ADSORPTION OF OLEATE ON APATITE STUDIED BY DIFFUSE REFLECTANCE INFRARED FOURIER-TRANSFORM SPECTROSCOPY

The adsorption of oleate on apatite was studied at pH values from 6 to 9.8 and oleate concentrations from 2 x 10(-5) to 3 x 10(-4) mol L-1. Diffuse reflectance infrared Fourier transform spectroscopy has been found to be superior to the transmission infrared technique for detecting adsorbed oleate species. Confusion in previous studies is clarified and a better understanding of the adsorption mechanism obtained. Chemisorbed oleate corresponds to a single peak at 1550 cm-1 and probably comprises one oleate ion bonding with one lattice calcium ion on the surface. Surface calcium oleate precipitate showing peaks at 1574 and 1538 cm-1 has a structure similar to that of bulk calcium oleate and probably adsorbs through ion-dipole interaction and hydrocarbon chain association. Oleic acid dimer and monomer adsorb via hydrocarbon chain association onto underlying chemisorbed oleate and correspond to a sharp peak at 1713 cm-1 and a shoulder at 1732 cm-1, respectively. Chemisorption of oleate on apatite occurred under all conditions studied and was accompanied by physical adsorption of calcium oleate precipitate and/or oleic acid monomer and/or oleic acid dimer, depending on the pH and concentration of the solution.