Interaction of mineral surfaces with simple organic molecules by diffuse reflectance IR spectroscopy (DRIFT)

The adsorption of simple organic molecules to mineral surfaces was investigated by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and by solution chemical analysis. Salicylic acid, myristic acid or octacosane were deposited from water or hexane onto γ-alumina and kaolinite, and examined without evacuation or blending with KBr. The DRIFTS results replicated those from ATR-IR, where available, supporting the validity of DRIFTS as a versatile and convenient alternative to ATR. Results were obtained for coverage as low as 0.02 molecules/nm2. Monolayer coverage on γ-alumina was determined from spectra as 0.7 molecules/nm2 for salicylic acid, confirmed by solution analysis, and 2 molecules/nm2 for myristic acid. Spectral features of salicylic acid deposited on γ-alumina from hexane were not distinguishable from those deposited from water. In almost all cases, spectral features associated with the carboxyl moiety were substantially altered, indicating a significant role in the adsorption mechanism. Adsorption of salicylic acid from hexane onto kaolinite was also as carboxylate, but myristic acid showed both carboxylate and carbonyl. The results from using hexane as solvent compared to water suggest that surface-adsorbed hydroxyl and molecular water, present on mineral oxides under ambient conditions, may be a key determinant of the adsorbate architecture.

Graphical abstractDRIFT IR shows that salicylic acid loading on alumina saturates (above) at a fixed value, corresponding to about 0.7 molecules/nm2 (lower), in agreement with solution depletion measurements: (×) phenolic OH 1263 cm−1, (♦) COO 1281 cm−1.Figure optionsDownload full-size imageDownload as PowerPoint slide