Structural characterisation of sodium behenate Langmuir-Blodgett films using X-ray diffraction techniques

Langmuir-Blodgett films of sodium behenate were deposited on silicon and analysed by X ray specular reflectivity and grazing incidence X-ray diffraction in order to investigate the surface normal ordering and the in-plane molecular packing. The results showed that the films were monophasic with uniform thickness. The average lattice periodicity along the surface normal was 563 Angstrom. A tilt angle between growth direction and surface normal in the range 0-26 degrees was detected. The surface diffraction measurements revealed three separate peaks. This indicates an oblique 2D unit cell as expected in the case of molecules tilted with respect to the surface normal (growth direction). From the width of. the in-plane peaks a lateral dimension of the diffracting crystallites of about 130 Angstrom was derived. These results related to sodium behenate, for which little information is reported in literature, are in accordance with the values obtained from the data available for different members of the homologous series of fatty acids.