An infrared study on Langmuir-Blodgett films of octadecylammonium octadecanoate: FWHM and the vibrational coupling

Themonolayer of themixture of octadecanoic acid and octadecylaminewithmolar ratio 1:1 has been investigated at the air-water interface. It was found that the monolayer shows a rather stable state at the surface pressure of 30 mN/m and this monolayer can be transferred onto a CaF₂ plate by Langmuir-Blodgett (LB) technique. The infrared spectra of LB films indicated that octadecylammonium octadecanoate is formed by an intermolecular proton exchange between adjacent carboxylic and aminic groups (COO^- and NH₃⁺). In three-layer LB film, the CH2 scissoring mode of the long hydrocarbon chains of octadecylammonium octadecanoate shows a broad band feature at about 1468 cm^-1 while this vibrationalmode of three-layer LB film of the mixture (1:1) of deuterated stearic acid and octadecylamine (octadecylammonium octadecanoate-d35, C₁₈H₃₇NH₃⁺C₁₇D_35COO^-) only shows a narrow band. The broad feature of the CH2 scissoring mode in octadecylammonium octadecanoate probably originates from the coupling between the chain of stearic acid and that of octadecylamine while this kind of coupling could be completely removed in octadecylammonium octadecanoate-d35. Another conclusion presented in this paper is that there are no couplings among the chains of fatty acid or among the chains of octadecylamine in LB films of octadecylammonium octadecanoate.