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

Abstract

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 gamma-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/nm(2). Monolayer coverage on gamma-alumina was determined from spectra as 0.7 molecules/nm(2) for salicylic acid, confirmed by solution analysis, and 2 molecules/nm(2) for myristic acid. Spectral features of salicylic acid deposited on gamma-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.

A study of competitive adsorption of organic molecules onto mineral oxides using DRIFTS

“…Studies with salicylic acid adsorption onto γ-alumina and kaolinite showed that consistent handling of the samples (so as to maintain the same particle size distribution) allowed selection of a peak and analysis of peak height (or area under the peak) to determine the surface loading of the organic up to the maximum possible with the washing step used. The quantitative spectral analysis was verified by analysis of the spent solution to determine the quantity of remaining salicylic acid [1,2]. The results of the quantitative studies using hexane as the solvent, as reported in our previous papers, are used as a calibration in this present study of competitive adsorption; the adsorption of salicylic acid onto γ-alumina and kaolinite [2] and myristic acid onto γ-alumina [1].…”

“…Both of the selected organic acids were found in previous associated studies [1,2] to adsorb from solution in hexane as carboxylate onto γ-alumina and kaolinite. The results suggest that the surface associated water, present on mineral oxides under ambient conditions, mediates the organic uptake.…”

“…The results suggest that the surface associated water, present on mineral oxides under ambient conditions, mediates the organic uptake. The nature of this water layer was discussed earlier [1,2]. The spectra obtained from sample sets prepared to determine the minimum detection level and maximum surface loading were used as a calibration tool in this present study of competitive adsorption together with the calibrated adsorption of myristic acid onto γ-alumina and kaolinite presented in this paper.…”

“…The technique used here and in our previous studies [1,2] added samples to the cup with no dilution with KBr and the spectra were recorded at ambient temperature under a flow of purified dry air. Spectra of γ-alumina and kaolinite samples treated with organic were collected together with that of a prepared blank for each.…”

“…The quantitative spectral analysis was verified by analysis of the spent solution to determine the quantity of remaining salicylic acid [1,2]. The results of the quantitative studies using hexane as the solvent, as reported in our previous papers, are used as a calibration in this present study of competitive adsorption; the adsorption of salicylic acid onto γ-alumina and kaolinite [2] and myristic acid onto γ-alumina [1]. The results of the calibration of the adsorption behavior of myristic acid onto kaolinite are presented in this paper.…”