Water Interfaces Group – UIC Department of Physics

The availability of synchrotron X-ray sources has radically transformed much of X-ray physics. Subsequent developments described in this book have led to substantial progress in our understanding of molecular ordering at liquid interfaces, with relevance to many areas of science and technology. This practical guide on the subject will enable graduate students and researchers to understand and carry out experimental investigations into the basic physical and chemical properties of liquid surfaces and interfaces. The book examines the surfaces of bulk liquids, thin wetting films, and buried liquid-liquid interfaces. It discusses experiments on simple and complex fluids, including pure water and organic liquids, liquid crystals, liquid metals, electrified liquid-liquid interfaces, and interfacial monolayers of amphiphiles, nanoparticles, polymers, and biomolecules. A detailed description of the apparatus and techniques required for these experiments is provided, and theoretical approaches to data analysis are described, including approximate methods such as the master formula, the Born approximation, Parratt’s algorithm, and the distorted-wave approximation. It is ideal for people working in physics, chemistry, biology, and materials science.

Liquid Surfaces and Interfaces: X-ray Synchrotron Methods

Table of Contents

1 Introduction
1.1 The intrinsic liquid/vapor interface
1.2 Surface-induced order
1.2.1 Surface layering
1.2.2 In-plane surface order
1.3 Capillary waves on the liquid surface
1.4 Thin liquid films
1.5 Buried interfaces
1.6 Optical methods
1.6.1 Ellipsometry
1.6.2 Non-linear optics
1.7 X-ray surface methods
1.8 Further reading
1.8.1 X-ray physics
1.8.2 Capillary wave theory
1.8.3 Soft matter
1.9 References

2 Instrumentation
2.1 Introduction
2.2 Liquid-surface reflectometers
2.3 Kinematics of scattering from liquid surfaces and interfaces
2.4 Overview of the first type of liquid-surface scattering instrument
2.4.1 Steering cyrstal: simple tracking
2.5 Detailed full alignment
2.5.1 Optical pre-alignment
2.5.2 X-ray alignment: first steps
2.5.3 Steering-crystal alignment
Centering the steering-crystal face on the synchrotron beam
Equations for the miscut and the effect of \(\eta\) not equal to zero
Correcting for the miscut
Correcting for \(\eta\) not equal to zero
2.5.4 Input-arm tracking: equations
Equations for the incident wave vector when using a base tilt stage
2.5.5 Input-arm, sample-stage, and output-arm alignment
Preliminary input-arm zero
Input-arm tracking and setting L1
First sample reflection and setting L2 and L3
Precise X-ray determination of \(\alpha_i=0\) for a flat sample
Alignment with curved samples
Aligning the guard slit S2 and setting the zero of the output-arm rotation
2.5.6 The output (detector) arm for grazing-incidence diffraction
Two-slit arm
Angular-selection arm
Area detector
2.5.7 Reflectometer motions as a function of \(Q\)
\(Q_z\) dependence for reflectometer with base-tilt stage
2.6 Overview of the second type of liquid-surface scattering instrument
2.6.1 Practical issues for the second type of reflectometer
2.6.2 First vs. second type of instrument
2.7 Detectors
2.7.1 Gas-ionization detectors
2.7.2 Scintillation detectors
2.7.3 Photodiodes
2.7.4 Position-sensitive linear detector (PSD-1D)
2.7.5 Position-sensitive area detector (PSD-2D)
2.8 Absorber calibration
2.9 Non-synchrotron-based liquid-surface reflectometer
2.9.1 Fixed X-ray sources
2.9.2 Butterfly-type reflectometers
2.10 Addenda
2.10.1 Bragg reflection
2.10.2 Alignment without a tilt stage
Equations for the incident wave vector in the absence of the base tilt stage
The \(Q_z\) dependence for a reflectometer without a base tilt stage
2.11 References

3 Theory of X-ray scattering from liquid surfaces
3.1 Introduction
3.2 Overview of the theory
3.3 Reflection from an idealized flat surface: the Fresnel reflectivity
3.4 Reflection from less idealized surfaces
3.4.1 The Parratt method
3.4.2 The master-formula approximation
3.5 The Born approximation
3.5.1 General development
3.6 Examples of applications for flat surfaces
3.6.1 The effective surface structure factor
3.6.2 The surface structure factor for layered surfaces
3.6.3 The Patterson function
3.7 The effect of thermal roughness
3.7.1 Resolution effects on the capillary-roughness model
Diffuse scattering: numerical integration
Specular reflectivity: numerical integration
Specular reflectivity: analytic approximation
Specular reflectivity: circular resolution function
Diffuse scattering: analytic approximation
3.8 The distorted-wave approximation (DWA)
3.8.1 Formal development
3.8.2 Sample applications of the DWA
Scattering from a thin surface layer
Height fluctuations
Grazing-incidence diffraction (GID) from two-dimensional surface order
3.9 Scattering from a thick film
3.9.1 The DWA cross section
3.9.2 Internal thermal fluctuations
3.9.3 The Born approximation
3.10 Effects of X-ray coherence and macroscopic surface inhomogeneities on specular reflectivity
3.11 X-ray photon-correlation spectroscopy
3.12 Addenda
3.12.1 Realistic integration of capillary fluctuations
3.12.2 Effective van der Waals interaction
3.13 References

4 Experiments on liquid surfaces and interfaces
4.1 Liquid/vapor interfaces without observable structure
4.1.1 The surface of water
4.1.2 Other non-structured liquid surfaces
4.2 Surfaces exhibiting structure
4.2.1 Liquid crystals
4.2.2 Liquid metals
Elemental metals
Alloys: Gibbs adsorption
Alloys: surface freezing
4.2.3 Two-dimensional molecular surface freezing
Surface freezing of alkanes and alcohols
4.2.4 Langmuir monolayers
Langmuir monolayers of amphiphilic molecules with long alkyl chains
Charged langmuir monolayers and interactions with subphase ions
Langmuir monolayers relevant to biological processes and biomaterials
Langmuir monolayers of nanoparticles
Langmuir monolayers of polymers
4.3 Interfaces between thin macroscopic liquid films and a bulk liquid
4.3.1 Ga–Bi
4.3.2 Vapor-controlling thin liquid wetting films
Controlled wetting
Thin macroscopic single-component liquid wetting layers on bulk substrate
Thin macroscopic binary wetting layers on bulk substrate
4.4 Deeply buried liquid/liquid interfaces
4.4.1 Neat liquid/liquid interfaces and special techniques for their study
4.4.2 Liquid/liquid interfaces with surfactants
Head-group packing: oil/water vs. air/water
Phase transitions
Reflectivity from inhomogeneous interfaces
Ultra-low-tension interfaces
4.4.3 Ion distributions
4.5 Time-dependent surface fluctuations
4.6 References