Neutron Reflectometry combined with Complementary … · 2014. 9. 23. · Neutron Reflectometry:...
Transcript of Neutron Reflectometry combined with Complementary … · 2014. 9. 23. · Neutron Reflectometry:...
Richard Campbell – Hercules – 16/09/2014
Neutron Reflectometry combined with
Complementary Techniques to solve Complex
Soft Matter Problems at the Air/Water Interface
Oxford Durham Lund Grenoble
Presentation Outline
• Part 1.
– polymers, surfactants & biomolecules at surfaces: relevant questions
– neutron reflectometry: focus on FIGARO for free liquid surfaces
– ellipsometry: background, instruments, analysis & strengths
– 3 other experimental techniques: background & applications
• Part 2.
– 4 examples of different problems that can be solved on FIGARO
• Part 3.
– 4 case studies where techniques help us understand a system
– summary: complementarity of neutron reflectometry & ellipsometry
– overview: attributes of the techniques in the case studies
Soft Matter & Biology at Liquid Interfaces
• Quantification.
– the surface excess, thickness, uniformity and roughness of
adsorbed, spread or coated material in an interfacial layer
• Composition.
– the proportion of different materials in a mixed interfacial layer
• Structure.
– preferred orientation of individual chemical bonds
– mean tilt angle of rigid chains
– lateral detail (e.g. domains or separation of attached particles)
– medial detail (e.g. stratified multilayer structures or inhomogeneity)
Soft Matter & Biology at Liquid Interfaces
– lateral detail (e.g. domains or separation of attached particles)
– medial detail (e.g. stratified multilayer structures or inhomogeneity)
Techniques for Studying Liquid Interfaces
• Neutron reflectometry (NR).
– structure & quantified composition
– contrast from isotopic labelling
• Ellipsometry (ELL).
– precision, sensitivity & fast kinetics
– calibration to physical parameters
• Other techniques.
– Infrared reflectometry (ER-FTIRS)
– Surface tensiometry (ST)
– Brewster angle microscopy (BAM)
Neutron Reflectometry: Physical Basis
• Specular reflection of neutrons close to grazing incidence.
• Reflectivity (R) is plotted
typically against the
momentum transfer (Q).
• Scattering lengths can
be considered as a
neutron refractive index.
• Scattering lengths of H & D
have opposite signs.
Q4 sin
Neutron Reflectometry: Physical Basis
• Neutron wavelengths similar to molecular length scales, so
interference fringes can help to fit precise layer thickness (d).
• Adsorbed amount (Γ) can be calculated directly for a uniform
isotropic thin film at the air/liquid interface.
• Isotopic substitution in the solvent or interfacial species leads to
more measured parameters and better data fits.
• Drawbacks: expensive materials & beam time.
i i
i
d
c b
Neutron Reflectometry: FIGARO
• Reasons.
– caters for increased demand of soft matter experiments
– provides a horizontal reflectometer for the study of liquid surfaces
• Specifications.
– time-of-flight reflectometer with variable resolution
– vertical scattering plane for air/liquid & liquid/liquid interfaces
– wide Q-range (0.005–0.4 Å–1) and λ-range (2–30 Å)
– high intensity to resolve kinetics and enhance detection
– two dimensional detector for off-specular scattering
– reflection upwards and downwards for flexible applications
– complementary measurements from ELL, BAM & ST
Neutron Reflectometry: FIGARO
Neutron Reflectometry: FIGARO
– four chopper system for
variable resolution & increased transmission
• Choppers.
Neutron Reflectometry: FIGARO
– inclined silicon plate removes
neutrons above a critical wavelength
• Frame Overlap Mirror.
Neutron Reflectometry: FIGARO
– supermirrors coated on both faces
to allow upward and downward reflection
• Deflection Mirrors.
Neutron Reflectometry: FIGARO
– need to remove off-specular
reflections from the deflection mirrors
• Collimation Guide.
Neutron Reflectometry: FIGARO
– two dimensional array
drilled from a single aluminium block
• Detector.
Neutron Reflectometry: FIGARO
• Sample stage.
– crude vertical translation stage
– dual goniometer tilt stage
– horizontal translation stage (500 mm)
– anti-vibration table
– fine vertical translation stage coupled to
optical sensing device (2 μm precision)
• Sample environment.
– adsorption troughs (six on one platform)
– Langmuir trough with in-situ BAM & ST
– expanding free liquid surface
Neutron Reflectometry: Sample Environment
Null Ellipsometry: Polarisation
• P: parallel to the plane of incidence
• S: perpendicular to the plane of incidence
i i j jn θ n θˆ ˆˆ ˆsin sin
2
2 j i i jij,p ij,p
i j j i
ˆ ˆˆ ˆcos cosˆ
ˆ ˆˆ ˆcos cos
n θ n θR r
n θ n θ
2
2 i i j jij,s ij,s
i i j j
ˆ ˆˆ ˆcos cosˆ
ˆ ˆˆ ˆcos cos
n θ n θR r
n θ n θ
Null Ellipsometry: Scheme
• Polariser and analyser are rotated to null detected signal
Null Ellipsometry: Scheme
• Two combinations at fixed C: and .
P P3 12
A A3 1
Null Ellipsometry: Physical Basis
• Interpretation of P & A:
• Relation of y & D to physical parameters of the interface:
• This quotient is defined as the coefficient of ellipticity:
D yp i
s
re
r
ˆtan
ˆ.
yA A1 3
2 D P P1 3
y D Dp
s
r
ri
ˆtan cos sin
ˆ.
Null Ellipsometry: Air–Liquid Interface
• Zero in p- reflectivity at ~ 53° for water substrate.
• Maximum sensitivity in D at both sides of the Brewster angle.
Null Ellipsometry: Solid–Liquid Interface
• Minimum in p- reflectivity at ~ 75° for silicon/silica substrate.
• Maximum sensitivity in and D close to the Brewster angle.
Null Ellipsometry: Interpretation
• Air–liquid interface.
– cannot extract adsorbed amount and thickness from D or ρ alone
– isotropic models can break down when there is order in the layer
– can relate precise changes in D or ρ to measured parameters
• Solid–liquid interface.
– numerical programs to calculate nx & d from & D
– care is again required when using an isotropic layer model
– de Feijter’s expression relates both nx & d to the surface excess:
2xd n n
dn dc/
External Reflection FTIR Spectroscopy
• Reflection of an incoherent
infrared beam at the
air–liquid interface.
• Two spectra shown.
– (A) = water
– (B) = surfactant solution
• Contributions.
– solution = dispersive-shaped
– adsorption = peak-shaped
External Reflection FTIR Spectroscopy
• Subtract the difference
between the two spectra.
• Monolayer peaks can be
positive or negative.
• ER-FTIRS / IRRAS / RAIRS =
spectroscopic reflectometry.
– structure
– quantification
– composition
0
DR
R S 02.3. A A≈
Surface Tensiometry
• Surface tension determination.
– force = plates & rings
– shape = bubbles & drops
• Surface tension of surfactant
solutions can be related to
the surface excess by
Gibb’s equation.
• Only valid if no complexation!
TT c
1
R ln
Brewster Angle Microscopy
• Simple concept: p-polarized light does not reflect at the Brewster
angle at all for a single interface or much for a thin isotropic film,
but a thin anisotropy film result in reflection.
• For example, p-polarized laser light is directed at the air–liquid
interface at ~ 53° (532 nm) then reflected into a CCD camera.
• White areas.
– anisotropic & liquid crystalline domains
• Black background.
– isotropic & liquid expanded regions
Part 2. Examples of Figaro Experiments: #1/4
Part 2. Examples of Figaro Experiments: #2/4
Part 2. Examples of Figaro Experiments: #3/4
Part 2. Examples of Figaro Experiments: #4/4
Part 3. Four Case Studies at Liquid Interfaces
• 1. Surfactant adsorption to a dynamic liquid surface (formulations)
NR & ELL ... + ER-FTIRS ► quantification & composition
• 2. Spread layers of lung surfactant mixtures (health)
NR & ELL ... + ST + BAM ► structure
• 3. Thick film growth in polymer/surfactant mixtures (membranes)
NR & ELL … + BAM ► quantification & structure
• 4. Atmospheric oxidation of organic monolayer films (environment)
NR & ELL … (+ BAM) ► quantification (... composition)
1. Surfactant adsorption
to a dynamic liquid interface
‘three platforms for dynamic liquid measurements’
• (A) max. bubble pressure; (B) liquid jet; (C) overflowing cylinder:
1. Surfactant adsorption
to a dynamic liquid interface
• The overflowing cylinder.
– steady state conditions
– well defined flow profile
– suited to scattering probes
• Dynamic surfactant adsorption.
– many practical applications
– Marangoni effects
– sub-surface controls diffusion
‘chosen platform & hydrodynamic construct’
1. Surfactant adsorption
to a dynamic liquid interface
• Three model surfactants.
– CTAB
– C10E8
– APFN
• Pure and mixed systems.
– different head groups
– different backbones
– different resonances
– different charges
– different interactions
– different chemistry
‘model surfactant systems’
1. Surfactant adsorption
to a dynamic liquid interface
• ER-FTIRS spectra [CTAB].
– bonds in a liquid-like environment
• ER-FTIRS coverage [CTAB].
– precision & sensitivity < 10%
‘structure and quantification from peaks’
1. Surfactant adsorption
to a dynamic liquid interface
• ELL data [CTAB].
– precise & sensitive but indirect
• NR data [CTAB].
– direct & accurate but insensitive
‘valuable quantification through complementarity’
1. Surfactant adsorption
to a dynamic liquid interface
• Calibration of NR & ELL.
– dependence almost linear
• Data from both techniques.
– now quantified at low coverage
‘valuable quantification through complementarity’
1. Surfactant adsorption
to a dynamic liquid interface
• Calibration to ER-FTIRS.
– dependence almost linear
• Data from all three techniques.
– surface coverage from spectra
‘valuable quantification through complementarity’
1. Surfactant adsorption
to a dynamic liquid interface
• C10E8:CTAB surface excess.
– competitive adsorption
• Surface vs bulk composition.
– hydrocarbons close to ideal mixing
‘composition of nonionic–cationic mixture’
1. Surfactant adsorption
to a dynamic liquid interface
• APFN:CTAB surface excess.
– only surfactant in excess adsorbs
• Free surfactant comparison.
– no aggregates at dynamic surface
‘composition of anionic–cationic mixture’
1. Surfactant adsorption
to a dynamic liquid interface
• Neutron reflectometry.
– accurate determination of the surface excess of surfactant
• Ellipsometry.
– precise and sensitive calibration to the surface excess
• External reflection FTIR spectroscopy.
– structural information from the position of peaks
– quantitative measure of the adsorbed amount of surfactant mixtures
after the calibration of the pure isotherms to NR/ELL
– characterization of mixed surfactant systems with greater access
to than can be achieved in several NR experiments
2. Spread layers of
lung surfactant mixtures
• Current representations of lung surfactant proteins in a lipid layer.
2. Spread layers of
lung surfactant mixtures
• Current representation of lung surfactant at high compression.
2. Spread layers of
lung surfactant mixtures
• NR profiles of exogenous (a) porcine
and (b) bovine lung surfactant.
• Bragg diffraction peaks show repeating
structures normal to the interface.
• Strong off-specular = lateral disorder.
2. Spread layers of
lung surfactant mixtures
• Provided the inspiration to work with human lung surfactant.
– material is extracted from amniotic fluid during caesarian section
• Spread layers of human pulmonary surfactant (HPS).
– probe the dynamic properties during compression & expansion
• Three lab-based techniques to pre-characterize the system.
– surface tensiometry + ellipsometry + Brewster angle microscopy
• Motivation = formulations for infant respiratory distress syndrome.
– need to understand the interaction of HPS with biological molcules.
2. Spread layers of
lung surfactant mixtures
• DPPC compression.
– anisotropy from LC islands
• HPS compression & expansion.
– hysteresis due to lipid reservoirs
2. Spread layers of
lung surfactant mixtures
• 1. HPS at 30 mN m–1.
– static surface and stable ~ 20 μm domains
• 2. HPS at 42 mN m–1.
– static surface and stable ~ 20 μm domains but with ring patterns
2. Spread layers of
lung surfactant mixtures
• Neutron reflectometry.
– Bragg peaks shows extended repeating structure normal to surface
for exogenous lung surfactant systems
• Surface tensiometry.
– kink in isotherms show onset of reservoir formation for HPS system
– extent of compression past the kink dictates degree of hysteresis
• Ellipsometry.
– sensitivity to anisotropy & thickness changes for complicated systems
• Brewster angle microscopy.
– polarized images reveal the lateral surface morphology
3. Thick film growth in
polymer/surfactant mixtures
• PEI and CTAB interact strongly at interfaces.
– bizarre mechanism results in thick films to form
– use of structured films as artificial membranes
• Films form and then disappear – but why?
– NR showed the transient presence of the films
– but no previous quantification of the film thickness
• Ellipsometry was also applied to the problem.
– challenge to quantify thicknesses > the wavelength!
– answer lay in the evolution of the dynamic properties
mw = 750k
[1 mM]
3. Thick film growth in
polymer/surfactant mixtures
• Opposite trends for growth & collapse.
– n = 1.40, 1.42 & 1.44 tested
• Experiments matches simulations.
3. Thick film growth in
polymer/surfactant mixtures
• Growth then collapse occurs.
• Micron-scale films quantified.
• NR peak related to thickness.
• Peak width reveals ordering.
3. Thick film growth in
polymer/surfactant mixtures
• Thin film evenly textured.
• Growth results in wrinkling.
• Circular defects on collapse.
• Final film smooth but mobile.
3. Thick film growth in
polymer/surfactant mixtures
• Ellipsometry.
– incredibly useful as a result of tracking the dynamics
– accurate quantification of the film thickness for the first time
– information can be used to harvest films at the optimum moment
• BAM.
– information from this technique gives information on the mechanism
– film morphology during growth & collapse very different to each other
• Neutron reflectometry.
– reliable (but not optimum!) technique to reveal the film lifetime
– quantitative information about the internal structure can be obtained
4. Atmospheric oxidation
of organic monolayer films
• Gases produced by mankind interact with droplets in clouds.
– the stability of cloud droplets is related to their surface organic films
– gases produced can destroy the films increasing evaporation
• Langmuir trough is used as a proxy for the droplet surface.
– films of organic material were spread and then exposed to oxidants
– the rate of loss of material was measured using NR
• Measurements are much faster than in past studies.
– typically 5 min scans were carried out previously
– scans as short as 1 s can be achieved on FIGARO
4. Atmospheric oxidation
of organic monolayer films
4. Atmospheric oxidation
of organic monolayer films
4. Atmospheric oxidation
of organic monolayer films
• Surface excess decays were measured in real time.
– oxidation of deuterated methyl oleate monolayers by ozone
• Second order rate constant could be calculated from the data.
– process much faster than in the bulk: atmospheric lifetime of 10 min
– insight into the important of surface vs bulk reactions was gained
4. Atmospheric oxidation
of organic monolayer films
• Reactions were also carried out in the new low-volume chamber.
– performance was validated with equivalent but better parameters
• Ellipsometry was also applied to the problem.
– here the data are not equivalent to those measured using NR
– differences attributed to transient anisotropic domains of products
4. Atmospheric oxidation
of organic monolayer films
• Neutron reflectometry.
– most direct quantification of the surface excess decays
– both the technique & reaction chamber are state-of-the-art
– isotopic substitution is now being exploited in mixed monolayers
• Ellipsometry.
– limited in its ability to contribute quantitative information
– comparison of morphology of reaction products can be revealing
• BAM.
– this technique was not applied to the problem in situ to date
– in principle it holds the potential for ellipsometry to be quantitative
Overview of the Case Studies
• 1. Surfactant adsorption to a dynamic liquid surface (formulations)
NR & ELL ... + ER-FTIRS ► quantification & composition
• 2. Spread layers of lung surfactant mixtures (health)
NR & ELL ... + ST + BAM ► structure
• 3. Thick film growth in polymer/surfactant mixtures (membranes)
NR & ELL … + BAM ► quantification & structure
• 4. Atmospheric oxidation of organic monolayer films (environment)
NR & ELL … (+ BAM) ► quantification (... composition)
Complementarity of NR with ELL
• Neutron reflectometry.
– probe structure of adsorbed species (isotopic labelling)
– reveal inhomogeneity (Bragg peaks & off-specular scattering)
– quantify accurately the adsorbed amount (direct measurement)
– measure composition of the surface layers (isotopic contrasts)
• Ellipsometry.
– optimize the use of neutron beam time (pre-screen systems)
– enhance precision & sensitivity (calibration to measured parameters)
– study detailed adsorption kinetics (fast acquisition rate)
– assess surface uniformity (small sampling size)
Assessment of the Techniques (Subjective!)
Pre-Screening Structure Quantification Composition
NR
ELL
ER-FTIRS
ST
BAM
Assessment of the Techniques (Subjective!)
Pre-Screening Structure Quantification Composition
NR
ELL
ER-FTIRS
ST
BAM
Thank you for your attention & good luck in your research!