Neutron Reflectometry combined with Complementary … · 2014. 9. 23. · Neutron Reflectometry: Physical Basis •Neutron wavelengths similar to molecular length scales, so interference

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



– four chopper system for

variable resolution & increased transmission

• Choppers.


– inclined silicon plate removes

neutrons above a critical wavelength

• Frame Overlap Mirror.


– supermirrors coated on both faces

to allow upward and downward reflection

• Deflection Mirrors.


– need to remove off-specular

reflections from the deflection mirrors

• Collimation Guide.


– two dimensional array

drilled from a single aluminium block

• Detector.


• 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 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:



• 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’



• 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’



• ER-FTIRS spectra [CTAB].

– bonds in a liquid-like environment

• ER-FTIRS coverage [CTAB].

– precision & sensitivity < 10%

‘structure and quantification from peaks’



• ELL data [CTAB].

– precise & sensitive but indirect

• NR data [CTAB].

– direct & accurate but insensitive

‘valuable quantification through complementarity’



• Calibration of NR & ELL.

– dependence almost linear

• Data from both techniques.

– now quantified at low coverage




• Calibration to ER-FTIRS.

– dependence almost linear

• Data from all three techniques.

– surface coverage from spectra




• C10E8:CTAB surface excess.

– competitive adsorption

• Surface vs bulk composition.

– hydrocarbons close to ideal mixing

‘composition of nonionic–cationic mixture’



• APFN:CTAB surface excess.

– only surfactant in excess adsorbs

• Free surfactant comparison.

– no aggregates at dynamic surface

‘composition of anionic–cationic mixture’



• 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


• Current representation of lung surfactant at high compression.

2. Spread layers of


• 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


• 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


• DPPC compression.

– anisotropy from LC islands

• HPS compression & expansion.

– hysteresis due to lipid reservoirs

2. Spread layers of


• 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



– 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]



• Opposite trends for growth & collapse.

– n = 1.40, 1.42 & 1.44 tested

• Experiments matches simulations.



• Growth then collapse occurs.

• Micron-scale films quantified.

• NR peak related to thickness.

• Peak width reveals ordering.



• Thin film evenly textured.

• Growth results in wrinkling.

• Circular defects on collapse.

• Final film smooth but mobile.



• 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


– 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







• 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



• 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




– 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


– 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!

Neutron Reflectometry combined with Complementary … · 2014. 9. 23. · Neutron Reflectometry: Physical Basis •Neutron wavelengths similar to molecular length scales, so interference

Documents