SAXS (small-angle X-ray scattering) is a small-angle scattering (SAS) technique where the scattering of X-rays (wavelength 0.1 ... 0.2 nm) by a sample which has inhomogeneities in the nm-range, is recorded at very low angles (typically < 3-5°). In this angular range, information about the shape and size of macromolecules, characteristic distances of partially ordered materials, pore sizes and the like is contained. SAXS is capable of delivering structural information of macromolecules between 5 and 25 nm, of repeat distances in partially ordered systems of up to 150 nm, using laboratory equipment. USAXS (ultra-small angle X-ray scattering) can resolve even larger dimensions.
The advantage over crystallography is that the samples need not be crystalline, and NMR methods cannot deal with macromolecules of high molecular mass (> 30000-40000). However, owing to the random orientation of dissolved or partially ordered molecules there occurs spatial averaging which leads to a loss of information.
SAXS is used for the determination of the size, shape and shape changes in macromolecules, the persistence length (a measure of flexibility) of polymers in solution, for the domain structure of solid polymers, for the determination of the specific inner surface of powders, long-range order of liquid crystals etc.
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