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

Mixed Langmuir monolayers and multilayers of cadmium heptadecanoate-ch lorohexadecane (CdHp-ClHx) and cadmium heptadecanoate-bromohexadecane (CdHp-BrHx), for molar fractions of alkyl halides up to 0.3, were tran sferred onto hydrophilic substrates (CaF2). Structural information was obtained from surface pressure-area (pi-A) isotherms and from FTIR tr ansmission spectra. Frequency shifts in the COO- antisymmetric stretch ing vibration (at 1535 cm(-1) for pure CdHp) were induced by the prese nce of the alkyl halides and have been qualitatively interpreted in te rms of local interactions involving the polar groups at the surface. H owever, the alkylhalides do not modify the in-plane organization of th e monolayer for any concentration; this was shown by the vibrational f requency of the CH2 antisymmetric stretch and also by a nonsplit band at 1467 cm(-1), assigned to the CH2 bending mode of the hydrocarbon ch ains 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 la yer 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 ban d assigned to the CD2 bending vibration (at 1090 cm(-1)) is clearly se parated from the two split components of the corresponding CH2 mode. A particular case of a pure CdHp monolayer transferred onto the hydroph obized substrate was also studied: it was proved that it rearranges in an orthorhombic structure, while the first layer (CdDSt) keeps the is otropic packing.