Contrast variation technique

Controversial discussions have surrounded the three-dimensional structure of flexible dendrimers in solution during the past two decades. Now, on the basis of numerous SANS experiments using the contrast variation technique, the idea that isolated flexible dendrimers in good solvents do not take on the originally predicted dense shell structure but instead assume a dense core structure [47] appears to be gaining general acceptance. This means that in such dendrimers the segment density reaches a maximum at the centre of the molecule and decreases towards the periphery (cf. Fig. 7.6). 

In the case of inhomogeneous particles, contrast variation techniques can be used to separate the shape and density fluctuation terms. D r) is now proportional to the number of pairs of scattering lengths separated by the distance r in the volume elements p and q, where each value of r is weighted by the product fp -f. Depending on the solvent scattering density, some volume elements can make a negative contribution to 7>(r). As previously, the contrast dependence of 7)(r) is readily derived in terms of three basic functions ... 

A more detailed treatment of contrast variation techniques, for both x ray and neutron scattering experiments, can be found in Reference 38 and in a book... 

The study of biological systems by neutron scattering is restricted by the difficulty, if not impossibility, of deuteriating the molecules. However, solutions or suspensions can still be studied using a contrast-variation technique, which involves altering the scattering density of the solvent by selective deuteriation. 

A recent and important development of SAS in the characterization of porous solids is the application on the contrast variation technique. Classically we consider a two phase system where the scattering dcr/dQ(q), expressed as the length of the scattering vector, is proportional to the contrast, i.e. the square of the scattering length densities difference between the two phases 1 and 2 ... 

Abstract Partial structure functions and mean curvatures of a ternary amphiphilic system consisting of water, n-octane and nonionic amphiphile C12E5 with equal volume fraction of oil and water have been studied by means of small-angle neutron scattering (SANS). Isotopic contrast variation technique was used to obtain suitable scattering contrasts of oil and water. We observed that the cross partial structure functions, that arise from the correlation between water-surfactant or oil-surfactant, change sign when the temperature... 

For the amphiphilic block copolymer in the non-polar selective solvent, the unpolar blocks form the corona, which provides solubilization and stabilization, while the polar or hydrophilic and functionalized blocks form the core, which is able to dissolve metal compounds due to coordination, followed by the nucleation and growth of metal particles upon reduction. Also the internal structure of block-copolymer micelles, as given by the size of core and corona and the density profile in each domain, has been carefully characterized by static and dynamic light scattering [146] and by small angle neutron scattering using contrast variation techniques [147], The micellar corona has many of the characteristics of a spherical polymer brush. 

Burkitt et al. [228,229] used SANS to examine the size and shape of micelles in solutions containing ammonium perfluorooctanoate or mixtures of ammonium perfluorooctanoate with ammonium decanoate. The SANS measurements were made by the external contrast variation technique using mixtures of water and D2O as the solvent. By selecting appropriate H2O-D2O ratios, it is possible to view hydrocarbon and fluorocarbon micelle species independently. At a match point, the scattering length density of the H2O-D2O mixture is equal to that of the surfactant and the surfactant is at zero contrast. If the surfactants in a binary mixture form separate micelles, two match points are found. If mixed micelles are formed, scattering would occur at the contrast match points for each surfactant, but another match point is found as well. 

Small-angle neutron scattering coupled with the contrast variation technique was used by Caponetti et al. [230] to study solutions of sodium perfluo-rooctaonate, sodium dodecanoate, and their mixture. Their data indicated the existence of mixed micelles having the same composition and a narrow size distribution. SANS data obtained by Berr and Jones [231] indicated that sodium perfluorooctanoate forms in water spherical micelles in which the fluorocarbon chains reside in a water-free core. 

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