Hsiu-Wen Wang

Amorphous magnesium carbonate (AMC) is recognized as a precursor material during the formation of crystalline magnesium carbonates, which are important materials, e.g., for next generation cements and CO2 sorbents, but its amorphous-to-crystalline transition is poorly understood. In this study, we combined in situ X-ray and neutron diffraction and total scattering to determine the activation energy of the amorphous-to-crystalline transition for AMC in water and identify the reaction mechanism. The calculated activation energy Ea of 41 ± 13 kJ/mol for the AMC to nesquehonite (MgCO3·3H2O) transformation matches previous studies indicating a dissolution–reprecipitation mechanism. Ea for AMC to hydromagnesite (Mg5(CO3)4(OH)2·4H2O) of 116 ± 19 kJ/mol is considerably higher and combined with our peak area analysis of total scattering data, we conclude that a second-order solid state transition is likely. Remarkably, we observed 2 orders of magnitude longer lifetime of deuterated AMC compared to protiated AMC in liquid.

Using small-angle neutron scattering, we studied the nanostructure of a prototypical D(H)-bonded network electrolyte, alkaline sodium aluminate bisalt, at concentrations up to solute volume fraction of ∼0.5. Analysis of the structure factor prepeak at 0.1 < Q < 1.3 Å–1 showed that its evolution is associated with nanoscopic species distribution in water–anion network, with differences in characteristic distance d between OD– and Al(OD)4– anions related to their distinct sizes and interactions. When the solute volume fraction approached 0.5, a common/maximum value of ∼4.5 Å was found for the correlation length ξ that characterizes the electrostatic force in concentrated electrolyte solution before precipitation occurred. This implies that the solutions’ morphology and behavior at high concentrations may be governed by geometric factors, rather than the chemistry of the specific anion. Furthermore, the scaling of ξ with concentration yielded an exponent of 1.25(1), suggesting that ξ is limited to a few Å. By combining the current observations of structural heterogeneity at the nanoscale, with dynamic heterogeneity at the microscopic scale from our previous quasi-elastic neutron scattering study, we have established a structural origin of local “caging” and restricted structural relaxation processes. These local solvent–solute interactions not only control dynamics heterogeneity in concentrated electrolytes but are also responsible for crystallization processes in industrial setting, such as aluminum production and radioactive waste treatment.

In highly concentrated electrolyte solutions, where classical models often fail, specific ion–solvent interactions dictate bulk properties. Here, we combine small-angle X-ray scattering (SAXS) and Raman spectroscopy to link molecular coordination to mesoscale structure across alkali nitrate and nitrite solutions. Our results reveal a structural hierarchy driven by cation identity in that smaller cations (Li⁺, Na⁺) form discrete contact ion pairs, while larger cations (K⁺, Rb⁺) promote increasingly disordered local coordination and extended solute–solvent networks. Using nitrite salts as a structural control, we confirm this organizing principle across different anion geometries. Cs⁺ undergoes a concentration-induced transition to a uniquely ordered state, resulting in a highly structured solution. This work provides a direct, mechanistic explanation for how cation choice dictates the ‘rules’ of solution architecture, offering a predictive basis for understanding phase behavior in complex industrial and environmental systems.

The upper librational band edge of –OH groups in aluminum (oxy)hydroxides reflects the strength-geometry of H-bonding. This blueshift mirrors (in opposite correlation) the redshift of intramolecular O–H stretching with increasing H-bond strength.

Thin fluid layers confined between nanoparticles play an important role in several natural and industrial systems, including radioactive wastes stored in tanks at the U.S. Department of Energy's Hanford site. Multimodal neutron and computational approaches have been integrated to examine the properties of one, two, and four layers of water (H₂O or D₂O) on nanoparticulate, hydrous, or deuterated boehmite (γ-AlOOH or γ-AlOOD). Exposure of deuterated boehmite to H₂O at 90 °C yielded rapid H/D isotopic exchange likely driven by a Grotthus-like proton-hopping mechanism. The in-plane and out-of-plane vibrations of the structural hydroxyls involved in this exchange were observed for both the hydrated and deuterated boehmite. These observations were confirmed by molecular dynamics simulations that also showed that single water molecules on the (010) surface bond to the structure via four bonds: two from hydrogens (deuteriums) on the water to surrounding oxygens and two from the water's oxygen to surrounding OD/OH. Bulk water-like properties began to appear once four monolayers of water had been added, but steric crowding limited water diffusion rates once two monolayers had been added. The super-Arrhenius temperature dependence observed at four monolayers indicated glass-like behavior in a well-formed hydrogen-bonding network. Such a network is not sufficiently developed, however, when the surface water coverage is less than four layers. The unique nature of these layers can provide critical information for understanding forces between particles in proximity, and resultant effects on suspension rheology.

Aluminum hydroxide polymorphs are of widespread importance yet their kinetics of nucleation and growth remain beyond the reach of current models. Here we attempt to unveil the reaction processes underlying the polymorphs formation at high chemical potential. We examine their formation in-situ from supersaturated alkaline sodium aluminate solutions using deuteration and time-resolved neutron pair distribution function analyses, which indicate the formation of individual Al(OD)₃ layers as an intermediate particle phase. These layers ultimately stack to form gibbsite- or bayerite-like layered heterostructures. Ex-situ characterization of the recovered precipitates using ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy, Raman, X-ray diffraction, and scanning electron microscopy, suggests the presence of additional intermediate states, an amorphous compound bearing both tetrahededrally- and penta-coordinated Al³⁺. These observations reveal the complex pathways to form Al(OD)₃ monolayers via either transient pentacoordinate species or amorphous-to-ordered transitions. The subsequent crystallization of admixed gibbsite/bayerite is followed by an Al(OD)₃ monolayer attachment process.

Using neutron scattering dynamics measurements to validate molecular dynamic simulations, it is found that the rates of solvent exchange are not likely to limit the rates of calcite crystal growth and dissolution.