Effect of temperature on oleate adsorption at a calcite surface: an FT-NIR/IRS study and review

Abstract

In-situ Fourier transform infrared/internal reflection spectroscopy (FT-IR/IRS) with reactive internal reflection elements (IREs) has been extended to study oleate adsorption at a calcite surface. Birefringent and transmission limitations of calcite were overcome with appropriate optics and a near-infrared (NIR) spectrometer. Adsorption isotherms were determined at pH 9.2 for temperatures of 20°C and 60°C and found to agree with data reported in the literature. The in-situ FT-NIR/IRS results indicate that chemisorption occurs at low oleate concentrations since the extent of adsorption increases at the higher temperature. At higher oleate concentrations, surface precipitation of calcium dioleate predominates but, in this case, the adsorption densities decrease with increasing temperature. This effect was related to surface-passivation due to increased chemisorption and to the increased solubility of calcium dioleate at 60°C. A thorough review of the literature substantiates this position. Finally, ex-situ FT-IR/IRS spectra of the chemisorbed and surface precipitated states were obtained in the mid-infrared (MIR) region and were found to support the in-situ FT-NIR/IRS results. Previous attempts with other infrared sampling methods were not successful in obtaining such MIR spectra due to the highly absorbing nature of carbonate vibrations.