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). 4 sin Q • Scattering lengths can be considered as a neutron refractive index.
• Scattering lengths of H & D have opposite signs. 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. d cbii i • Isotopic substitution in the solvent or interfacial species leads to more measured parameters and better data fits.
• Drawbacks: expensive materials & beam time. 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
• Choppers.
– four chopper system for variable resolution & increased transmission Neutron Reflectometry: FIGARO
• Frame Overlap Mirror.
– inclined silicon plate removes neutrons above a critical wavelength Neutron Reflectometry: FIGARO
• Deflection Mirrors.
– supermirrors coated on both faces to allow upward and downward reflection Neutron Reflectometry: FIGARO
• Collimation Guide.
– need to remove off-specular reflections from the deflection mirrors Neutron Reflectometry: FIGARO
• Detector.
– two dimensional array drilled from a single aluminium block 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
ˆˆ nˆˆisinθ i n j sin θ j
2 ˆˆ 2 nˆˆjcosθ i n i cos θ j Rrˆ ij,p ij,p ˆˆ nˆˆicosθ j n j cos θ i
2 ˆˆ 2 nˆˆicosθ i n j cos θ j • P: parallel to the plane of incidence Rrˆ ij,s ij,s ˆˆ nˆˆicosθ i n j cos θ j • S: perpendicular to the plane of incidence Null Ellipsometry: Scheme
• Polariser and analyser are rotated to null detected signal Null Ellipsometry: Scheme
• Two combinations at fixed C: PP 31 and AA31 . 2 Null Ellipsometry: Physical Basis
• Interpretation of P & A:
AA13 y D PP13 2 • Relation of y & D to physical parameters of the interface: rˆ p tany.eiD rˆs • This quotient is defined as the coefficient of ellipticity: rˆ p tany. cos D i sin D rˆs 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:
d nx n 2 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. DR ≈ 2.3.AAS0 R0 • Monolayer peaks can be positive or negative.
• ER-FTIRS / IRRAS / RAIRS = spectroscopic reflectometry. – structure – quantification – composition 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. 1 RTc ln T • Only valid if no complexation! 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
‘chosen platform & hydrodynamic construct’
• The overflowing cylinder. • Dynamic surfactant adsorption. – steady state conditions – many practical applications – well defined flow profile – Marangoni effects – suited to scattering probes – sub-surface controls diffusion 1. Surfactant adsorption to a dynamic liquid interface
‘model surfactant systems’
• Three model surfactants. – CTAB
– C10E8 – APFN
• Pure and mixed systems. – different head groups – different backbones – different resonances – different charges – different interactions – different chemistry 1. Surfactant adsorption to a dynamic liquid interface
‘structure and quantification from peaks’
• ER-FTIRS spectra [CTAB]. • ER-FTIRS coverage [CTAB].
– bonds in a liquid-like environment – precision & sensitivity < 10% 1. Surfactant adsorption to a dynamic liquid interface
‘valuable quantification through complementarity’
• ELL data [CTAB]. • NR data [CTAB].
– precise & sensitive but indirect – direct & accurate but insensitive 1. Surfactant adsorption to a dynamic liquid interface
‘valuable quantification through complementarity’
• Calibration of NR & ELL. • Data from both techniques.
– dependence almost linear – now quantified at low coverage 1. Surfactant adsorption to a dynamic liquid interface
‘valuable quantification through complementarity’
• Calibration to ER-FTIRS. • Data from all three techniques.
– dependence almost linear – surface coverage from spectra 1. Surfactant adsorption to a dynamic liquid interface
‘composition of nonionic–cationic mixture’
• C10E8:CTAB surface excess. • Surface vs bulk composition.
– competitive adsorption – hydrocarbons close to ideal mixing 1. Surfactant adsorption to a dynamic liquid interface
‘composition of anionic–cationic mixture’
• APFN:CTAB surface excess. • Free surfactant comparison.
– only surfactant in excess adsorbs – no aggregates at dynamic surface 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. • HPS compression & expansion.
– anisotropy from LC islands – 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 mw = 750k
• Ellipsometry was also applied to the problem. – challenge to quantify thicknesses > the wavelength! – answer lay in the evolution of the dynamic properties [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. • NR peak related to thickness. • Micron-scale films quantified. • Peak width reveals ordering. 3. Thick film growth in polymer/surfactant mixtures
• Thin film evenly textured. • Circular defects on collapse. • Growth results in wrinkling. • 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!