The Impact of Salinity on the Interfacial Structuring of an Aromatic Acid at the Calcite/Brine Interface: An Atomistic View on Low Salinity Effect

This study aims to elucidate the impact of salinity on the interactions governing the adsorption of polar aromatic oil compounds onto calcite. To this end, molecular dynamics simulations were employed to assess adsorption of a model polar organic molecule (deprotonated benzoic acid, benzoate) on the calcite surface in NaCl brines of different concentration levels, namely, deionized water (DW), low-salinity water (LS, 5000 ppm), and sea water (SW; 45,000 ppm). Calcite was found to be completely covered by several well-ordered water layers. The top hydration layer is very compact and prevents direct adsorption of benzoates onto the substrate. Instead, Na⁺ ions form a distinct positively charged layer by adhering on the calcite substrate through inner-sphere complexion mode. Cl^- ions mostly lodge on top of the adsorbed sodium cations, forming a negatively charged layer. The distribution of ions at the calcite/brine interface thus exhibits the features of an electrical double layer, composed of a Stern-like positive layer followed by a negative one. The positive charged layer attracts benzoates toward the surface. As such, the sodium ions attached onto the calcite can act as adsorption sites to connect benzoates to the surface. By increasing the salinity, more Na⁺ ions adsorb onto the calcite surface, and the density of benzoate molecules at the interface is enhanced as a result of more Na⁺ bridging ions. The monotonic salinity-dependent adsorption of benzoate molecules on calcite offers a mechanism driving additional oil recovery upon injection of diluted brine into subsurface carbonate reservoirs.

Correlations of crystal shape and lateral orientation in bioinspired CaCO3mineralization

Our study shows that in bioinspired mineralization, the crystal shape and lateral orientation are highly correlated instead of being independent.

Molecular Dynamics Simulation to Understand the Ability of Anionic Polymers to Alter the Morphology of Calcite

Molecular dynamics was utilized to investigate the ability of anionic macromolecules to drastically change the morphology of calcite in the presence of magnesium ions. Anionic poly(acrylic acid) and poly(methacrylic acid) were compared with cationic poly(ethylene imine) in their binding behavior on calcite (104) and (110) surfaces. Poly(acrylic acid) and poly(methacrylic acid) showed preferential binding on (110) with strong electrostatic attractions, whereas poly(ethylene imine) was only weakly attracted to (104). The extent of the charge imbalance on the surfaces appeared responsible for the current results, which originated from the deficient number of the coordinating oxygen atoms of carbonate around the surface calcium. The results of the current study were in accordance with the previous experimental observations, where the {hk0} surfaces of calcite were elongated under the coexistence of the anionic polymers and magnesium ions. These results could be generally utilized in the polymer-controlled crystallization with broad implications in the specific interactions with crystal surfaces.

Adsorption Properties of Triethylene Glycol on a Hydrated {101̅4} Calcite Surface and Its Effect on Adsorbed Water

Molecular dynamics (MD) and Born-Oppenheimer MD (BOMD) simulations were employed to investigate adsorption of aqueous triethylene glycol (TEG) on a hydrated {101̅4} calcite surface at 298 K. We analyzed the orientation of TEG adsorbed on calcite, as well as the impact of TEG on the water density and adsorption free energy. The adsorption energies of TEG, free energy profiles for TEG, details of hydrogen bonding between water and adsorbed TEG, and dihedral angle distribution of adsorbed TEG were estimated. We found that while the first layer of water was mostly unaffected by the presence of adsorbed TEG, the density of the second water layer was decreased by 71% at 75% surface coverage of TEG. TEG primarily attached to the calcite surface via two adjacent adsorption sites. Hydrogen bonds between water and adsorbed TEG in the second layer almost exclusively involved the hydroxyl oxygen of TEG. The adsorption energy of TEG on calcite in a vacuum environment calculated by classical MD amounted to 217 kJ/mol, which agreed very well with estimates found by using BOMD. Adsorption on hydrated calcite yielded a drastically lower value of 33 kJ/mol, with the corresponding adsorption free energy of 55.3 kJ/mol, giving an entropy increase of 22.3 kJ/mol due to adsorption. We found that the presence of TEG resulted in a decreased magnitude of the adsorption free energy of water, thus decreasing the calcite wettability. This effect can have a profound effect on oil and gas reservoir properties and must be carefully considered when evaluating the risk of hydrate nucleation.