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Neutron Reflectometry Combined with Complementary Techniques to Solve Complex Soft Matter Problems at the Air/Water Interface

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Neutron Reflectometry Combined with Complementary Techniques to Solve Complex Soft Matter Problems at the Air/Water Interface 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
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