Etude aux rayons x des modifications structurales de films langmuirblodgett d'acide behenique par insertion d'argent in situ

Service de Physique du Solide et de Résonance Magnétique, Centre d'Etudes Nucléaires de Saclay, 91191 Gif-sur-Yvette Cedex, France.

Résumé

Les changements structuraux liés à l'insertion d'ions Ag⁺ dans un film Langmuir-Blodgett d'acide béhénique puis à leur réduction in situ en Ag⁺ sont étudiés par Rayons X. Des mesures de paramètres et d'intensité permettent de conclure à la formation de béhénate d'argent. Sa réduction conduit à un acide de structure différente et les amas d'argent formés sont incohérents avec la matrice de l'acide. Ces modifications structurales impliquent des rotations collectives de molécules.

Abstract

Structure changes of Langmuir-Blodgett films of behenic acid induced by adding Ag+ ions within the layers and further in situ reduction are studied by X-ray diffraction at small glancing angles.

— The multilayers remain regular during the chemical processes.

— During the first stage, silver soap is created as shown by the long spacing value. From intensity measurements, a 3 step model of electron density profile is deduced. All Ag⁺ ions are located in the CO₂-layer which width is given.

Reduction of Ag⁺ into Ag° and growth of silver intercalated clusters restores behenic acid which cristallographic form depends upon the reduction conditions. The clusters are not coherent with the matrix of behenic acid.

These structural changes involve collective rotations of the zig-zag planes of the carbon chains which also explain the diphasic growth process of the silver salt within behenic acid.