Adsorption of ethanol and water on calcite: dependence on surface geometry and effect on surface behavior

Low-Temperature Synthesis of Disordered Dolomite and High-Magnesium Calcite in Ethanol-Water Solutions: The Solvation Effect and Implications

How dolomite [CaMg(CO₃)₂] forms is still underdetermined, despite over a century of efforts. Challenges to synthesizing dolomite at low temperatures have hindered our understanding of sedimentary dolomite formation. Unlike calcium, magnesium's high affinity toward water results in kinetic barriers from hydration shells that prevent anhydrous Ca-Mg carbonate growth. Previous synthesis studies show that adding low-dielectric-constant materials, such as dioxane, dissolved sulfide, and dissolved silica, can catalyze the formation of disordered dolomite. Also, polar hydrophilic amino acids and polysaccharides, which are very common in biomineralizing organisms, could have a positive role in stimulating Mg-rich carbonate precipitation. Here, we show that disordered dolomite and high-magnesium calcite can be precipitated at room temperature by partially replacing water with ethanol (which has a lower dielectric constant) and bypassing the hydration barrier. Increasing the ethanol volume percentage of ethanol results in higher Mg incorporation into the calcite structure. When the ethanol volume percentage increases to 75 vol %, disordered dolomite (>60 mol % MgCO₃) can rapidly precipitate from a solution with [Mg²⁺] and [Ca²⁺] mimicking seawater. Thus, our results suggest that the hydration barrier is the critical kinetic inhibitor to primary dolomite precipitation. Ethanol synthesis experiments may provide insights into other materials that share similar properties to promote high-Mg calcite precipitation in sedimentary and biomineral environments.

A Deep Look into the Dynamics of Saltwater Imbibition in a Calcite Nanochannel: Temperature Impacts Capillarity Regimes

This research concerns fundamentals of spontaneous transport of saltwater (1 mol·dm^-3 NaCl solution) in nanopores of calcium carbonates. A fully atomistic model was adopted to scrutinize the temperature dependence of flow regimes during solution transport under CaCO₃ nanoconfinement. The early time of capillary filling is inertia-dominated, and the solution penetrates with a near-planar meniscus at constant velocity. Following a transition period, the meniscus angle falls to a stabilized value, characterizing the capillary-viscous advancement in the calcite channel. At this stage, brine displacement follows a parabolic relationship consistent with the classical Lucas-Washburn (LW) theory. Approaching the slit outlet, the meniscus contact lines spread widely on the solid substrate and brine leaves the channel at a constant rate, in oppose to the LW law. The brine imbibition rate considerably increases at higher temperatures as a result of lower viscosity and greater tendency to form wetting layers on slit walls. We also pointed out a longer primary inertial regime and delayed onset of the viscous-capillary regime at higher temperatures. Throughout the whole span of capillary displacement, transport of sodium and chloride ions is tied to dynamics and diffusion of the water phase, even at the mineral interface. The results presented in this study are of broad implications in diverse science and technological applications.

Molecular dynamics simulation of the interaction of water and humic acid in the adsorption of polycyclic aromatic hydrocarbons

Humic acid (HA) and water play an important role in polycyclic aromatic hydrocarbons (PAHs) adsorption and biodegradation in soil. In this work, molecular dynamics (MD) and electrostatic potential surfaces (EPSs) simulations are conducted to research the contribution of quartz surface, leonardite humic acid (LHA), and water to PAH adsorption. The adsorption energies between PAHs and LHA are much higher than that between PAHs and quartz. Simulation shows that the hydroxyl and carboxyl groups' attraction by LHA is the main adsorption force between PAHs and LHA. The π-π interaction between PAHs and LHA also contributes to the adsorption process. In addition, the mobility of water on quartz surface is much higher than that of LHA. Water should be regarded as an adsorbate in the system as well as PAHs. However, the presence of water has a remarkable negative effect on the adsorption of PAHs on LHA and quartz. The bridging effect of water could only enhance the stability of the aggregation system. The adsorption contribution of quartz and LHA to PAHs in the soil model tends to 0 if the water layer reaches 2.0 nm. Graphical abstract.

Wetting Properties of the CO2-Water-Calcite System via Molecular Simulations: Shape and Size Effects

Assessment of the risks and environmental impacts of carbon geosequestration requires knowledge about the wetting behavior of mineral surfaces in the presence of CO₂ and the pore fluids. In this context, the interfacial tension (IFT) between CO₂ and the aqueous fluid and the contact angle, θ, with the pore mineral surfaces are the two key parameters that control the capillary pressure in the pores of the candidate host rock. Knowledge of these two parameters and their dependence on the local conditions of pressure, temperature, and salinity is essential for the correct prediction of structural and residual trapping. We have performed classical molecular dynamics simulations to predict the CO₂-water IFT and the CO₂-water-calcite contact angle. The IFT results are consistent with previous simulations, where simple point charge water models have been shown to underestimate the water surface tension, thus affecting the simulated IFT values. When combined with the EPM2 CO₂ model, the SPC/Fw water model indeed underestimates the IFT in the low-pressure region at all temperatures studied. On the other hand, at high pressure and low temperature, the IFT is overestimated by ∼5 mN/m. Literature data regarding the CO₂/water/calcite contact angle on calcite are contradictory. Using our new set of force field parameters, we performed NVT simulations at 323 K and 20 MPa to calculate the contact angle of a water droplet on the calcite {10.4} surface in a CO₂ atmosphere. We performed simulations for both spherical and cylindrical droplet configurations for different initial radii to study the size dependence of the water contact angle on calcite in the presence of CO₂. Our results suggest that the contact angle of a cylindrical droplet, is independent of droplet size, for droplets with a radius of 50 Å or more. On the contrary, spherical droplets make a contact angle that is strongly influenced by their size. At the largest size explored in this study, both spherical and cylindrical droplets converge to the same contact angle, 38°, indicating that calcite is strongly wetted by water.

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.

Construction of LiMn2O4 microcubes and spheres via the control of the (104) crystal planes of MnCO3 for high rate Li-ions batteries

LiMn₂O₄ cathode materials with different morphologies were produced as a result of the addition of ethanol that interacts with the MnCO₃ (104) crystal planes.

The effect of solvation and temperature on the adsorption of small organic molecules on calcite

The effect of multicomponent mixture on small organic molecule adsorption on calcite at nonzero temperature was investigated.

Interactions of the Calcite {10.4} Surface with Organic Compounds: Structure and Behaviour at Mineral - Organic Interfaces

The structure and the strength of organic compound adsorption on mineral surfaces are of interest for a number of industrial and environmental applications, oil recovery, CO₂ storage and contamination remediation. Biomineralised calcite plays an essential role in the function of many organisms that control crystal growth with organic macromolecules. Carbonate rocks, composed almost exclusively of calcite, host drinking water aquifers and oil reservoirs. In this study, we examined the ordering behaviour of several organic compounds and the thickness of the adsorbed layers formed on calcite {10.4} surfaces. We used X-ray reflectivity (XRR) to study calcite {10.4} surfaces that were prepared in three alcohols: methanol, isopropanol and pentanol and one carboxylic acid: octanoic acid. All molecules adsorbed in self-assembled layers, where thickness depended on the density and the length of the molecule. For methanol and isopropanol, molecular dynamic simulations (MD) provided complementary information, which allowed us to develop a surface model. Branching in isopropanol induced slightly less ordering because of the additional degree of freedom. Pentanol and octanoic acid adsorbed as single monolayers. The results of this work indicate that adhered organic compounds from the surrounding environment can affect the surface behaviour, depending on properties of the organic compound.

Kinetics and Mechanism of Calcium Hydroxide Conversion into Calcium Alkoxides: Implications in Heritage Conservation Using Nanolimes

Nanolimes are alcohol dispersions of Ca(OH)2 nanoparticles used in the conservation of cultural heritage. Although it was believed that Ca(OH)2 particles were inert when dispersed in short-chain alcohols, it has been recently shown that they can undergo transformation into calcium alkoxides. Little is known, however, about the mechanism and kinetics of such a phase transformation as well as its effect on the performance of nanolimes. Here we show that Ca(OH)2 particles formed after lime slaking react with ethanol and isopropanol and partially transform (fractional conversion, α up to 0.08) into calcium ethoxide and isopropoxide, respectively. The transformation shows Arrhenius behavior, with apparent activation energy Ea of 29 ± 4 and 37 ± 6 kJ mol(-1) for Ca-ethoxide and Ca-isopropoxide conversion, respectively. High resolution transmission electron microscopy analyses of reactant and product phases show that the alkoxides replace the crystalline structure of Ca(OH)2 along specific [hkl] directions, preserving the external hexagonal (platelike) morphology of the parent phase. Textural and kinetic results reveal that this pseudomorphic replacement involves a 3D diffusion-controlled deceleratory advancement of the reaction front. The results are consistent with an interface-coupled dissolution-precipitation replacement mechanism. Analysis of the carbonation of Ca(OH)2 particles with different degree of conversion into Ca-ethoxide (α up to 0.08) and Ca-isopropoxide (α up to 0.04) exposed to air (20 °C, 80% relative humidity) reveals that Ca-alkoxides significantly reduce the rate of transformation into cementing CaCO3 and induce the formation of metastable vaterite, as opposed to stable calcite which forms in untransformed Ca(OH)2 samples. Similar effects are obtained when a commercial nanolime partially transformed into Ca-ethoxide is subjected to carbonation. Such effects may hamper/delay the strengthening or consolidation effects of nanolimes, thus having important implications in the conservation of cultural heritage.

Visualising the molecular alteration of the calcite (104) - water interface by sodium nitrate

The reactivity of calcite, one of the most abundant minerals in the earth’s crust, is determined by the molecular details of its interface with the contacting solution. Recently, it has been found that trace concentrations of NaNO₃ severely affect calcite’s (104) surface and its reactivity. Here we combine molecular dynamics (MD) simulations, X-ray reflectivity (XR) and in situ atomic force microscopy (AFM) to probe the calcite (104) – water interface in the presence of NaNO₃. Simulations reveal density profiles of different ions near calcite’s surface, with NO₃⁻ able to reach closer to the surface than CO₃²⁻ and in higher concentrations. Reflectivity measurements show a structural destabilisation of the (104) surfaces’ topmost atomic layers in NaNO₃ bearing solution, with distorted rotation angles of the carbonate groups and substantial displacement of the lattice ions. Nanoscale AFM results confirm the alteration of crystallographic characteristics, and the ability of dissolved NaNO₃ to modify the structure of interfacial water was observed by AFM force spectroscopy. Our experiments and simulations consistently evidence a dramatic deterioration of the crystals’ surface, with potentially important implications for geological and industrial processes.

Amorphous and crystalline calcium carbonate phases during carbonation of nanolimes: implications in heritage conservation

Alcohol dispersions of Ca(OH)₂nanoparticles, the so-called nanolimes, carbonate in air following first order kinetics,viaa multistep, non-classical crystallization process involving amorphous and crystalline CaCO₃phases.