Neutron scattering contrast variation experiments

When neutron scattering of a sample is combined with contrast variation, information can be obtained not only about the shape and size of the micelle but also about its detailed (internal) molecular architecture. Because of the unique level of information about micellar systems that can be obtained from SANS experiments, the technique is now an extremely well-established tool for investigating the shape, size, and, to a lesser extent, the internal structure of micellar aggregates with several hundreds of papers being published since the 1970s when micelles were the first colloidal systems to be studied using SANS. 

Fig. 19. Dependence of 7(0) on the volume fraction of HjO and H20 present in neutron scattering experiments. The vertical scales in (a), (b) and (c) are not to the same scale, (a) The I(O) values for myoglobin in 10 different contrasts are a linear function of the solvent scattering density [44]. This is the usual result of neutron contrast variation and indicates the monodispersity of the sample. The match-point corresponds to the so-called dry unhydrated volume of the protein. Typical Stuhrmann plots for proteins and glycoproteins are exempUfied in Fig. 21. (b) The values tor ferritin in 10 different... 

Time resolved SAXS/SANS allow a structural observation of kinetic processes on the nanoscale (1-100 nm) on a time scale ranging from milliseconds to hours. This allows micellar kinetics to be followed in real time, giving direct structural information of the process and its evolution. Synchrotron SAXS can reach smaller time scales and exhibits better resolution compared to neutron-based methods. However, SANS offers the possibility for contrast variation via simple H/D exchange chemistry, which opens up a world of possibilities for the investigation of kinetics in soft matter systems, in particular transport and exchange processes that otherwise would be invisible in scattering experiments. As most of these techniques have become available over recent years with advancements in both instrumentation and sample environments, there is a need for an overview of the development and the possibilities that are now available in the field of soft matter in general and micellar systems in particular. 

A more detailed treatment of contrast variation techniques, for both x ray and neutron scattering experiments, can be found in Reference 38 and in a book... 

The most important benefit of contrast variation lies in the isolation of the contour scattering function Ip(h), which lends itself directly to shape analysis. It has to be critically ensured, however, that the addition of contrast enhancing agents, as e.g. salts or sucrose, does not by itself lead to auiy structural changes. If D O is used in neutron experiments, the effects of H/D exchange have to be taken into account. 

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