Spectroscopic and Structural Studies on Adsorption of Azo Dye in Cationic Langmuir–Blodgett Films

The monolayer properties of octadecylamine at various pHs on the Naphthalene Red (NR) aqueous solution were studied from the measurements of surface pressure–area isotherms. It was observed that the presence of the azo dye substituted with the –OH group (NR) in the subphase affects the compressibility of the octadecylamine monolayer. The adsorbability of the cationic Langmuir–Blodgett (LB) films was investigated by NR as an adsorbate. The cationic LB films before and after the azo dye adsorption were characterized by UV–vis absorption and emission spectroscopy, polarized visible absorption, X-ray diffraction, FTIR spectroscopy, and atomic force microscopy. Strong dipole–dipole interaction between the NR molecules in functionalized LB films was observed in accordance with the exciton model. It was found that the adsorption of the azo dye induced rearrangement of the layer structure of the cationic LB films where the hydrocarbon chains of the amine molecules exhibited an interdigitated structure and the azo dye molecules adsorbed in the LB films were oriented in an edge-on configuration. Furthermore, the substituent position of the –OH group of NR plays a crucial role in the different ways on the orientation of the other azo dyes in organized assembly.