surfactant (monolayer of phospholipids and proteins) covers the pulmonary alveoli and reduces the surface tension of the liquid film.
During the inspiration, the surface area of the alveoli increases as large as possible for an efficient gas exchange.
Premature babies do not get enough lung surfactant, because it is produced only after 30 weeks of gestation − respiratory distresssyndrome
DPPC in lung surfactant
Application:
Development of synthetic lung surfactants to be administrated in premature babies with respiratory distress syndrome
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III - Experimental Studies
� Binary mixture (DPPC + Stearonitrile)
O-
O
O
POO
O
O
O
CH3
CH3
N+
CH3
CH3CH3
C17H35CN
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10
15
20
25
30
35
40
0 0.2 0.4 0.6 0.8 1XDPPC
Are
a pe
r ch
ain
(Å2)
AoLA25A40
10 ºC
10
15
20
25
30
35
40
0 0.2 0.4 0.6 0.8 1XDPPC
Are
a pe
r cha
in (
Å2)
A0LA25A40
30 ºC
Thermodynamic analysis
Negative deviations from ideal behaviour: AE < 0
DPPC and SN are miscible at 10 ºC and 30 ºC
0
10
20
30
40
50
60
70
0 20 40 60 80 100
Mean Molecular Area (Å 2/molecule)
Sur
face
Pre
ssur
e (m
N/m
)
2
SN
4
6 8 DPPC
10 ºC
XDPPC
0.2
0.4
0.6
0.8
0
10
20
30
40
50
60
70
0 20 40 60 80 100Mean Molecular Area (Å 2/molecule)
Sur
face
Pre
ssur
e (m
N/m
)
2
SN
4 6 8 DPPC
30 ºC
XDPPC
0.2
0.4
0.6
0.8
DPPC / Stearonitrile (SN)(Mixing and temperature effects)
SNDPPC
Condensing effect:
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0
10
20
30
40
50
60
70
0 20 40 60 80 100Mean Molecular Area (Å 2/molecule)
Sur
face
Pre
ssur
e (m
N/m
)
2
SN
4 6 8 DPPC
30 ºC
BAM images of DPPC/SN mixtures
53 16 35
XDPPC = 0.6:
π 630 x 470 µm2
XDPPC
SN 0.0
2 0.2
4 0.4
6 0.6
8 0.8
DPPC 1.022
DPPC
SN
18π
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Aqueous solution:
Lower critical solution temperature (LCST):
LCST of PDEA (aq) = 32 ºC
Coil Globule
(w. soluble) (w. insoluble)
32ºC
Poly(N,N-diethylacrylamide) (PDEA)
Thermoresponsive polymer in aqueous solution
H bonding
The thermoresponsive behavior of PDEA at the air-water interface
Gonçalves da Silva et al., J. Colloid Interface Sci. 2008, 327, 129-137.
10ºC
40ºC
0
5
10
15
20
25
30
35
0 10 20 30 40 50
Area per segment (Å2)
Sur
face
Pre
ssur
e (m
N/m
)
10ºC
20ºC
35ºC
40ºC
T
T
A 0p
ALB
AFM images and cross-section profile of
LB films transferred onto mica at 5 mN/m
Air-water interface:T < 32ºC T > 32ºC
LB film:
(on mica at 5 mN/m)
T < 32ºC T > 32ºC
T
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T
0
10
20
30
0 10 20 30 40 50
Area per segment (Å2)
Sur
face
Pre
ssur
e (m
N/m
) T= 40ºC
T= 30ºC
T= 20ºC
T= 10ºC
DHBC
RhB-PDMA 207-b-PDEA177 Double Hydrophilic Block Copolymer (DHBC)
Thermoresponsive diblock copolymer
RhB – rhodamine is a fluorescent dye, attached at the end of PDMA block
to follow the polymer organization in a LB film deposited on glass at different temperatures
Characterized by Laser Scanning Confocal Fluorescence Microscopy (LSCFM)
PDEAPDMARhB
PDMA
PDEA
LB
PDEAPDMA
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T
T = 20ºC < LCST
Thermoresponsive behavior of RhB-PDMA 207-b-PDEA177
10 µm
T = 35ºC > LCST
Laser Scanning Confocal Fluorescence Microscope (LSCFM)
LB films transferred onto glass at 3 mN/m
Core-shell inversion
Romão et al., Langmuir 2010, 26, 1807-1815.
PDEAPDMARhB
Application of stimuli polymers: drug deliver systems
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• J.L. Gaines, Insoluble Monolayers at Liquid-Gas Interfaces, Wiley-Interscience, New York, 1966.
• G. Roberts, Langmuir-Blodgett Films, Plenum Press, New York, 1990.
• M.C. Petty, Langmuir-Blodgett Films, An introduction, Cambridge Univ.Press, 1996
• M.J. Jaycock and G.D.Parfitt, Chemistry of Interfaces, Ellis Horwood, Chichester, 1986.
Bibliography
