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Small angle X-ray scattering (SAXS) is a low resolution method to investigate structures of the order of 1-100 nm.
Information of such large objects is contained in the scattering intensity at small scattering angles, typically
less than 2 degrees. In principle, the fundamental phenomenon of scattering is the same as in the diffraction techniques.
However, the small-angle scattering method requires both a specialized experimental set-up to measure such low scattering
angles and the methods of data analysis are special for the technique. As shown in the photo, a long vacuum tube between
sample and detector is typical for a SAXS beamline. The long distance is needed to obtain good resolution at the
required low angles.
In brief, we may study three kinds of systems by small angle X-ray scattering.
Firstly, dilute particulate systems, in these cases one material is dispersed in a uniform matrix of second material.
A typical example could be colloids or proteins that are dissolved in a solvent and the solution is so dilute that
individual colloid or protein particles are far apart from each other, that is to say uncorrelated. The analysis
of the data is focused on the shape and size of the dissolved particles.
Secondly, non-particulate two-phase systems, in these cases two materials are irregularly intermixed but neither one
is considered as solvent. This could typically be a semi-crystalline polymer that contains crystalline and amorphous domains.
The analysis is focused on the interfaces of these domains. It is important to note that the crystalline structure of
crystalline domains is studied by crystallography, whereas small-angle scattering arises from the contrast between the domains.
The crystalline structure takes place in Ångström scale whereas the size of the crystalline domain is of the order
of tens of nanometers, i.e., 100 times bigger.
Thirdly, also periodic systems are studied. In these cases the material shows long range periodicities that are too large
to be studied by standard crystallographic instruments.
Overall, small-angle scattering finds its applications in areas as diverse as physics, chemistry, materials science and life
sciences. It is yet important to note that small-angle X-ray scattering forms usually only a part of the research and the data
are typically complemented by various other methods such as neutron scattering and electron microscopy.
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