VARIATIONS ON CONTRAST IN SANS - DETERMINATION OF SELF AND DISTINCT CORRELATION-FUNCTIONS

Studies of colloid solutions by scattering techniques use the very imp ortant notion of contrast; its origin is described. The interest of Sm all Angle Neutron Scattering (SANS) associated with the isotopic subst itution as a labelling method is shown. A survey of the usual labellin g methods used in order to separate the form factor and the interactio n term in the intensity scattered by a concentrated colloid solution i s given. The more recent ones, the zero average contrast (ZAG) and the triple isotopic substitution (TIS) methods, are described. Examples a re taken in the fields of polymers, micelles and interfaces. The ZAC m ethod is applied to the determination of short range interactions betw een monomers belonging to different polymers. The result can be a mode l 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 labell ed and from another solution where all the colloids have this part par tially labelled (labelling ratio x). How to obtain a similar result fr om only one isotopic substitution and using the contrast variation of the solvent is discussed. In conclusion, the problems of the isotopic substitution are raised.