NATURE AND STRUCTURE OF ADSORPTION LAYER ON APATITE CONTACTED WITH OLEATE SOLUTIONS .2. IN-SITU AND EX-SITU FOURIER-TRANSFORM INFRARED, NMR, AND X-RAY PHOTOELECTRON-SPECTROSCOPY STUDIES

The chemical composition, molecular structure, and aggregation state of oleate molecules in the adsorption layers on apatite were examined using Fourier transform infrared (FTIR) in situ and ex situ techniques, solid state C-13 CP/MAS NMR, and X-ray photoelectron spectroscopy (XPS) methods. Two different samples of oleate with different amounts of isomers and homologues (''impurities'') were used. The infrared reflection, solid-state C-13 NMR, XPS, and X-ray diffraction characterization of sodium, calcium, and cadmium oleate precipitates were carried out in order to provide references for discussion of the spectra of the adsorbed oleate layers. For adsorption submonolayers fabricated with oleate I (lower ''impurity'' level), it was found that the chemisorbed oleate molecules prefer to assemble into well-organized close-packed patches with hydrocarbon chain directed toward the solution. Size of the patches increases with coverage. Oleate II (higher ''impurity'' level), on the other hand, maintains a partially organized state without a clear formation of patchwise structure. Highest ordered and packed structure of oleate were found at about 0.7 and 1.0 of statistical monolayer of oleate I and oleate II, respectively. The presence of water molecules was found in the well-organized structures of the adsorbed layers. This water stabilizes the organized structures of chemisorbed oleate which are responsible for hydrophobicity of apatite. Higher coverages are formed by a three-dimensional condensation (surface precipitation) mechanism, where precipitation of calcium oleate takes place directly on surface of apatite or with the use of chemisorbed covered sites as nuclei. It was found that the structure of surface precipitated calcium oleate, for both oleate samples, shows uneven surface distribution and, in majority, amorphous structure. Furthermore, it was found that this adsorption layer contains other ions such as sodium and phosphate. Surface structure of the adsorption layer at different coverages for both types of the oleate samples used has been proposed.