Structural characterization of Langmuir–Blodgett films of 4,5-bis(dodecyloxy)phthalic acid

Abstract

In this work, the formation of Langmuir monolayer of 4,5-bis(dodecyloxy)phthalic acid (DPA) molecule at the air–water interface, surface-pressure area ( π– A) isotherm and stability of this monolayer was investigated. The structural characterization of Langmuir–Blodgett (LB) films of DPA deposited on aluminium covered glass substrate was studied using Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) measurements. For FTIR measurements, p-polarized grazing angle (GAIR) and horizontal attenuated total reflectance (HATR) techniques have been employed to characterize the LB films. The molecular structure of LB films was investigated by comparing the GAIR and HTAR spectra. The two extra bands that appeared in the FTIR spectra of LB films suggest that the carbonyl groups are bonded to the adjacent carbons of head groups and the carboxylic acids of head groups are capable of forming five-membered cyclic anhydrides in all LB films. The slightly downward shifts of the ν(CH 2) stretching bands are indicative of the increase in conformational order from gauche conformers to highly ordered transconformers in the hydrocarbon chain with increasing number of layers. Changes in intensity between the p-polarized GAIR and HATR spectra may indicate that the alkyl chains are almost perpendicular to the substrate in LB films. The singlet absorption due to the δ(CH 2) and ρ r(CH 2) modes indicated that in LB films of DPA alkyl chains are packed in hexagonal structure. The XRD results show that all the multilayer structure is Y-type and the alkyl chains aligned almost vertically in the LB films.