A TRANSMISSION FTIR SPECTROSCOPIC STUDY ON MIXED LANGMUIR-BLODGETT-FILMS OF CADMIUM HEPTADECANOATE CHLORO/BROMOHEXADECANE

Mixed Langmuir monolayers and multilayers of cadmium heptadecanoate-chlorohexadecane (CdHp-ClHx) and cadmium heptadecanoate-bromohexadecane (CdHp-BrHx), for molar fractions of alkyl halides up to 0.3, were transferred onto hydrophilic substrates (CaF2). Structural information was obtained from surface pressure-area (pi-A) isotherms and from FTIR transmission spectra. Frequency shifts in the COO- antisymmetric stretching vibration (at 1535 cm(-1) for pure CdHp) were induced by the presence of the alkyl halides and have been qualitatively interpreted in terms of local interactions involving the polar groups at the surface. However, the alkylhalides do not modify the in-plane organization of the monolayer for any concentration; this was shown by the vibrational frequency of the CH2 antisymmetric stretch and also by a nonsplit band at 1467 cm(-1), assigned to the CH2 bending mode of the hydrocarbon chains and very sensitive to lateral interactions. For mixed multilayers, this band is a doublet, indicating that the structure has changed to a two-parameter (orthorhombic) arrangement, with the same parameters as pure LB multilayers. The structural difference between the first layer and the subsequent ones was proved by studying the spectra of four layers deposited on a CaF2 substrate previously hydrophobized with a layer of perdeuterated cadmium stearate (CdDSt); in this case, the band assigned to the CD2 bending vibration (at 1090 cm(-1)) is clearly separated from the two split components of the corresponding CH2 mode. A particular case of a pure CdHp monolayer transferred onto the hydrophobized substrate was also studied: it was proved that it rearranges in an orthorhombic structure, while the first layer (CdDSt) keeps the isotropic packing.