Energies of the Adsorption of Functional Groups to Calcium Carbonate Polymorphs: The Importance of −OH and −COOH Groups

Abstract

The adsorption behavior of calcium carbonate is an important factor in many processes in nature, industry, and biological systems. We determined and compared the adsorption energies for a series of small molecules of different sizes and polarities (i.e., water, several alcohols, and acetic acid) on three synthetic CaCO3 polymorphs (calcite, aragonite, and vaterite). We measured isosteric heats of adsorption from vapor adsorption isotherms for 273 < T < 293 K, and we used XRD and SEM to confirm that samples did not change phase during the experiments. Density functional calculations and molecular dynamics simulations complemented the experimental results and aided interpretation. Alcohols with molecular mass greater than that of methanol bind more strongly to the calcium carbonate polymorphs than water and acetic acid. The adsorption energies for the alcohols are typical of chemisorption and indicate alcohol displacement of water from calcium carbonate surfaces. This explains why organisms favor biomolecules that contain alcohol functional groups (-OH) to control which polymorph they use, the crystal face and orientation, and the particle shape and size in biomineralization processes. This new insight is also very useful in understanding organic molecule adsorption mechanisms in soils, sediments, and rocks, which is important for predicting the behavior of mineral-fluid interactions when the challenge is to remediate contaminated groundwater aquifers or to produce oil and gas from reservoirs.

Probing the energetics of organic–nanoparticle interactions of ethanol on calcite

“…Recently, the binding energies between alcohols and calcium carbonate phases were studied using both DFT and isosteric heat calculation (Q st ) from adsorption isotherm data between 0 and 20°C. It was found that alcohols heavier than methanol (including ethanol) tend to bind to the calcite surface more strongly than water and acetic acid (31). Very recently, the interactions of a series of alcohols with the calcite surface were studied using X-ray photoelectron spectroscopy coupled with MD simulation, in which all alcohols were found to bond with calcite surface through the hydroxyl group forming closely packed, ordered monolayer (33).…”

“…Further adsorption results in less exothermic heat effects. The differential enthalpy curve reaches its first plateau at −98.3 ± 4.8 kJ/mol ethanol, which spans 0.3-3.5 ethanol molecules per nm 2 (the latter being approximately a monolayer of ethanol), before ending with a sharp rise to the next plateau (−3.2 ± 1.6 kJ/mol ethanol) above 3.5 molecules per nm 2 (31). Surprisingly, the position of the second plateau is much less exothermic than that of the condensation enthalpy of ethanol [−42.3 kJ/mol, at 25°C (32)].…”

Calcium phosphate coatings for marble conservation: Influence of ethanol and isopropanol addition to the precipitation medium on the coating microstructure and performance

“…Therefore, we investigated the effect of using IPA instead of EtOH. IPA was selected because it has lower adsorption affinity to calcite (i.e., it binds less strongly) than EtOH [44]. In principle, methanol would have an even lower adsorption affinity [44], but it was discarded because of its toxicity.…”

“…IPA was selected because it has lower adsorption affinity to calcite (i.e., it binds less strongly) than EtOH [44]. In principle, methanol would have an even lower adsorption affinity [44], but it was discarded because of its toxicity. Moreover, the adsorption onto calcite by IPA molecules is expected to be lower than that by ethanol, because of steric effects.…”

Polymorphs of pure calcium carbonate prepared by the mineral carbonation of flue gas desulfurization gypsum

“…Metastable vaterite transforms into stable calcite upon exposure to water [30]. However, both ammonia and ethanol stabilize vaterite in aqueous systems.…”