Neutron reflectivity and soft condensed matter

doi:10.1016/S1359-0294(02)00015-8

Current Opinion in Colloid & Interface Science

Volume 7, Issues 1–2, March 2002, Pages 139-147

https://doi.org/10.1016/S1359-0294(02)00015-8 Get rights and content

Abstract

During the last 10–15 years neutron reflectivity has emerged as a powerful and important technique for the study of surfaces and interfaces. The selectivity and sensitivity afforded by deuterium/hydrogen exchange makes the technique particularly attractive for application to the broad field of colloid and interface science. The development of the instrumentation, specialised sample environment equipment and analysis techniques has resulted its application to complex interfaces and environments and in the study of complex multi-component systems. This review provides a summary of those developments in the last two years.

Introduction

The specular reflectivity of neutrons provides information about the refractive index or scattering length density distribution normal to the surface or interface, and is directly related to the composition or concentration profile in the interfacial region. Grazing incidence geometry and the wavelength range of cold neutrons provides a wave-vector transfer, Q, range (where $Q= 4π λ sin θ$ , θ is the glancing angle of incidence, and λ, the neutron wavelength) well matched to the length scales of interest (∼10 to 4000 Å). The scattering powers of hydrogen and deuterium are vastly different, and provide the opportunity to manipulate the refractive index distribution by H/D isotopic substitution, without substantially altering the chemistry. The refractive index for neutrons is defined as $n=1− Nb 2π λ^{2}$ , where N is the atomic number density, and b the neutron scattering length (0.6674×10⁻¹² cm for deuterium, and −0.374×10⁻¹² cm for hydrogen). The ability to manipulate the ‘contrast’ is a powerful feature and extensively exploited. It provides the contrast to highlight the interface of a polymer bilayer, and the selectivity to study the adsorption of complex multi-component mixtures. Cold neutrons are also a penetrating probe, and this provides access to ‘buried’ interfaces. Studies are hence not limited to the air–solution and air–solid interfaces, but can also be made at the solid–solid, solid–solution and liquid–liquid interfaces.

The emerging patterns of the application of neutron reflectivity in colloid and interface science are summarised in the main themes of this review. They involve the investigation of more complex interfaces, including bio-membranes, in-situ electrochemistry, and adsorption at the liquid–solid and liquid–liquid interfaces, and more complex environments, surfaces under shear or confinement. In the study of polymer and surfactant adsorption at interfaces, which have been predominantly the domain of neutron reflectivity, the trend is towards complex structures, mixtures and the development of nano-structures. The greater sophistication of experimental design is also reflected in the studies on solid polymer films. The article will briefly review progress over the broad area of applications relevant to colloid and interface science under the following categories; surfactant adsorption, polymer adsorption, polymer films, polymer–surfactant mixtures, bio-membranes, electrochemistry, and nano-structured films.

Section snippets

Surfactant adsorption

The study of the adsorption of surfactants at interfaces has been one of the startling successes of the applications of neutron reflectivity, where the combination with H/D isotopic substitution has enabled not only adsorbed amounts but also the detailed structure of the monolayer to be determined. Lu et al. [1] have produced a comprehensive review of the broad range of investigations of surfactant adsorption at the air–water interface. In particular they highlight the detailed structural

Adsorption of polymer–surfactant mixtures

Until recently most of the experimental investigations into polymer/surfactant complexes have focussed on the solution aggregate behaviour and not their adsorption properties. This is because there have been few techniques which measure directly the surface composition of such layers, and techniques such as surface tension are difficult to interpret comprehensively. Neutron reflection is capable of providing both structural and compositional information, and this is now transforming our

Biomembranes

In the last few years there has been a greatly increased interest in using neutron reflectivity to study surfaces and thin films of biological or biomimetic interest. These involve protein adsorption (including protein/surfactant mixtures, already discussed in the previous section because of their similarity with polymer–surfactant mixtures), model bio-membranes, and the nature of protein–membrane interactions. Krueger [20] and Fragneto-Cusani [21] have recently reviewed aspects of the

In-situ electrochemistry and other complex environments

The favourable transmission of thermal and cold neutrons through crystalline materials (such as silicon and quartz) provide the opportunity to investigate ‘buried’ interfaces and exploit more complex environments. This has been exploited in some in-situ electrochemistry measurements. Although the amount of material electrochemically deposited can be measured straightforwardly using techniques such as the quartz micro-balance, the structure of the deposited film, and the solvent penetration into

Polymer adsorption

The nature of polymer adsorption is important for a wide range of technologies, and has been extensively studied both experimentally and theoretically. Neutron reflectivity has emerged as a powerful technique for determining adsorbed amounts and the structure of the adsorbed layer at both the air–solution and liquid–solid interfaces.

Kent [36] has summarised a comprehensive study of the nature of tethered chains under a variety of solution conditions, using Langmuir monolayers of the diblock

Thin polymer films

Neutron reflectivity has been extensively used to study a variety of phenomena in polymer thin films, including the nature of polymer–polymer interfaces, micro-phase separated structures, partitioning or segregation at interfaces, and the effects of confinement. The examples discussed in this section summarise some of the recent developments and studies in these areas.

Sferrazza et al. [45] have reviewed a number of recent results on the nature of polymer–polymer interfaces, and on the

Summary

A review of the literature over just the last two years shows an extensive and exciting range applications of neutron reflectivity in colloid and surface science. The emphasis on more complex interfaces, complex environments, and complex multi-component systems is clear. The combination of techniques, neutron and X-ray reflectivity with other surface techniques is also increasingly prevalent.

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Citation Excerpt :
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Neutron reflectivity and soft condensed matter

Abstract

Introduction

Section snippets

Surfactant adsorption

Adsorption of polymer–surfactant mixtures

Biomembranes

In-situ electrochemistry and other complex environments

Polymer adsorption

Thin polymer films

Summary

Adv Con Int Sci

Coll Surf

Adv Coll Int Sci

Cur Opin Coll Int Sci

Physica B

Physica B

J Electroanal Chem

Physica B

Coll Surf A

Langmuir

PCCP

Langmuir

Langmuir

J Phys Chem B

PCCP

Langmuir

Langmuir

PCCP

Langmuir

J Phys: Condens Matt

Langmuir

Macromolecules

Langmuir

PCCP

J Phys Chem B

J Phys: Codens Matt

Europhys Lett

Langmuir