Evidence of a tilted and herringbone structure in cadmium behenate Langmuir–Blodgett ultrathin films: Comparison with Langmuir monolayers

Abstract

The molecular arrangement of monolayers and multilayers of cadmium behenate deposited at the air–water interface or on air-oxidized silicon wafers was investigated by means of grazing incidence x-ray diffraction. Even though it is frequently mentioned that the Langmuir–Blodgett (LB) monolayers of fatty acid salts have usually an in-plane hexagonal packing, we obtained two first-order peaks in all the investigated films, indicating a distorted hexagonal lattice. Whereas these peaks are in-plane in the Langmuir or LB monolayers implying no molecular tilt, they are slightly out of the plane in the 3 and 5 layers samples indicating tilted molecules. In these few layers LB films, second-order peaks are detected and the molecules are tilted towards a next-nearest neighbor (NNN structure), as in the L2′ phase of fatty acids. Using two angles of incidence of the x-ray beam allowing to probe either all the layers or only the external layers, a distribution of the tilt angle in the range 1°–6° is evidenced within each layer of the LB films. Moreover, the herringbone peak is measured in this NNN structure corresponding to the L2′ phase, implying that the herringbone order is long-range in the LB films.