The Electronic Absorption Spectra of Fatty Acid Mono-, Bi-, and Multilayers with Bivalent Metal Ions

Abstract

Abstract The electronic absorption spectra of fatty acid multilayers with bivalent metal ions were studied in the vacuum ultraviolet (VUV) to ultraviolet (UV) energy region below 8 eV; they are due to transitions in the carboxylic acid and metal carboxylate moieties. Infrared (IR) absorptions due to the C–O stretching vibration in the carboxylate moiety are also studied. Stearic acid multilayers without metal ions are shown to form hydrogen-bonded dimers. The spectra of multilayers with metal ions show a marked dependence on the subphase pH which is different for different kinds of adsorbed metal ions. Their VUV spectra are conspicuously dependent on the kind of metal ions adsorbed. All the multilayers with alkaline earth metals, metals of the 2B group, transition metals, Cu, and Pb have at least two bands in the VUV region, 7.3–6.3 eV. The polarization spectra of the Ba stearate multilayer suggest that the two transitions may be the intramolecular π-π* and n-π* transitions in the carboxylate anion moiety. The multilayers with Cu and Pb have another band in the UV region. The spectra of the UV bands of the Pb and Cu stearate mono-, bi-, and multilayers were studied. In the Pb stearate layers, the positions of the UV bands in the bi- and multilayers are the same, but those in the monolayers are at considerably shorter wavelengths. The positions of the UV band in the Cu stearate mono-, bi-, and multilayers are successively red shifted. The polarization spectra of the UV bands of the mono- and bilayers showed that they are charge-transfer bands between the Pb or Cu ion and the coordinated carboxylate moiety.