A study of the layer structure in a calix-8-arene Langmuir–Blodgett film by reflectometry

Abstract

X-ray reflectometry measurements have been made on Langmuir–Blodgett films of tertiary-octyl calix-8-arene. The samples have additional cadmium ions incorporated in the film and the typical X-ray reflectivity profile exhibits both interference fringes and a well defined Bragg peak. The total film thickness and the bilayer spacing were obtained from these. Simulations of the experimental reflectivity profile were made for three structural models, using a program based on Parratt's recursive relations. The traditional “bowl and chain” conformation and the “wheel-like” pleated loop conformation of calix-8-arene were both unable to provide an effective model for the structure of the multilayer film. In contrast, the more complex “pinched loop” molecular conformation produced a film with the correct total thickness and bilayer spacing. The reflectivity profile calculated from this model was in excellent agreement with the experimental profile. The principle features of the model are a partial interdigitation of the hydrophobic R-groups and an interpenetration of the hydrocarbon X-groups, which leads to a high value of electronic scattering length density in the plane of the incorporated cadmium ions.