Interaction of Water with the Gypsum (010) Surface: Structure and Dynamics from Nonlinear Vibrational Spectroscopy and Ab Initio Molecular Dynamics

Study of the Structural Changes and Internal Activator Transport Behavior after Activation of Aluminum-Based Flameless Ration Heaters: Experimental and Molecular Dynamics Simulations

Aluminum-based flameless ration heaters (AFRHs) are heating elements in food packaging. Water is used to activate AFRHs. The material properties of each region of AFRHs were determined by X-ray diffraction, scanning electron microscopy, and hydrogen and heat generation. The results show that the internal cross-section shows stratification with hydrogen and heat production capacities of 105.2 ± 9.7 mL/g and 1435.0 ± 30.3 J/g for the outer layer, 27.1 ± 4.4 mL/g and 80.4 ± 3.1 J/g for the inner layer, and 1.1 ± 0.01 mL/g and 1.2 ± 0.05 J/g for the middle layer, respectively. According to the correspondence between aluminum and hydrogen in the aluminum-water reaction relationship, the reaction efficiency of the outer layer and the inner layer is as low as 64 and 80%, which is an indication of low reaction efficiency. To analyze the reasons for low reaction efficiency, a pore channel model of 3.5 nm tricalcium aluminate (C3A) was developed using molecular dynamics (MD) to reveal the adsorption behavior of the activator in the pore channel. The results show that the activator is subject to solid surface adsorption in the pore channel with a low diffusion coefficient. Oxygen atoms on the surface adsorb hydrogen atoms to form hydrogen bonds and sodium ions to form ionic bonds with calcium ions. This increases the retention time of the activator on the surface. The MD results explain the low reaction efficiency of AFRHs at the microscopic scale. Moreover, it provides ideas and a basis for the optimization of AFRHs.

Unveiling the structure of aqueous magnesium nitrate solutions by combining X-ray diffraction and theoretical calculations

The structure of aqueous magnesium nitrate solution is gaining significant interest among researchers, especially whether contact ion pairs exist in concentrated solutions. Here, combining X-ray diffraction experiments, quantum chemical calculations and ab initio molecular dynamics simulations, we report that the [Mg(NO₃)₂] molecular structure in solution from the coexistence of a free [Mg(H₂O)₆]²⁺ octahedral supramolecular structure with a free [NO₃(H₂O)_n]^- (n = 11-13) supramolecular structure to an [Mg²⁺(H₂O)_n(NO₃^-)_m] (n = 3, 4, 5; m = 3, 2, 1) associated structure with increasing concentration. Interestingly, two hydration modes of NO₃^--the nearest neighbor hydration with a hydration distance less than 3.9 Å and the next nearest neighbor hydration with hydration distance ranging from 3.9 to 4.3 Å-were distinguished. With an increase in the solution concentration, the hydrated NO₃^- ions lost outer layer water molecules, and the hexagonal octahedral hydration structure of [Mg(H₂O)₆²⁺] was destroyed, resulting in direct contact between Mg²⁺ and NO₃^- ions in a monodentate way. As the concentration of the solution further increased, NO₃^- ions replaced water molecules in the hydration layer of Mg²⁺ to form three-ion clusters and even more complex chains or linear ion clusters.

HONO Production from Gypsum Surfaces Following Exposure to NO2 and HNO3: Roles of Relative Humidity and Light Source

Nitrous acid (HONO) is a toxic household pollutant and a major source of indoor OH radicals. The high surface-to-volume ratio and diverse lighting conditions make the indoor photochemistry of HONO complex. This study demonstrates surface uptake of NO₂ and gaseous HNO₃ followed by gas-phase HONO generation on gypsum surfaces, model system for drywall, under reaction conditions appropriate for an indoor air environment. Tens of parts per billion of steady-state HONO are detected under these experimental conditions. Mechanistic insight into this heterogeneous photochemistry is obtained by exploring the roles of material compositions, relative humidities, and light sources. NO₂ and HNO₃ are adsorbed onto drywall surfaces, which can generate HONO under illumination and under dark conditions. Photoenhanced HONO generation is observed for illumination with a solar simulator as well as with the common indoor light sources such as compact fluorescence light and incandescent light bulbs. Incandescent light sources release more HONO and NO₂ near the light source compared to the solar radiation. Overall, HONO production on the gypsum surface increases with the increase of RH up to 70% relative humidity; above that, the gaseous HONO level decreases due to surface loss. Heterogeneous hydrolysis of NO₂ is predicted to be the dominant HONO generation channel, where NO₂ is produced through the photolysis of surface-adsorbed nitrates. This hydrolysis reaction predominantly occurs in the first layer of surface-adsorbed water.

Hydrogen bonding and molecular orientations across thin water films on sapphire

HYPOTHESIS

Water vapor binding to metal oxide surfaces produces thin water films with properties controlled by interactions with surface hydroxo sites. Hydrogen bonding populations vary across films and induce different molecular orientations than at the surface of liquid water. Identifying these differences can open possibilities for tailoring film-mediated catalytic reactions by choice of the supporting metal oxide substrate.

EXPERIMENTS

The (0001) face of a single sapphire (α-Al₂O₃) sample exposed to water vapor and the surface of liquid water were probed by polarization dependent Sum Frequency Generation-Vibration Spectroscopy (SFG-VS). Molecular dynamics (MD) provided insight into the hydrogen bond populations and molecular orientations across films and liquid water.

FINDINGS

SFG-VS revealed a submonolayer film on sapphire exposed to 43% relative humidity (R.H.), and a multilayer film at 78% R.H. Polarization dependent SFG-VS spectra showed that median tilt angles of free OH bonds on the top of films are at ∼43° from the normal of the (0001) face but at 38° on neat liquid water. These values align with MD simulations, which also show that up to 36% of all OH bonds on films are free. This offers new means for understanding how interfacial reactions on sapphire-supported water films could contrast with those involving liquid water.

Quantitative Assessment of Vapor Molecule Adsorption to Solid Surfaces by Flow Rate Monitoring in Microfluidic Channels

Measuring parameters related to gas adsorption on the effective surfaces of solid samples is important in catalyst studies. Further attention on the subject has appeared due to the materials and methods required to concentrate the gaseous biomarkers for detection. The conventional methods are mainly based on the volumetric and gravimetric analyses, which are applicable to bulk samples. No standard method has yet been provided for such measurements on thin films, which are the most commonly used samples for material screening. Here, a novel method is presented for the adsorption coefficient measurement on thin-film samples. This method comprises coating of the inner walls of a microfluidic channel with the thin film under test. The recorded diffusion rates for a trace gas along this microchannel are compared with the solutions of the adsorption-diffusion equation of the channel for determining the adsorption coefficient of the gas molecule to the inner walls of the channel. The high ratio of surface-to-volume in such channels magnifies the gas sorption effects and improves accuracy. The method is fast, versatile, and cost-effective, allowing measurements at different temperatures and atmospheric pressures. The adsorption coefficients of different isomers of butanol on poly(methyl methacrylate) sheets, zinc oxide thick films, and gold thin films are determined as examples.