Ab Initio Studies of Calcium Carbonate Hydration

Ion association behaviors in the initial stage of calcium carbonate formation: An ab initio study

In this work, we performed static density functional theory calculations and ab initio metadynamics simulations to systematically investigate the association mechanisms and dynamic structures of four kinds of ion pairs that could be formed before the nucleation of CaCO3. For Ca2+-HCO3- and Ca2+-CO32- pairs, the arrangement of ligands around Ca2+ evolves between the six-coordinated octahedral structure and the seven-coordinated pentagonal bipyramidal structure. The formation of ion pairs follows an associative ligand substitution mechanism. Compared with HCO3-, CO32- exhibits a stronger affinity to Ca2+, leading to the formation of a more stable precursor phase in the prenucleation stage, which promotes the subsequent CaCO3 nucleation. In alkaline environments, excessive OH- ions decrease the coordination preference of Ca2+. In this case, the formation of Ca(OH)+-CO32- and Ca(OH)2-CO32- pairs favors the dissociative ligand substitution mechanism. The inhibiting effects of OH- ion on the CaCO3 association can be interpreted from two aspects, i.e., (1) OH- neutralizes positive charges on Ca2+, decreases the electrostatic interactions between Ca2+ and CO32-, and thus hinders the formation of the CaCO3 monomer, and (2) OH- decreases the capacity of Ca2+ for accommodating O, making it easier to separate Ca2+ and CO32- ions. Our findings on the ion association behaviors in the initial stage of CaCO3 formation not only help scientists evaluate the impact of ocean acidification on biomineralization but also provide theoretical support for the discovery and development of more effective approaches to manage undesirable scaling issues.

Oxygen Isotope Fractionation between Carbonate Minerals and Carbonic Acid Systems and Constraints for Environmental Science and Geological Processes

The equilibrium oxygen isotope fractionation factor is widely used in geological thermometry. However, under most natural conditions, the oxygen isotope exchange is rare to reach equilibrium. Especially for the complex water-rock interaction process, the contribution of the H2CO3 solution, CO32- solution, Ca(HCO3)2 solution, and CaCO3 solution to the equilibrium oxygen isotope fractionation factor of this process is poorly understood. In view of this predicament, these key parameters are obtained by ab initio calculations. The results showed that the contributions of different carbonate minerals and different aqueous solutions to the equilibrium oxygen isotope fractionation factor were different. Among all nine carbonate minerals (dolomite, calcite, aragonite, magnesite, siderite, otavite, smithsonite, ankerite, and strontianite), the minerals with the highest and lowest reduced partition function ratios (RPFR) were siderite and strontianite, respectively. At the same time, the RPFR of nitratine, which has the same structure as carbonate, was studied. The RPFRs of the three most widely distributed carbonates in nature (dolomite, calcite, and aragonite) were dolomite > calcite > aragonite. Among the H2CO3 solution, CO32- solution, Ca(HCO3)2 solution, and CaCO3 solution, the H2CO3 solution had the strongest ability to enrich 18O. In addition, the equilibrium oxygen isotope fractionation factors between aqueous solutions and gas phase species (CO2(g), H2O(g), and O2(g), etc.) were calculated systematically. The results showed that the oxygen isotope fractionation factors between solutions and gas phases were often inconsistent with the temperature change direction and that the kinetic effects played a key role. These theoretical parameters obtained in this study will provide key equilibrium oxygen isotope constraints for water-rock interaction processes.

Self-calcifying lipid nanocarrier for bone tissue engineering

BACKGROUND

A nanoemulsion with specific surface properties (such as charge and functional groups) can initiate the deposition of calcium phosphate (CaP) on its surface, leading to formation of CaP nanoparticles with a lipid core. The lipid core can carry lipophilic compounds based on the function of the nanoemulsion. Therefore, a dual purpose nanoemulsion of lipid nanoparticles (LNPs) exhibiting self-calcifying and carrier abilities can be developed.

METHODS

We employed an emulsification process to formulate LNPs with a specific charged surface. The LNPs were tested for their ability to calcify in simulated body fluid and encapsulate cholecalciferol (a model of active compound). The self-calcifying LNP was successfully fabricated using the emulsification process and stabilized using a mixture of polysorbate 80 and polysorbate 20.

RESULTS

The LNPs incubated in simulated body fluid bound to calcium and phosphate, subsequently forming CaP on the particle surface and resulting in approximately 180-nm CaP spheres with a lipid core. The LNPs facilitated calcium phosphate deposition in the collagen scaffolds. In addition, LNPs can be used as carriers of lipophilic compounds without impeding the self-calcifying ability.

Ab initio quantum chemical studies of isotopic fractionation during acid digestion reaction of dolomite for clumped isotope application

RATIONALE

In 'clumped isotope paleothermometry' carbonates are reacted with anhydrous phosphoric acid to extract CO₂ that carries the isotopic signature of the reacting carbonates, and the amount of clumping in the product CO₂ is measured. Previous theoretical models for determining clumped isotopic fractionation in product CO₂ during acid digestion of carbonates are independent of the cations present in the carbonate lattice. Hence further study is required to understand the cationic effect.

METHODS

We studied the acid reaction mechanism based on the protonation of carbonates, calculated the acid fractionation factor for dolomite using the partition functions and vibrational frequencies obtained for the transition state structure, and determined the effect of cations on the acid fractionation factor. Experimentally, carbonates are reacted using the modified sealed vessel method and analyzed in the dual inlet of a ThermoFinnigan MAT 253 isotope ratio mass spectrometer.

RESULTS

The oretically obtained acid fractionation factor can be expressed as Δ_{47 acid fractionation in dolomite}  = -0.28563 + 0.49508 * (10⁵ /T² ) - 0.08231 * (10⁵ /T² )² for a temperature range between 278.15 and 383.15 K. The theoretical slope of the dolomite-acid digestion curve is lower than that of the calcite-acid digestion curve obtained using the identical reaction mechanism. Our theoretical slope is consistent with the result from the common acid bath experiments but higher than the slope obtained in our experimental study using the modified sealed vessel method and in a previous theoretical study using the H₂ CO₃ model.

CONCLUSIONS

The transition state structure, obtained in our study, includes the cations present in the carbonate minerals and provides distinct acid fractionation factors for calcite and dolomite. The observed gentler slope of the theoretically calculated dolomite-acid digestion curve than of the calcite curve is expected considering the stronger Mg-O bond. Our experimental approach invokes post-digestion isotopic exchange and agrees with the previous theoretical estimates where post-digestion isotopic fractionation was considered.

Isotopic fractionation during acid digestion of calcite: A combined ab initio quantum chemical simulation and experimental study

RATIONALE

Carbonate clumped isotope analysis involves the reaction of carbonate minerals with phosphoric acid to release CO₂ for measurement in a gas-source isotope ratio mass spectrometer. Although the clumped isotope proxy is based on the temperature dependence of ¹³ C-¹⁸ O bonding preference in the mineral lattice, which is captured in the product CO₂ , there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO₃ ^2- to mass 47 in CO₂ ) during the acid digestion.

METHODS

We studied the reaction mechanism for the phosphoric acid digestion of calcite using first-principles density functional theory. We identified the transition state structures for each reaction involving different isotopologues and used the corresponding vibrational frequencies in reduced partition function theory to estimate the Δ₄₇ acid fractionation. Experimental Δ₄₇ data were acquired by processing the sample CO₂ gas through the dual-inlet peripheral of a ThermoFinnigan MAT253 isotope ratio mass spectrometer.

RESULTS

We showed that the acid digestion reaction, which results in the formation of CO₂ enriched with ¹³ C-¹⁸ O bonds, began with the protonation of calcium carbonate in the presence of water. Our simulations yielded a relationship between the Δ₄₇ acid fractionation and reaction temperature as Δ₄₇ = -0.30175 + 0.57700 × (10⁵ /T² ) - 0.10791 × (10⁵ /T² )² , with T varying between 298.15 and 383.15 K.

CONCLUSIONS

We propose a reaction mechanism that shows a higher slope (Δ₄₇ acid fractionation vs. 1/T² curve) for the phosphoric acid digestion of calcite than in previous studies. The theoretical estimates from the present and earlier studies encapsulate experimental observations from both "sealed vessel" and "common acid bath" acid digestion methods.

Ab initio calculations and reduced density gradient analyses of the structure and energetics of hydrated calcium fluoride and calcium carbonate

We studied microhydrated calcium fluoride, calcium carbonate and their ions at the MP2/6-311++G** level of theory and found that water–water non-covalent interactions destabilize the solvation shell, and are compensated by cooperative hydrogen bonds.

Effective Antiscaling Performance of Reverse-Osmosis Membranes Made of Carbon Nanotubes and Polyamide Nanocomposites

The antiscaling properties of multiwalled carbon nanotube (MWCNT)-polyamide (PA) nanocomposite reverse-osmosis (RO) desalination membranes (MWCNT-PA membranes) were studied. An aqueous solution of calcium chloride (CaCl₂) and sodium bicarbonate (NaHCO₃) was used to precipitate in situ calcium carbonate (CaCO₃) to emulate scaling. The MWCNT contents of the studied nanocomposite membranes prepared by interfacial polymerization ranged from 0 wt % (plain PA) to 25 wt %. The inorganic antiscaling performances were compared for the MWCNT-PA membranes to laboratory-made plain and commercial PA-based RO membranes. The scaling process on the membrane surface was monitored by fluorescence microscopy after labeling the scale with a fluorescent dye. The deposited scale on the MWCNT-PA membrane was less abundant and more easily detached by the shear stress under cross-flow compared to other membranes. Molecular dynamics simulations revealed that the attraction of Ca²⁺ ions was hindered by the interfacial water layer formed on the surface of the MWCNT-PA membrane. Together, our findings revealed that the observed outstanding antiscaling performance of MWCNT-PA membranes results from (i) a smooth surface morphology, (ii) a low surface charge, and (iii) the formation of an interfacial water layer. The MWCNT-PA membranes described herein are advantageous for water treatment.

Viscous interfacial layer formation causes electroosmotic mobility reversal in monovalent electrolytes

Using molecular dynamics simulations, the ion density, shear viscosity and electroosmotic mobility of an aqueous monovalent electrolyte at a charged solid surface are studied as a function of the surface charge density.

A First-Principle Theoretical Study of Mechanical and Electronic Properties in Graphene Single-Walled Carbon Nanotube Junctions

The new three-dimensional structure that the graphene connected with SWCNTs (G-CNTs, Graphene Single-Walled Carbon Nanotubes) can solve graphene and CNTs′ problems. A comprehensive study of the mechanical and electrical performance of the junctions was performed by first-principles theory. There were eight types of junctions that were constituted by armchair and zigzag graphene and (3,3), (4,0), (4,4), and (6,0) CNTs. First, the junction strength was investigated. Generally, the binding energy of armchair G-CNTs was stronger than that of zigzag G-CNTs, and it was the biggest in the armchair G-CNTs (6,0). Likewise, the electrical performance of armchair G-CNTs was better than that of zigzag G-CNTs. Charge density distribution of G-CNTs (6,0) was the most homogeneous. Next, the impact factors of the electronic properties of armchair G-CNTs were investigated. We suggest that the band gap is increased with the length of CNTs, and its value should be dependent on the combined effect of both the graphene’s width and the CNTs’ length. Last, the relationship between voltage and current (U/I) were studied. The U/I curve of armchair G-CNTs (6,0) possessed a good linearity and symmetry. These discoveries will contribute to the design and production of G-CNT-based devices.