Phase transitions in Langmuir-Blodgett films of cadmium stearate: Grazing incidence X-ray diffraction studies

Variations in the structure of multilayer Langmuir-Blodgett (LB) films of cadmium stearate with temperature have been monitored by grazing incidence synchrotron X-ray diffraction. At room temperature the film structure comprises a centered rectangular unit cell in the plane of the substrate with the hydrocarbon chains normal to the substrate surface. As the temperature increases up to 90 degrees C, the diffraction pattern due to this in-plane structure becomes diffuse and the intensity decreases. Changes in the Q-values of the in-plane spots indicate an anisotropic expansion of the unit cell. At around 100 degrees C the film undergoes a phase transition from a layered three-dimensional crystalline structure to an array of rod structures packed hexagonally. The rods are oriented parallel to the surface of the film. The core of each rod is 6.7 Angstrom in diameter and appears to be composed of cadmium cations and the carboxylate groups of the fatty acid chains. The aliphatic chains are randomly arranged around these rods. The rod structure is retained on cooling, but the observed correlation length is reduced.