Variations on contrast in SANS: Determination of self and distinct correlation functions

Studies of colloid solutions by scattering techniques use the very important notion of contrast; its origin is described. The interest of Small Angle Neutron Scattering (SANS) associated with the isotopic substitution as a labelling method is shown. A survey of the usual labelling methods used in order to separate the form factor and the interaction term in the intensity scattered by a concentrated colloid solution is given. The more recent ones, the zero average contrast (ZAG) and the triple isotopic substitution (TIS) methods, are described. Examples are taken in the fields of polymers, micelles and interfaces. The ZAC method is applied to the determination of short range interactions between monomers belonging to different polymers. The result can be a model for interactions between diffuse particles. The TIS method allows to determine the structure of a specific part of a complex colloid from a solution containing a mixture with x of them having this part labelled and from another solution where all the colloids have this part partially labelled (labelling ratio x). How to obtain a similar result from only one isotopic substitution and using the contrast variation of the solvent is discussed. In conclusion, the problems of the isotopic substitution are raised.