Texture of fatty acid Langmuir films studied by means of Brewster angle reflectometry

The compression induced change of the integral reflected intensity of an arachidic acid Langmuir film at the Brewster angle — the so-called reflection isotherm — is analysed to get information about the in-plane anisotropy of the Langmuir film caused by compression and expansion cycles. Based on the matrix method of Berreman a theoretical model, connecting the shape of the reflection isotherms with the domain orientation distribution in the monofilm, is developed. Furthermore, the influence of different counter-ions on the reflection isotherms is studied. The data indicate the formation of ion-specific domain orientation distributions (textures). The reflection isotherms are sensitive to changes in the Langmuir films supramolecular structure which are not reflected by the surface pressure-area isotherms.