A Practical Guide to Quasi-elastic Neutron Scattering

Mark T F Telling

• Publisher: Royal Society of Chemistry
• Year: 2020
• ISBN:
  9781788019262
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Condensed Matter

The technique of Quasi-Elastic Neutron Scattering (QENS) is a powerful experimental tool for extracting temporal and spatial information at the nanoscale from both soft and hard condensed matter systems. However, while seemingly simple, the method is beset with sensitivities that, if ill considered, can hinder data interpretation and possibly publication. By highlighting key theoretical and data evaluation aspects of the technique, this specialised 'primer style' training resource encourages research success by guiding new researchers through a typical QENS experiment; from planning and sample preparation considerations to data reduction and subsequent analysis. Research examples are referenced throughout to illustrate the concepts addressed, with the book being written in such a way that it remains accessible to chemists, biologists, physicists, and materials scientists.

Part 1
Basics

1

If You Read Nothing Else…

The technique of quasi-elastic neutron scattering (QENS) is a powerful experimental tool for extracting dynamical information at the nanoscale from both soft and hard condensed matter systems. However, while seemingly simple, the method is beset with sensitivities that, if ill considered, can hinder data interpretation and possibly publication. To start, however, this chapter simply asks “What can QENS do for me?” Key parameters and preliminary experimental considerations necessary to plan a successful neutron scattering experiment are presented, as are research case studies in the areas of chemistry, biology, physics and materials science which expand upon the information that might be extracted using the QENS method.

In this chapter we will consider:

1. Whether the QENS method is suitable for your research.
2. Key parameters and preliminary experimental considerations necessary to plan a successful experiment.
3. Research examples in the areas of chemistry, biology, physics and materials science.

1.1 “What can Quasi-elastic Neutron Scattering Do for Me?”

Since you have picked up this book it is probably safe to assume that you want to know, first and foremost, if the technique of quasi-elastic neutron scattering (QENS) can aid your research. To start, therefore, let's forget the where and how and simply ask:

“What can QENS do for me?”

Quasi-elastic neutron scattering (QENS) is a well-established experimental method for exploring low-energy collective (i.e. species move as an ensemble), or self (i.e. a specie moves alone), atomic fluctuations and/or molecular reorientation from both soft and hard condensed matter systems; dynamic phenomena that may be thought of as ‘diffusive’. When we think of ‘diffusive’ here we might consider processes that are underpinned by atoms or molecules moving freely through a medium (e.g. the translational diffusion of molecules in a bulk liquid) or alternatively along a geometrically constrained path (e.g. the restricted movement of side groups on a polymer backbone). For those interested in magnetism, the quasi-elastic scattering signal of interest might arise from magnetic fluctuations that are perhaps not harmonic and therefore do not occur at a well-defined frequency.

QENS experiments allow researchers to access motions that occur on picosecond (ps, 10⁻¹² second) to nanosecond (ns, 10⁻⁹ second) timescales, and to explore said motions over length scales from 1 to ∼500 Ångström (Å, where 1 Å = 1 × 10⁻¹⁰ m); length scales that cover both inter and intra molecular distances. Figure 1.1 illustrates the temporal and spatial ranges covered by the QENS technique relative to other experimental methods.

Figure 1.1Temporal and spatial ranges explored using quasi-elastic neutron scattering relative to other experimental methods. Figure adapted from Telling.²⁶ The diagram correlates length (l) and time (t). Those techniques that do not directly provide length scale information are indicated as bars along the time axis.

Numerous research fields have benefitted from a QENS investigation. A comprehensive overview is beyond the scope of this book. However, if we were to group themes under the broad headings of Biology, Materials Science, Chemistry and Physics then areas of study include:

Biology: protein structure–function-dynamics relationships;^1,2 phase behaviour of lipid bilayers and membrane shape fluctuations;^3,4 hydration shell⁵ and cell crowding effects.⁶

Materials Science: hydrogen storage materials;⁷ organic solar cell photo-voltaics;⁸ built heritage;⁹ nano-composites;¹⁰ ion transport optimisation.¹¹

Chemistry: ionic liquids;^12,13 surfactant mixtures;¹⁴ micro emulsions;¹⁵ physisorption;¹⁶ polymer topology¹⁷ and confinement effects;¹⁸ proton conduction.¹⁹

Physics: spin fluctuations;²⁰ magnetic monopoles;²¹ phonon lifetimes;²² liquids under sheer and confinement;^23,24 theoretical predictions.²⁵

In terms of the information extracted, and depending upon the nature of the investigation, QENS measurements allow researchers access to:

relaxation rates and associated distributions; bending moduli; immobile/mobile volume fractions; reaction kinetics; diffusion coefficients; transition temperatures; Debye–Waller factors; activation and binding energies; molecular rigidity and/or elastic compressibility; geometry and associated dimensionality of motion.

To further accentuate research effort in these broad subject areas, case studies from peer-reviewed works are summarized in Table 1.1. These research examples expand upon the information that may be obtained. Detailed information about each example can be found in the accompanying reference.

Table 1.1Research case studies

Biology	High hydrostatic pressure specifically affects molecular dynamics and shape of low-density lipoprotein particles.^27
Research aim	To study the effect of hydrostatic pressure (20–3000 bar) on the molecular dynamics, and shape, of low-density lipoprotein (LDL) particles; the physicochemical characteristics being relevant for proper functioning of lipid transport in blood circulation.
Why QENS?	To assign motions observed on different timescales to different dynamical populations, specific molecules or molecular groups within LDL and under pressure.
Additional characterisation/supporting methods	Small angle neutron scattering (SANS-II @ the Paul Scherrer Institut (PSI), Switzerland) Sodium dodecyl sulfate polyacrylamide gel Electrophoresis (SDS-PAGE)
Main result reported	LDL copes well under high pressure conditions, although the lipid composition impacts the molecular dynamics and shape arrangement of the lipoprotein.
Instrument(s)/facility(ies) used and reported upper experimental observation time(s)	IN5 (up to 100 ps) @ the Institut Laue–Langevin (ILL), France IN6 (up to 15 ps) @ ILL, France IN13 (up to 100 ps) @ ILL, France

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