Atomistic insight into salinity dependent preferential binding of polar aromatics to calcite/brine interface: implications to low salinity waterflooding.

This paper resolve the salinity-dependent interactions of polar components of crude oil at calcite-brine interface in atomic resolution. Molecular dynamics simulations carried out on the present study showed that ordered water monolayers develop immediate to a calcite substrate in contact with a saline solution. Carboxylic compounds, herein represented by benzoic acid (BA), penetrate into those hydration layers and directly linking to the calcite surface. Through a mechanism termed screening effect, development of hydrogen bonding between -COOH functional groups of BA and carbonate groups is inhibited by formation of a positively-charged Na+ layer over CaCO3 surface. Contrary to the common perception, a sodium-depleted solution potentially intensifies surface adsorption of polar hydrocarbons onto carbonate substrates; thus, shifting wetting characteristic to hydrophobic condition. In the context of enhanced oil recovery, an ion-engineered waterflooding would be more effective than injecting a solely diluted saltwater.

Ion-specific interactions at calcite-brine interfaces: a nano-scale study of the surface charge development and preferential binding of polar hydrocarbons.

This research provides an atomic-level insight into the synergic contribution of mono- and divalent ions to interfacial characteristics of calcite surfaces exposed to electrolyte solution containing organic compounds. The emphasis was placed on the ionic interactions responsible for charge developing mechanisms of calcite surfaces and also the capacity for adsorption of polar hydrocarbons, represented by benzoic acid (BA), at different brine compositions. For this purpose, molecular dynamics (MD) simulations were employed to explore the interplay of the main constituent ions of natural brines (Na+, Cl-, Mg2+, and SO42-) and BA at the interface of CaCO3. It was observed that surface accumulation of Na+ cations produces a positively charged layer immediate to the basal plane of calcite, validating the typical positive surface charge of carbonates reported by laboratory experiences. Meanwhile, a negatively charged layer appears beyond the sodium layer as a result of direct and solvent-mediated pairing of anions with Na+ cations lodged on the calcite substrate. In this process, sulfate adsorption severely diminishes surface charge to even a negative value in the case of a SO42--rich solution, providing an interpretation for the measurements reported in the literature. Our results revealed the inhibition of direct binding of BA molecules onto the calcite surface through complexation with protruding oxygen atoms of basal carbonates by the residing Na+ cations. Further, we noticed the sulfate-mediated pairing of BA molecules to the Na+ layer, which in effect intensifies surface adsorption of BA. However, BA-SO42- interaction is considerably reduced by magnesium cations shielding sulfate sites in the Mg2+-augmented brine. The findings presented in this study are of fundamental importance to advance our microscopic understanding of interfacial interactions in brine/oil/carbonate systems; with broad scientific and applied implications in the context of mobilizing organic contaminants trapped in aquifer sediments and enhancement of hydrophilicity of subsurface oil-bearing carbonate reservoirs by injecting ion-modified brine solutions.

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.

How do ions contribute to brine-hydrophobic hydrocarbon Interfaces? An in silico study.

HYPOTHESIS: The saltwater-oil interface is of broad implication in geochemistry and petroleum disciplines. To date, the main focus has been on the surface contribution of polar, heavy compounds of crude oil, widely neglecting the role of non-polar hydrocarbons. However, non-polar compounds are expected to contribute to characteristics of oil-brine interfaces. METHODOLOGY: Utilizing molecular dynamics simulation, we aim to characterize ion behavior adjacent to hydrophobic organic phases. Concerning natural environments, NaCl, CaCl2 and Na2SO4 electrolytes at low (5 wt%) and high (15 wt%) concentrations were brought in contact with heptane and/or toluene which account for aliphatic and aromatic constituents of typical crude oils, respectively. The reproduced experimental data for interfacial tension, brines density and ions' diffusivities adequately verify our molecular calculations. FINDINGS: Ions accumulate nearby the intrinsically charge-neutral oil surfaces. A disparate surface-favoring propensity of ions causes the interfacial region to resemble an electrical layer and impose an effective surface charge onto the oil surface. Despite absence of any polar site, the effective surface charge density is hydrocarbon-dependent, with the highest and lowest values observed for toluene and heptane interfaces, respectively. Due to accumulation of toluene molecules nearby the brines, the interfacial characteristics of heptol (toluene-heptane mixture) is comparable to that of the toluene phase.

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.

Bonding, structural and thermodynamic analysis of dissociative adsorption of H3O+ ion onto calcite (101⁻4) surface: CPMD and DFT calculations.

We used density functional theory (DFT) and Car-Parrinello molecular dynamics (CPMD) simulation to investigate the adsorption and bond formation of hydronium ion (H3O+) onto a [Formula: see text] calcite surface. For surface coverage of 25% to 100%, the nature of H3O+ interaction was explored through electron density and energetics in the context of bond critical points. The adsorbate-adsorbent structure was studied by simulation of pair correlation function. The results revealed that dissociation into water molecule(s) and proton(s) complements H3O+ ion(s) adsorbtion. The H2O molecule adsorbs onto the surface via its O atom, and interacts with surface calcium in a closed-shell mode; the H+ ion makes a covalent bond to the surface oxygen while maintaining H-bonding with water. Adsorption energies were diminished by 70-90 kJ mol-1 when Obridge-bonded H+ ions transferred to the Oterminal manually. While dissociative adsorption of H3O+ ions is spontaneous at all surface coverages tested, the free energy was lowest at 75% coverage. Also, protonation of a completely pre-hydrated calcite surface leads to stronger interaction of water molecules with the surface. This unique outlook on hydrating calcite provides specific insights into biomineralization of this mineral, and helps depict further pH consequences in the field of biomaterial adsorption. Graphical abstract Dissociative adsorption of hydronium ion onto the surface of calcite.

Carbon nanostructures as catalytic support for chemiluminescence of sulfur compounds in a molecular emission cavity analysis system.

The effect of different substrates including stainless steel, activated carbon, single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), fullerenes (C60, C70, etc.) and SWCNTs doped with iron and palladium nanoparticles were compared for catalytic chemiluminescence reaction of sulfur compounds in a flame-containing cavity of molecular emission cavity analysis (MECA) system. Different forms of CNT substrates were fabricated using electric arc-discharge method. The blue emission of excited S2 was monitored using a CCD camera. The results demonstrate that, due to the high surface area, plenty of basal planes, high thermal conductivity, and high flexibility of the carbon nanostructure as appropriate support, carbon nanostructures play an important role in catalytic chemiluminescence emission of sulfur compounds in MECA. Moreover, the presence of metallic nanoparticles doped on carbon nanostructures enhances their catalytic effect. The results revealed that under similar conditions, SWCNTs/Pd doped nanoparticles, SWCNTs/Fe doped nanoparticles, SWCNTs, MWCNTs and fullerenes have the most catalytic effects on chemiluminescence of sulfur compounds, respectively.