NATURE AND STRUCTURE OF ADSORPTION LAYER ON APATITE CONTACTED WITH OLEATE SOLUTION .1. ADSORPTION AND FOURIER-TRANSFORM INFRARED REFLECTIONSTUDIES

Interfacial structures formed by adsorption of oleate on hydroxyapatit e were characterized by adsorption isotherms, electrokinetic and hydro phobicity measurements, and FTIR reflection techniques. Two different samples of oleate with different amounts of isomers and homologues ('' impurities'') were used. The infrared reflection (ATR, DRIFT) studies showed that the observation of a doublet or singlet positioned at abou t 1560 cm-1, due to the asymmetric stretching vibration of the carboxy l group, depends on organization of the adsorbed molecules rather than , as was suggested previously, on the mechanism of the adsorption of o leate. Close relationships between the adsorption, spectroscopic, elec trokinetic, and hydrophobicity data were found. The chemisorbed oleate molecules form a well-organized close-packed structure (strong latera l interaction) probably of two distinguished structural forms at submo nolayer (about 0.7 of statistical monolayer) coverages. These structur al forms (showed by doublet with maxima at 1572 and 1540 cm-1) are res ponsible for the hydrophobic properties of the sample. The presence of ''impurities'' causes a perturbation of the chemisorbed layer structu re (broad band at about 1550 cm-1) and, in this case (perturbated late ral interaction), the formation of a more organized structure was obse rved at higher, close to monolayer, coverage at which maximum hydropho bicity was alw found. Higher than monolayer coverage is formed by the three-dimensional condensation of calcium oleate on apatite (surface p recipitation mechanism) which forms a poorly organized structure that does not produce high hydrophobicity. Close relationships between the structures of the adsorption layers and the changes in infrared spectr a are remarkable.