Small angle scattering methods - A guide to neutron and X ray scattering experiments on surfactant solutions

At present (1985), small angle scattering is the only method which allows distances to be measured in the range 5-500 A. The structural information is obtained in the form of interference patterns, where all spatial correlations are averaged over the irradiated volume and over the duration of the experiment. For disordered samples such as surfactant solutions, where thermal agitation results in large fluctuations of all distances and orientations, this averaging reduces drastically the amount of information contained in the scattering patterns; this reduction is at the same time a tremendous help and a severe limitation. It is a help because the information derived from scattering patterns is the true thermodynamic average of the solution; this is in contrast with the images obtained from microscopy, which relate to different fluctuations observed at different point and times in the sample. It is a limitation, because this information may not always be sufficient to reconstruct an image of a typical structure at the desired level of resolution. All recent developments of small angle scattering are attempts to partially overcome this limitation: extending the scattering patterns to high scattering vectors and therefore high resolution, selective labelling of some distances and contrast variation, and finally orienting the solutions with high magnetic fields or shear gradients. Beyond these developments, the fundamental constraints of small angle scattering (averaging and loss of the phases of the scattered amplitudes) will probably remain until X-ray holography or microscopy with image processing take over