Adsorption of carboxylate on calcium carbonate (10 1¯ 4) surface: Molecular simulation approach

Molecular Dynamics Simulation of Polyacrylamide Adsorption on Calcite

In poorly consolidated carbonate rock reservoirs, solids production risk, which can lead to increased environmental waste, can be mitigated by injecting formation-strengthening chemicals. Classical atomistic molecular dynamics (MD) simulation is employed to model the interaction of polyacrylamide-based polymer additives with a calcite structure, which is the main component of carbonate formations. Amongst the possible calcite crystal planes employed as surrogates of reservoir rocks, the (1 0 4) plane is shown to be the most suitable surrogate for assessing the interactions with chemicals due to its stability and more realistic representation of carbonate structure. The molecular conformation and binding energies of pure polyacrylamide (PAM), hydrolysed polyacrylamide in neutral form (HPAM), hydrolysed polyacrylamide with 33% charge density (HPAM 33%) and sulfonated polyacrylamide with 33% charge density (SPAM 33%) are assessed to determine the adsorption characteristics onto calcite surfaces. An adsorption-free energy analysis, using an enhanced umbrella sampling method, is applied to evaluate the chemical adsorption performance. The interaction energy analysis shows that the polyacrylamide-based polymers display favourable interactions with the calcite structure. This is attributed to the electrostatic attraction between the amide and carboxyl functional groups with the calcite. Simulations confirm that HPAM33% has a lower free energy than other polymers, presumably due to the presence of the acrylate monomer in ionised form. The superior chemical adsorption performance of HPAM33% agrees with Atomic Force Microscopy experiments reported herein.

Adsorption behavior of multicomponent volatile organic compounds on a citric acid residue waste-based activated carbon: Experiment and molecular simulation

A considerable amount of volatile organic compounds (VOCs) is emitted, and a vast amount of citric acid residue (CAR) waste is simultaneously produced during citric acid production. Thus, a suitable method realizing the clean production of citric acid must be developed. This study investigated the adsorption of the multicomponent VOCs in a homemade CAR waste-based activated carbon (CAR-AC). A fixed-bed experimental setup was used to explore the adsorption and desorption of single- and multi-component VOCs. Surface adsorption and diffusion molecular models with different defects were built to study the underlying adsorption and diffusion mechanisms of multicomponent VOCs on CAR-AC. The adsorption amount of ethyl acetate in CAR-AC from multicomponent VOCs was 3.04 and 5.91 times higher than those of acetone and acetaldehyde, respectively, and the interaction energy between ethyl acetate and C surfaces was low at -13.41 kcal/mol. During desorption, the most weakly adsorbed acetaldehyde desorbed from the surface of CAR-AC first, followed by acetone and ethyl acetate. The regeneration efficiencies of acetaldehyde, acetone, and ethyl acetate reached 88.77, 85.55, and 91.46 %, respectively, after four adsorption/desorption cycles. We aimed to provide a new strategy to realize the recycle use of CAR and the clean production of citric acid.

Comparison of CH4 and CO2 Adsorptions onto Calcite(10.4), Aragonite(011)Ca, and Vaterite(010)CO3 Surfaces: An MD and DFT Investigation

The interaction between greenhouse gases (such as CH₄ and CO₂) and carbonate rocks has a significant impact on carbon transfer among different geochemical reservoirs. Moreover, CH₄ and CO₂ gases usually associate with oil and natural gas reserves, and their adsorption onto sedimentary rocks may influence the exploitation of fossil fuels. By employing the molecular dynamics (MD) and density functional theory (DFT) methods, the adsorptions of CH₄ and CO₂ onto three different CaCO₃ polymorphs (i.e., calcite(10.4), aragonite(011)Ca, and vaterite(010)CO₃) are compared in the present work. The calculated adsorption energies (E _ad) are always negative for the three substrates, which indicates that their adsorptions are exothermic processes and spontaneous in thermodynamics. The E _ad of CO₂ is much more negative, which suggests that the CO₂ adsorption will form stronger interfacial binding compared with the CH₄ adsorption. The adsorption precedence of CH₄ on the three surfaces is aragonite(011)Ca > vaterite(010)CO₃ > calcite(10.4), while for CO₂, the sequence is vaterite(010)CO₃ > aragonite(011)Ca > calcite(10.4). Combining with the interfacial atomic configuration analysis, the Mulliken atomic charge distribution and overlap bond population are discussed. The results demonstrate that the adsorption of CH₄ is physisorption and that its interfacial interaction mainly comes from the electrostatic effects between H in CH₄ and O in CO₃ ^2-, while the CO₂ adsorption is chemisorption and the interfacial binding effect is mainly contributed by the bonds between O in CO₂ and Ca²⁺ and the electrostatic interaction between C in CO₂ and O in CO₃ ^2-.

Mechanistic Study of Wettability Changes on Calcite by Molecules Containing a Polar Hydroxyl Functional Group and Nonpolar Benzene Rings

Oil extraction efficiency strongly depends on the wettability status (oil- vs water-wet) of reservoir rocks during oil recovery. Aromatic compounds with polar functional groups in crude oil have a significant influence on binding hydrophobic molecules to mineral surfaces. Most of these compounds are in the asphaltene fraction of crude oil. This study focuses on the hydroxyl functional group, an identified functional group in asphaltenes, to understand how the interactions between hydroxyl groups in asphaltenes and mineral surfaces begin. Phenol and 1-naphthol are used as asphaltene surrogates to model the simplest version of asphaltenes. Adsorption of oil molecules on the calcite {101̅4} surface is described using static quantum-mechanical density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations. DFT calculations indicate that adsorption of phenol and 1-naphthol occurs preferentially between their hydroxyl group and calcite step edges. 1-Naphthol adsorbs more strongly than phenol, with different adsorption geometries due to the larger hydrophobic part of 1-naphthol. MD simulations show that phenol can behave as an agent to separate oil from the water phase and to bind the oil phase to the calcite surface in the water/oil mixture. In the presence of phenol, partial separation of water/oil with an incomplete lining of phenol at the water/oil boundary is observed after 0.2 ns. After 1 ns, perfect separation of water/oil with a complete lining of phenol at the water/oil boundary is observed, and the calcite surface becomes oil-wet. Phenol molecules enclose decane molecules at the water-decane boundary preventing water from repelling decane molecules from the calcite surface and facilitate further accumulation of hydrocarbons near the surface, rendering the surface oil-wet. This study indicates phenol and 1-naphthol to be good proxies for polar components in oil, and they can be used in designing further experiments to test pH, salinity, and temperature dependence to improve oil recovery.