Functional Group Adsorption on Calcite: II. Nitrogen and Sulfur Containing Organic Molecules

Sulfur and nitrogen are two common constituents of natural and synthetic organic molecules, especially in systems where organisms play a role. There is evidence in the literature that nitrogen and sulfur containing functional groups have an influence on adsorption of organic molecules to calcite surfaces. The purpose of this work was to investigate the interaction of these functional groups with CaCO₃ and to explore how adsorption is affected by various side groups and the H atom. First, we used density functional theory with semiempirical dispersion corrections (DFT-D2) to determine the energy of adsorption on the dominant calcite face, {10.4} for molecules containing nitrogen (ammonia, methylamine, ethylamine, aniline, hydrogen cyanide, acetonitrile, propionitrile, benzonitrile, dimethylamine, pyrrole, trimethylamine, and pyridine) and sulfur (hydrogen sulfide, methanethiol, ethanethiol, thiophenol, dimethyl sulfide, and thiophene). Second, based on the determined adsorption energies, we predicted desorption temperature for each molecule within the transition state theory approximation. Finally, we used X-ray photoelectron spectroscopy (XPS) to determine the desorption temperature for four molecules for comparison with the predicted values. Our results show that ammonia and primary amines (R-NH₂) adsorb more strongly than nitriles (R-CN) and hydrogen sulfide and thiols (R-SH). On average, the adsorption energy of nitriles is slightly higher than hydrogen sulfide and thiols. Attachment of side groups or a H atom changes the strength of the surface-molecule interactions and significantly affects the adsorption behavior of all three functional groups.