Oleate Adsorption at an Apatite Surface Studied by Ex-Situ FTIR Internal Reflection Spectroscopy

Abstract

Oleate adsorption at an apatite surface was investigated by ex-situ Fourier transform infrared internal reflection spectroscopy (FTIR/IRS). Adsorption isotherms have been determined using an apatite internal reflection element (IRE) and it has been found that pH has a significant influence on oleate adsorption by apatite. At pH 8.0 and 20°C, oleate adsorption density increases monotonically as equilibrium oleate concentration increases from 5 × 10−6to 1 × 10−3M. These results are in contrast to the results at pH 9.5 and 20°C in which case the adsorption density is limited to that corresponding to approximately monolayer coverage. Oleate adsorption by apatite was compared to oleate adsorption by fluorite and calcite and the different adsorption behavior at these three mineral surfaces is attributed to the differences in the densities of surface calcium sites and to the differences in the solubilities of these minerals.The spectral results at pH 8.0 and 20°C, suggest that the dominant adsorbed species at the apatite surface is oleic acid as indicated by the appearance of a strong IR absorbance peak at about 1710 cm−1for the whole oleate concentration range investigated from 5 × 10−6to 1 × 10−3M. On the other hand at pH 9.5, 20°C, and low surface coverage, it appears that oleate is chemisorbed as indicated by a peak at 1554 cm−1and an increased adsoption density at higher temperatures. Above what corresponds to monolayer coverage, surface precipitation of calcium dioleate salt is suggested by the doublet at 1575 and 1540 cm−1. At pH 9.5 and elevated temperature, 65°C, both chemisorption of oleate and surface precipitation of calcium dioleate at the apatite surface are enhanced. These FTIR/IRS spectra are compared with those obtained by the diffuse reflection technique, and the difference between the spectra are discussed.Finally, contact angles have been measured at the apatite IRE surface and it has been demonstrated that both the amount and the nature of the adsorbed species influence the hydrophobic state of the surface.