Infrared external reflection spectroscopy of adsorbed monolayers in a region of strong absorption of substrate

There are a number of systems in which the substrate shows a spectral stron g absorption in the region of the characteristic vibration of the surface s pecies. These systems are extremely difficult to study because of the overl apping of a small spectroscopic signal from surface species with a very str ong absorption signal from the substrate. For example, major mineral compon ents of hard tissue, carbonates and phosphates, show very strong absorbance in a relatively wide spectral region in which the characteristic vibration s of the carboxylate, amide, C-O, or other groups of soft tissue, having a biological importance, take place. The system chosen for the study was calc ite with very strong absorption between 1600 and 1400 cm(-1) due to carbona te vibrations and oleate which forms a calcium oleate surface complex with characteristic carboxylate absorbance bands at about 1500 cm(-1). It is sho wn, for the first time, that the experimental spectra of the oleate adsorbe d species can be recorded successfully, at different incidence angles and t wo polarizations, in the spectral region of very strong substrate absorptio n. Moreover, the experimental spectra can be reproduced by spectral simulat ions, giving detailed information about the surface composition and structu re of the produced adsorbed layers even for those with lateral heterogeneou s structure. The adsorbed oleate forms surface calcium carboxylate species with two preferential conformations: unidentate-like (a band at 1537 cm(-1) ) and bidentate-like (a band at 1575 cm(-1)). At multilayer coverage, the a dsorbed layer forms a patchlike structure produced by a pillar growth mecha nism. This spectroscopic structural finding was supported by atomic force m icroscope measurements. The various experimental and theoretical aspects of the infrared external reflection technique within a very strong absorbing region of substrate are also discussed. For comparison, a discussion of the aliphatic stretching vibration region in the recorded spectra, at about 30 00 cm(-1), in which calcite is transparent is also performed.