Molecular Mechanisms of Sorbed Ion Effects during Boehmite Particle Aggregation.

Classical theories of particle aggregation, such as Derjaguin-Landau-Verwey-Overbeek (DLVO), do not explain recent observations of ion-specific effects or the complex concentration dependence for aggregation. Thus, here, we probe the molecular mechanisms by which selected alkali nitrate ions (Na+, K+, and NO3-) influence aggregation of the mineral boehmite (γ-AlOOH) nanoparticles. Nanoparticle aggregation was analyzed using classical molecular dynamics (CMD) simulations coupled with the metadynamics rare event approach for stoichiometric surface terminations of two boehmite crystal faces. Calculated free energy landscapes reveal how electrolyte ions alter aggregation on different crystal faces relative to pure water. Consistent with experimental observations, we find that adding an electrolyte significantly reduces the energy barrier for particle aggregation (∼3-4×). However, in this work, we show this is due to the ions disrupting interstitial water networks, and that aggregation between stoichiometric (010) basal-basal surfaces is more favorable than between (001) edge-edge surfaces (∼5-6×) due to the higher interfacial water densities on edge surfaces. The interfacial distances in the interlayer between aggregated particles with electrolytes (∼5-10 Å) are larger than those in pure water (a few Ångströms). Together, aggregation/disaggregation in salt solutions is predicted to be more reversible due to these lower energy barriers, but there is uncertainty on the magnitudes of the energies that lead to aggregation at the molecular scale. By analyzing the peak water densities of the first monolayer of interstitial water as a proxy for solvent ordering, we find that the extent of solvent ordering likely determines the structures of aggregated states as well as the energy barriers to move between them. The results suggest a path for developing a molecular-level basis to predict the synergies between ions and crystal faces that facilitate aggregation under given solution conditions. Such fundamental understanding could be applied extensively to the aggregation and precipitation utilization in the biological, pharmaceutical, materials design, environmental remediation, and geological regimes.

Armoring of MgO by a Passivation Layer Impedes Direct Air Capture of CO2.

It has been proposed to use magnesium oxide (MgO) to separate carbon dioxide directly from the atmosphere at the gigaton level. We show experimental results on MgO single crystals reacting with the atmosphere for longer (decades) and shorter (days to months) periods with the goal of gauging reaction rates. Here, we find a substantial slowdown of an initially fast reaction as a result of mineral armoring by reaction products (surface passivation). In short-term experiments, we observe fast hydroxylation, carbonation, and formation of amorphous hydrated magnesium carbonate at early stages, leading to the formation of crystalline hydrated Mg carbonates. The preferential location of Mg carbonates along the atomic steps on the crystal surface of MgO indicates the importance of the reactive site density for carbonation kinetics. The analysis of 27-year-old single-crystal MgO samples demonstrates that the thickness of the reacted layer is limited to ∼1.5 µm on average, which is thinner than expected and indicates surface passivation. Thus, if MgO is to be employed for direct air capture of CO2, surface passivation must be circumvented.

Predicting Outcomes of Nanoparticle Attachment by Connecting Atomistic, Interfacial, Particle, and Aggregate Scales.

Predicting nanoparticle aggregation and attachment phenomena requires a rigorous understanding of the interplay among crystal structure, particle morphology, surface chemistry, solution conditions, and interparticle forces, yet no comprehensive picture exists. We used an integrated suite of experimental, theoretical, and simulation methods to resolve the effect of solution pH on the aggregation of boehmite nanoplatelets, a case study with important implications for the environmental management of legacy nuclear waste. Real-time observations showed that the particles attach preferentially along the (010) planes at pH 8.5 and the (101) planes at pH 11. To rationalize these results, we established the connection between key physicochemical phenomena across the relevant length scales. Starting from molecular-scale simulations of surface hydroxyl reactivity, we developed an interfacial-scale model of the corresponding electrostatic potentials, with subsequent particle-scale calculations of the resulting driving forces allowing successful prediction of the attachment modes. Finally, we scaled these phenomena to understand the collective structure at the aggregate-scale. Our results indicate that facet-specific differences in surface chemistry produce heterogeneous surface charge distributions that are coupled to particle anisotropy and shape-dependent hydrodynamic forces, to play a key role in controlling aggregation behavior.

Grain boundary widening controls siderite (FeCO3) replacement of limestone (CaCO3).

The microstructure of minerals and rocks can significantly alter reaction rates. This study focuses on identifying transport paths in low porosity rocks based on the hypothesis that grain boundary widening accelerates reactions in which one mineral is replaced by another (replacement reaction). We conducted a time series of replacement experiments of three limestones (CaCO3) of different microstructures and solid impurity contents using FeCl2. Reacted solids were analyzed using chemical imaging, small angle X-ray and neutron scattering and Raman spectroscopy. In high porosity limestones replacement is reaction controlled and complete replacement was observed within 2 days. In low porosity limestones that contain 1-2% dolomite impurities and are dominated by grain boundaries, a reaction rim was observed whose width did not change with reaction time. Siderite (FeCO3) nucleation was observed in all parts of the rock cores indicating the percolation of the solution throughout the complete core. Dolomite impurities were identified to act as nucleation sites leading to growth of crystals that exert force on the CaCO3 grains. Widening of grain boundaries beyond what is expected based on dissolution and thermal grain expansion was observed in the low porosity marble containing dolomite impurities. This leads to a self-perpetuating cycle of grain boundary widening and reaction acceleration instead of reaction front propagation.

Effect of Fluid Properties on Contact Angles in the Eagle Ford Shale Measured with Spontaneous Imbibition.

Models of fluid flow are used to improve the efficiency of oil and gas extraction and to estimate the storage and leakage of carbon dioxide in geologic reservoirs. Therefore, a quantitative understanding of key parameters of rock-fluid interactions, such as contact angles, wetting, and the rate of spontaneous imbibition, is necessary if these models are to predict reservoir behavior accurately. In this study, aqueous fluid imbibition rates were measured in fractures in samples of the Eagle Ford Shale using neutron imaging. Several liquids, including pure water and aqueous solutions containing sodium bicarbonate and sodium chloride, were used to determine the impact of solution chemistry on uptake rates. Uptake rate analysis provided dynamic contact angles for the Eagle Ford Shale that ranged from 51 to 90° using the Schwiebert-Leong equation, suggesting moderately hydrophilic mineralogy. When corrected for hydrostatic pressure, the average contact angle was calculated as 76 ± 7°, with higher values at the fracture inlet. Differences in imbibition arising from differing fracture widths, physical liquid properties, and wetting front height were investigated. For example, bicarbonate-contacted samples had average contact angles that varied between 62 ± 10° and ∼84 ± 6° as the fluid rose in the column, likely reflecting a convergence-divergence structure within the fracture. Secondary imbibitions into the same samples showed a much more rapid uptake for water and sodium chloride solutions that suggested alteration of the clay in contact with the solution producing a water-wet environment. The same effect was not observed for sodium bicarbonate, which suggested that the bicarbonate ion prevented shale hydration. This study demonstrates how the imbibition rate measured by neutron imaging can be used to determine contact angles for solutions in contact with shale or other materials and that wetting properties can vary on a relatively fine scale during imbibition, requiring detailed descriptions of wetting for accurate reservoir modeling.

Degradation of Adsorbed Bisphenol A by Soluble Mn(III).

Bisphenol A (BPA), a high production volume chemical and potential endocrine disruptor, is found to be associated with sediments and soils due to its hydrophobicity (log KOW of 3.42). We used superfine powdered activated carbon (SPAC) with a particle size of 1.38 ± 0.03 µm as a BPA sorbent and assessed degradation of BPA by oxidized manganese (Mn) species. SPAC strongly sorbed BPA, and desorption required organic solvents. No degradation of adsorbed BPA (278.7 ± 0.6 mg BPA g-1 SPAC) was observed with synthetic, solid α-MnO2 with a particle size of 15.41 ± 1.35 µm; however, 89% mass reduction occurred following the addition of 0.5 mM soluble Mn(III). Small-angle neutron scattering data suggested that both adsorption and degradation of BPA occurred in SPAC pores. The findings demonstrate that Mn(III) mediates oxidative transformation of dissolved and adsorbed BPA, the latter observation challenging the paradigm that contaminant desorption and diffusion out of pore structures are required steps for degradation. Soluble Mn(III) is abundant near oxic-anoxic interfaces, and the observation that adsorbed BPA is susceptible to degradation has implications for predicting, and possibly managing, the fate and longevity of BPA in environmental systems.

One-Dimensional Glassy Behavior of Ultraconfined Water Strings.

Water is renowned for its anomalous behaviors, which can be linked to a distributed H-bond network in bulk water. Ultraconfinement of the water molecule can remove H-bonding, leaving only molecular water. In natural cordierite crystals, water is trapped in an orthorhombic channel with an average diameter of 5.7 Å, running through the center of the unit cell parallel to the c-axis. Calorimetric measurements reveal the existence of a one-dimensional (1D) glass linked to this water. In these channels, water molecules in truncated, sparse 1D strings interact only via dipole-dipole correlations. A physical 1D glass is formed from these strings. This unusual state can be explained by a modified Ising model. This model predicts a dependence of the glass transition temperature, Tg, on the size of these domains. This is confirmed experimentally.

Correlating inter-particle forces and particle shape to shear-induced aggregation/fragmentation and rheology for dilute anisotropic particle suspensions: A complementary study via capillary rheometry and in-situ small and ultra-small angle X-ray scattering.

HYPOTHESIS: Understanding the stability and rheological behavior of suspensions composed of anisotropic particles is challenging due to the complex interplay of hydrodynamic and colloidal forces. We propose that orientationally-dependent interactions resulting from the anisotropic nature of non-spherical sub-units strongly influences shear-induced particle aggregation/fragmentation and suspension rheological behavior. EXPERIMENTS: Wide-, small-, and ultra-small-angle X-ray scattering experiments were used to simultaneously monitor changes in size and fractal dimensions of boehmite aggregates from 6 to 10,000 Å as the sample was recirculated through an in-situ capillary rheometer. The latter also provided simultaneous suspension viscosity data. Computational fluid dynamics modeling of the apparatus provided a more rigorous analysis of the fluid flow. FINDINGS: Shear-induced aggregation/fragmentation was correlated with a complicated balance between hydrodynamic and colloidal forces. Multi-scale fractal aggregates formed in solution but the largest could be fragmented by shear. Orientationally-dependent interactions lead to a relatively large experimental suspension viscosity when the hydrodynamic force was small compared to colloidal forces. This manifests even at low boehmite mass fractions.

Structure and dynamics of water on the forsterite surface.

The behavior of water on mineral surfaces is the key to understanding interfacial and chemical reaction processes. Olivine is one of the major rock-forming minerals and its interaction with water is a ubiquitous phenomenon both on Earth's surface and in the subsurface. This work presents a combined study using molecular dynamics (MD) simulations and quasi-elastic neutron scattering (QENS) experiments conducted using three different instruments to study the structure and dynamics of water on the forsterite (Mg-end member of olivine) surface at 270 K. A combination of three different QENS instruments probes dynamical processes occurring across a broad range of time scales (∼1 ps to ∼1 ns in this study). The water structure on the hydroxylated surface is composed of three distinct water layers, transitioning from well-ordered and nearly immobile closest to the surface to a less structured layer. The energies of three motions (including translation and rotation) derived from simulations agree well with the experiments, covering the energy range from a few to hundreds of micro electron volts.

Impact of Solution Chemistry and Particle Anisotropy on the Collective Dynamics of Oriented Aggregation.

Although oriented aggregation of particles is a widely recognized mechanism of crystal growth, the impact of many fundamental parameters, such as crystallographically distinct interfacial structures, solution composition, and nanoparticle morphology, on the governing mechanisms and assembly kinetics are largely unexplored. Thus, the collective dynamics of systems exhibiting OA has not been predicted. In this context, we investigated the structure and dynamics of boehmite aggregation as a function of solution pH and ionic strength. Cryogenic transmission electron microscopy shows that boehmite nanoplatelets assemble by oriented attachment on (010) planes. The coagulation rate constants obtained from dynamic light scattering during the early stages of aggregation span 7 orders of magnitude and cross both the reaction-limited and diffusion-limited regimes. Combining a simple scaling analysis with calculations for stability ratios and rotational/translational diffusivities of irregular particle shapes, the effects of orientation for irregular-shaped particles on the early stages of aggregation are understood via angular dependencies of van der Waals, electrostatic, and hydrodynamic interactions. Using Monte Carlo simulations, we found that a simple geometric parameter, namely, the contact area between two attaching nanoplatelets, presents a useful tool for correlating nanoparticle morphologies to the emerging larger-scale aggregates, hence explaining the unusually high fractal dimensions measured for boehmite aggregates. Our findings on nanocrystal transport and interactions provide insights toward the predictive understanding of nanoparticle growth, assembly, and aggregation, which will address critical challenges in developing synthesis strategies for nanostructured materials, understanding the evolution of geochemical reservoirs, and addressing many environmental problems.

Author Correction: Facile emulsion mediated synthesis of phase-pure diopside nanoparticles.

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Facile emulsion mediated synthesis of phase-pure diopside nanoparticles.

Diopside is a common natural pyroxene that is rarely found in a pure state, since magnesium is often partially substituted by iron, and other elements (sodium and aluminum) are often present. This pyroxene, along with feldspars and olivines, is common in concrete. As the prospective license renewal of light water reactors to 80 years of operation has raised concerns on the effects of radiation in the concrete biological shield surrounding the reactors, mineral nanoparticles can be valuable to perform amorphization studies to inform predictive models of mechanical properties of irradiated concrete. The synthesis of diopside nanoparticles was achieved in this study using a reverse-micelle sol-gel method employing TEOS, calcium chloride and Mg(MeO)2 in a methanol/toluene solution. Tert-butylamine and water were used as hydrolysis agents, and dodecylamine as a surfactant. The resulting amorphous precursor was centrifuged to remove organics and fired at 800 °C. Additional reaction with hydrogen peroxide was used to remove amine remnants. TEM and SEM examinations revealed a product comprised of 50-100 nm diameter nanoparticles. XRD indicated phase pure diopside and BET indicated a surface area of 63.5 m2/g before peroxide treatment, which at a bulk density of 3.4 g/cm3 is equivalent to particles with diameter of 28 nm.

Boehmite and Gibbsite Nanoplates for the Synthesis of Advanced Alumina Products.

Boehmite (γ-AlOOH) and gibbsite (α-Al-(OH)3) are important archetype (oxy)hydroxides of aluminum in nature that also play diverse roles across a plethora of industrial applications. Developing the ability to understand and predict the properties and characteristics of these materials, on the basis of their natural growth or synthesis pathways, is an important fundamental science enterprise with wide-ranging impacts. The present study describes bulk and surface characteristics of these novel materials in comprehensive detail, using a collectively sophisticated set of experimental capabilities, including a range of conventional laboratory solids analyses and national user facility analyses such as synchrotron X-ray absorption and scattering spectroscopies as well as small-angle neutron scattering. Their thermal stability is investigated using in situ temperature-dependent Raman spectroscopy. These pure and effectively defect-free materials are ideal for synthesis of advanced alumina products.

Rapid Diffusion and Nanosegregation of Hydrogen in Magnesium Alloys from Exposure to Water.

Hydrogen gas is formed when Mg corrodes in water; however, the manner and extent to which the hydrogen may also enter the Mg metal is poorly understood. Such knowledge is critical as stress corrosion cracking (SCC)/embrittlement phenomena limit many otherwise promising structural and functional uses of Mg. Here, we report via D2O/D isotopic tracer and H2O exposures with characterization by secondary ion mass spectrometry, inelastic neutron scattering vibrational spectrometry, electron microscopy, and atom probe tomography techniques direct evidence that hydrogen rapidly penetrated tens of micrometers into Mg metal after only 4 h of exposure to water at room temperature. Further, technologically important microalloying additions of <1 wt % Zr and Nd used to improve the manufacturability and mechanical properties of Mg significantly increased the extent of hydrogen ingress, whereas Al additions in the 2-3 wt % range did not. Segregation of hydrogen species was observed at regions of high Mg/Zr/Nd nanoprecipitate density and at Mg(Zr) metastable solid solution microstructural features. We also report evidence that this ingressed hydrogen was unexpectedly present in the alloy as nanoconfined, molecular H2. These new insights provide a basis for strategies to design Mg alloys to resist SCC in aqueous environments as well as potentially impact functional uses such as hydrogen storage where increased hydrogen uptake is desired.

Nano-scale synthesis of the complex silicate minerals forsterite and enstatite.

Olivine is a relatively common family of silicate minerals in many terrestrial and extraterrestrial environments, and is also useful as a refractory ceramic. A capability to synthesize fine particles of olivine will enable additional studies on surface reactivity under geologically relevant conditions. This paper presents a method for the synthesis of nanocrystalline samples of the magnesium end-member, forsterite (Mg2SiO4) in relatively large batches (15-20g) using a sol-gel/surfactant approach. Magnesium methoxide and tetraethylorthosilicate (TEOS) are refluxed in a toluene/methanol mixture using dodecylamine as a surfactant and tert-butyl amine and water as hydrolysis agents. This material is then cleaned and dried, and fired at 800°C. Post-firing reaction in hydrogen peroxide was used to remove residual organic surfactant. X-ray diffraction showed that a pure material resulted, with a BET surface area of up to 76.6m2/g. The results of a preliminary attempt to use this approach to synthesize nano-scale orthopyroxene (MgSiO3) are also reported.

Wellbore Cement Porosity Evolution in Response to Mineral Alteration during CO2 Flooding.

Mineral reactions during CO2 sequestration will change the pore-size distribution and pore surface characteristics, complicating permeability and storage security predictions. In this paper, we report a small/wide angle scattering study of wellbore cement that has been exposed to carbon dioxide for three decades. We have constructed detailed contour maps that describe local porosity distributions and the mineralogy of the sample and relate these quantities to the carbon dioxide reaction front on the cement. We find that the initial bimodal distribution of pores in the cement, 1-2 and 10-20 nm, is affected differently during the course of carbonation reactions. Initial dissolution of cement phases occurs in the 10-20 nm pores and leads to the development of new pore spaces that are eventually sealed by CaCO3 precipitation, leading to a loss of gel and capillary nanopores, smoother pore surfaces, and reduced porosity. This suggests that during extensive carbonation of wellbore cement, the cement becomes less permeable because of carbonate mineral precipitation within the pore space. Additionally, the loss of gel and capillary nanoporosities will reduce the reactivity of cement with CO2 due to reactive surface area loss. This work demonstrates the importance of understanding not only changes in total porosity but also how the distribution of porosity evolves with reaction that affects permeability.

Quantum Tunneling of Water in Beryl: A New State of the Water Molecule.

Using neutron scattering and ab initio simulations, we document the discovery of a new "quantum tunneling state" of the water molecule confined in 5 Å channels in the mineral beryl, characterized by extended proton and electron delocalization. We observed a number of peaks in the inelastic neutron scattering spectra that were uniquely assigned to water quantum tunneling. In addition, the water proton momentum distribution was measured with deep inelastic neutron scattering, which directly revealed coherent delocalization of the protons in the ground state.

Strong anisotropic dynamics of ultra-confined water.

Dynamics of water confined in ∼5 Å diameter channels of beryl and cordierite single crystals were studied by using inelastic (INS) and quasielastic (QENS) neutron scattering. The INS spectra for both samples were similar and showed that there are no hydrogen bonds acting on water molecule, which experiences strong anisotropic potential, steep along the channels and very soft perpendicular to it. The high-resolution (3.4 µeV) QENS data revealed gradual freezing out of the water molecule dynamics for both minerals at temperatures below about 80 K when the scattering momentum transfer was parallel to the channels, but not when it was perpendicular to the channels. The QENS study with medium energy resolution (0.25 meV) of the beryl with the scattering momentum transfer along the channels showed gradual freezing out of water molecule dynamics at temperatures below about 200 K, whereas at higher temperatures the data could be described as 2-fold rotational jumps about the axis coinciding with the direction of the dipole moment (that is, perpendicular to the channels), with a residence time of 5.5 ps at 225 K. The energy resolution dependence of the apparent dynamics freezing temperature suggests gradual slowing down of the rotational jumps as the temperature is decreased, until the associated QENS broadening can no longer be detected, rather than actual freezing.

Pore-size-dependent calcium carbonate precipitation controlled by surface chemistry.

Induced mineral precipitation is potentially important for the remediation of contaminants, such as during mineral trapping during carbon or toxic metal sequestration. The prediction of precipitation reactions is complicated by the porous nature of rocks and soils and their interaction with the precipitate, introducing transport and confinement effects. Here X-ray scattering measurements, modeling, and electron microscopies were used to measure the kinetics of calcium carbonate precipitation in a porous amorphous silica (CPG) that contained two discrete distributions of pore sizes: nanopores and macropores. To examine the role of the favorability of interaction between the substrate and precipitate, some of the CPG was functionalized with a self-assembled monolayer (SAM) similar to those known to enhance nucleation densities on planar substrates. Precipitation was found to occur exclusively in macropores in the native CPG, while simultaneous precipitation in nanopores and macropores was observed in the functionalized CPG. The rate of precipitation in the nanopores estimated from the model of the X-ray scattering matched that measured on calcite single crystals. These results suggest that the pore-size distribution in which a precipitation reaction preferentially occurs depends on the favorability of interaction between substrate and precipitate, something not considered in most studies of precipitation in porous media.

Anisotropic dynamics of water ultraconfined in macroscopically oriented channels of single-crystal beryl: a multifrequency analysis.

The properties of fluids can be significantly altered by the geometry of their confining environments. While there has been significant work on the properties of such confined fluids, the properties of fluids under ultraconfinement, environments where, at least in one plane, the dimensions of the confining environment are similar to that of the confined molecule, have not been investigated. This paper investigates the dynamic properties of water in beryl (Be(3)Al(2)Si(6)O(18)), the structure of which contains approximately 5-Å-diam channels parallel to the c axis. Three techniques, inelastic neutron scattering, quasielastic neutron scattering, and dielectric spectroscopy, have been used to quantify these properties over a dynamic range covering approximately 16 orders of magnitude. Because beryl can be obtained in large single crystals we were able to quantify directional variations, perpendicular and parallel to the channel directions, in the dynamics of the confined fluid. These are significantly anisotropic and, somewhat counterintuitively, show that vibrations parallel to the c-axis channels are significantly more hindered than those perpendicular to the channels. The effective potential for vibrations in the c direction is harder than the potential in directions perpendicular to it. There is evidence of single-file diffusion of water molecules along the channels at higher temperatures, but below 150 K this diffusion is strongly suppressed. No such suppression, however, has been observed in the channel-perpendicular direction. Inelastic neutron scattering spectra include an intramolecular stretching O-H peak at ~465 meV. As this is nearly coincident with that known for free water molecules and approximately 30 meV higher than that in liquid water or ice, this suggests that there is no hydrogen bonding constraining vibrations between the channel water and the beryl structure. However, dielectric spectroscopic measurements at higher temperatures and lower frequencies yield an activation energy for the dipole reorientation of 16.4 ± 0.14 kJ/mol, close to the energy required to break a hydrogen bond in bulk water. This may suggest the presence of some other form of bonding between the water molecules and the structure, but the resolution of the apparent contradiction between the inelastic neutron and dielectric spectroscopic results remains uncertain.