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The lack of reliable predictive modeling methods and robust experimental techniques has hindered the rational design of hierarchical materials with desired structure−property−performance attributes suitable for extreme environments. With this context in mind, we explore the utility of ReaxFF reactive molecular dynamics (MD) simulations in combination with in-operando wide-angle X-ray scattering (WAXS) and X-ray pair distribution function (PDF) analyses. To demonstrate the method, we consider kaolinite, a natural hierarchical material, as the candidate and determine thermally induced chemical and structural transformations when heated from 298 to 1673 K. We first compare the key structural features from the PDF data and WAXS peaks obtained experimentally to those calculated from MD simulations. Upon observing excellent agreement, we proceed to elucidate the underlying chemical reaction mechanisms associated with dehydroxylation and sintering, identify intermediate and transition states, and also estimate energy barriers of individual reactions and their effects on the structural organization of kaolinite obtained using MD simulations. On heating from 298 to 873 K, dehydroxylation reactions lead to the transformation of crystalline kaolinite with octahedrally coordinated aluminum atoms to semicrystalline metakaolin with ∼90% tetrahedrally coordinated aluminum atoms. Sintering reactions and the subsequent emergence of mullite (a high-temperature phase of kaolinite) are observed on heating metakaolin from 1055 to 1673 K. We also find that heating rates have a significant effect on the onset temperature of dehydroxylation and sintering reactions. A rapid heating rate leads to early dehydroxylation (425 K) and sintering (1055 K), whereas a 10 times slower heating rate delays dehydroxylation (622 K) and sintering reactions (1100 K). An outcome of our method is a regime map that illustrates the degree of agreement between the experimental data and simulation results in describing the thermally induced onset of atomic-level reorganizations in materials. Herein, quantitative agreement between simulation predictions and experimental data is noted at lower temperatures (T < 1000 K), and minor deviations between these methods is noted for T > 1000 K. The remarkable agreement between the methods observed in our study reiterates the reliability of the combined ReaxFF approach in predicting material properties under chemically reactive and extreme conditions.

Layered H2TiO3 has been studied as an ionic sieve material for the selective concentration of lithium from solutions. The accepted mechanism of lithium adsorption on H2TiO3 ion sieves is that it occurs via Li+–H+ ion exchange with no chemical bond breakage. However, in this work, we demonstrate that lithium adsorption on H2TiO3 occurs via O–H bond breakage and the formation of O–Li bonds, contrary to previously proposed mechanisms. Thermogravimetric analysis results show that the weight loss due to dehydroxylation decreases from 2.96 wt % to 0.8 wt % after lithium adsorption, indicating that surface hydroxyl groups break during lithium adsorption. Raman and Fourier transform infrared spectroscopy studies indicate that H2TiO3 contains isolated OH groups and hydrogen-bonded OH groups. Among these two hydroxyl groups, isolated OH groups present in the HTi2 layers are more actively involved in lithium adsorption than hydrogen-bonded OH groups. As a result, the actual adsorption capacity is limited by the number of isolated OH groups, whereas hydrogen-bonded OH groups involved are for stabilizing the layered structure. We also show that H2TiO3 contains a high concentration of stacking faults and structural disorders which play a crucial role in controlling lithium adsorption properties.

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Aqueous amine solvents (e.g., monoethanolamine) coupled with reactive alkaline sorbents (e.g., MgO) favor low temperature CO2 removal as solid carbonates.

To achieve tunable controls on the interactions of siliceous materials in subsurface environments bearing water and hydrocarbons, it is essential to determine the influence of hydrocarbon−water interfaces on the self-assembly of the silica particles. The hydrophilic silica particle have a tendency to aggregate on the water front of the interface. Self-assembly of silica nanoparticles proceeds via the migration of these nanoparticles to the water−hydrocarbon interface followed by aggregation at the interface. Fractal-like morphologies of assembled silica nanoparticles at water−hydrocarbon interfaces are observed. Rapid assembly of the hydrophilic silica nanoparticles at water−hydrocarbon interfaces corresponds to an overall reduction in the surface tension of water− toluene and water−heptane systems. These studies demonstrate the silica aggregation that is the precursor of silica polymerization, or nucleation and growth is influenced by the presence of hydrocarbons in subsurface geologic environments. These insights were derived from in operando ultrasmall and small-angle X-ray scattering (USAXS/SAXS) measurements, cryo-scanning electron microscopy (Cryo-SEM) imaging, and classical molecular dynamics simulations. These studies are intended to inform current and future efforts aimed at tuning silica reactivity in subsurface geologic environments to enhance permeability, the design and use of silica-based proppants to enhance fractures, and the development of effective strategies to control silica-based scaling behavior in subsurface reservoirs.

The ability to direct subsurface fluid flow or achieve a high level of control on permeability in subsurface environments necessitates the development of tunable novel subsurface fluids. These multifunctional fluids should have the potential to form hydrogels to divert flow or enhance fracture networks at elevated pressures, ability to transport proppants, undergo reversible transformations from gel-like to fluid-like in response to chemical-or pressure-based perturbations, and have improved CO 2 carrying capacity for enhancing the miscibility and flowability of oil and gas. In this paper, we discuss the development of multifunctional nanofluids constructed from silica (SiO 2 ) nanoparticles and poly(allylamine) (PAA), aminebearing polymer chains with high affinity for CO 2 . A 2-fold increase in CO 2 absorption in SiO 2 −PAA nanofluids compared to the pure polymer was noted. Upon CO 2 absorption, the weakly interacting polymeric chains around the nanoparticles formed relatively compact hydrogels. Time-resolved ultrasmall-angle X-ray scattering/small-angle X-ray scattering measurements showed the transition from swollen branched polymers to Gaussian coils with an increased exposure to CO 2. Further, CO 2induced hydrogel formation in aqueous fluids bearing 1 wt % SiO 2 −PAA nanofluids occurred at room temperature, unlike in fluids bearing 1 wt % PAA. These observations point to the feasibility of forming hydrogels at lower temperatures and pressures using novel nanofluids as opposed to using the pure polymer. The ability to tune the structures and morphologies of these fluids expands the potential applications of these fluids in a wide range of subsurface environments.