Water-Catalyzed Formation of Reactive Oxygen Species from NO2 on a Weakly Hydrated Calcite Surface.

The interfaces of weakly hydrated mineral substrates have been shown to serve as catalytic sites for chemical reactions that may not be accessible in the gas phase or under bulk conditions. Currently known mechanisms for the formation of reactive oxygen species (ROS) from nitrogen dioxide (NO2) involve NO2 dimerization. Here, we report the formation of the ROS HONO via a mechanism involving simple adsorption of a single NO2 molecule on a weakly hydrated calcite substrate. First-principles molecular dynamics simulations coupled with enhanced sampling techniques show how an adsorbed water sublayer can enhance NO2 adsorption on calcite compared to adsorption on a bare dry substrate. On the weakly hydrated calcite surface, an interfacial electric field facilitates proton extraction from water, thus allowing HONO formation from a single adsorbed NO2, i.e., without the need for the formation of a NO2 dimer precomplex. HONO formation on calcite is kinetically more favorable than that in the gas phase, with a reaction barrier of 14 kcal/mol on the weakly hydrated calcite surface compared to 27 kcal/mol in the gas phase. Further photocatalysed HONO production by visible light and HONO dissociation are hampered on calcite, unlike the process on silica. NO2 is a significant anthropogenic pollutant, and understanding its chemistry is crucial for explaining the high ROS levels and haze formation in polluted areas or prebiotic ROS generation. These findings emphasize how mineral substrates under water-restricted hydration conditions can trigger chemical pathways that are unexpected in the gas phase or under bulk conditions.

Fast Hydroxyl Radical Generation at the Air-Water Interface of Aerosols Mediated by Water-Soluble PM2.5 under Ultraviolet A Radiation.

Due to the adverse health effects and the role in the formation of secondary organic aerosols, hydroxyl radical (OH) generation by atmospheric fine particulate matter (PM) has been of particular research interest in both bulk solutions and the gas phase. However, OH generation by PM at the air-water interface of atmospheric water droplets, a unique environment where reactions can be accelerated by orders of magnitude, has long been overlooked. Using the field-induced droplet ionization mass spectrometry methodology that selectively samples molecules at the air-water interface, here, we show significant oxidation of amphiphilic lipids and isoprene mediated by water-soluble PM2.5 at the air-water interface under ultraviolet A irradiation, with the OH generation rate estimated to be 1.5 × 1016 molecule·s-1·m-2. Atomistic molecular dynamics simulations support the counter-intuitive affinity for the air-water interface of isoprene. We opine that it is the carboxylic chelators of the surface-active molecules in PM that enrich photocatalytic metals such as iron at the air-water interface and greatly enhance the OH generation therein. This work provides a potential new heterogeneous OH generation channel in the atmosphere.

Uptake and reactivity of NO2 on the hydroxylated silica surface: A source of reactive oxygen species.

We report state-of-the-art first-principles molecular dynamics results on the heterogeneous chemical uptake of NO2, a major anthropogenic pollutant, on the dry and wet hydroxylated surface of α-quartz, which is a significant component of silica-based catalysts and atmospheric dust aerosols. Our investigation spotlights an unexpected chemical pathway by which NO2 (i) can be adsorbed as HONO by deprotonation of interfacial silanols (i.e., -Si-OH group) on silica, (ii) can be barrierless converted to nitric acid, and (iii) can finally dissociated to surface bounded NO and hydroxyl gas phase radicals. This chemical pathway does not invoke any previously experimentally postulated NO2 dimerization, dimerization that is less likely to occur at low NO2 concentrations. Moreover, water significantly catalyzes the HONO formation and the dissociation of nitric acid into surface-bounded NO and OH radicals, while visible light adsorption can further promote these chemical transformations. This work highlights how water-restricted solvation regimes on common mineral substrates are likely to be a source of reactive oxygen species, and it offers a theoretical framework for further and desirable experimental efforts, aiming to better constrain trace gases/mineral interactions at different relative humidity conditions.

Uptake and hydration of sulfur dioxide on dry and wet hydroxylated silica surfaces: a computational study.

We present a first-principles molecular dynamics study on the uptake and hydration of sulfur dioxide on the dry and wet fully hydroxylated surfaces of (0001) α-quartz, which are a proxy for suspended silica dust in the atmosphere. The average adsorption energy for SO2 is about -10 kcal mol-1 on both dry and wet surfaces. The adsorption is driven by hydrogen bond formation between SO2 and the interfacial hydroxyl groups (on dry silica), or with water molecules (in the wet case). In the dry system, we report an additional electrostatic interaction between the interfacial hydroxyl oxygen and the sulfur atom, which further stabilizes the adsorbate. On dry silica, the interfacial hydroxyl group coordinates to SO2 yielding a surface bound bisulfite (Si-SO3H) complex. On the wet surface, SO2 reacts with water forming bisulfite (HSO3-), and the latter remains solvated inside the adsorbed water layer. The hydration barrier for sulfur dioxide is 1 kcal mol-1 and 3 kcal mol-1 on dry and wet silica, respectively, while for the backward reaction (i.e., bisulfite to SO2) the barrier is 6 kcal mol-1 on both surfaces. The modest backward barrier rationalizes earlier experimental findings showing no SO2 uptake on silica. These results underline the importance of the surface hydroxylation and/or adsorbed water layers for the SO2 uptake and its hydration on silica. Moreover, the hydration to bisulfite may prevent direct SO2 photochemistry and be an additional source of sulfate; this is especially relevant in atmospheres subject to a high level of suspended mineral dust, intense solar radiation and atmospheric oxidizers.

Hydration, Solvation, and Isomerization of Methylglyoxal at the Air/Water Interface: New Mechanistic Pathways.

Aqueous-phase processing of methylglyoxal (MG) has been suggested to play a key role in the formation of secondary organic aerosols and catalyze particle growth in the atmosphere. However, the details of these processes remain speculative owing to the lack of a complete description of the physicochemical behavior of MG on atmospheric aerosols. Here, the solvation and hydrolysis of MG at the air/liquid water interface is studied via classical and first-principles molecular dynamics simulations combined with free-energy methods. Our results reveal that the polarity of the water solvent catalyzed the trans-to-cis isomerization of MG at the air/liquid water interface relative to the gas phase. Despite the presence of a hydrophobic group, MG often solvates with both the ketone and methyl groups parallel to the water interface. Analysis of the instantaneous water surface reveals that when MG is in the trans state, the methyl group repels interfacial water to maintain the planarity of the molecule, indicating that lateral and temporal inhomogeneities of interfacial environments are important for fully characterizing the solvation of MG. The counterintuitive behavior of the hydrophobic group is ascribed to a tendency to maximize the number of hydrogen bonds between MG and interfacial water while minimizing the torsional free energy. This drives MG hydration, and our simulations indicate that the formation of MG diol is catalyzed at the air/liquid water interface compared to the gas phase and occurs through nucleophilic attack of water on the carbonyl carbon.

Adsorption and isomerization of glyoxal and methylglyoxal at the air/hydroxylated silica surface.

We present results from molecular dynamics simulations coupled with enhanced sampling techniques on the adsorption and isomerization of glyoxal (GL) and methylglyoxal (MG) at the air/hydroxylated silica (α-Quartz) interface. GL and MG are two organic compounds present in the atmosphere as oxidation products of both biogenic and anthropogenic precursors. By adsorption and hydration on liquid droplets or wetted dust particles, they can enable aerosol growth in the atmosphere. Moreover, thanks to the different polar characters of their trans and cis conformers, GL and MG have been suggested as possible molecular switches capable of responding to changes in solvent polarity. Here, we show that the hydroxylated silica surface does not significantly catalyze the trans-to-cis isomerization, but it stabilizes the cis-isomers, indicating a higher interfacial cis/trans relative concentration compared to the gas phase. Moreover, adsorbed GL prefers to lie parallel on the silica surface, while adsorbed MG shows a tilted orientation. In particular, we report the aldehyde group pointing upward (downward) to the gas phase (to the silica surface) in trans-MG (cis-MG). These results will help in the rationalization of upcoming experimental and modeling work on the adsorption of ketonic compounds on dust aerosols, while it clarifies the catalytic role of the solid substrate surface in promoting conformational changes.

Tuning the Stereoselectivity and Solvation Selectivity at Interfacial and Bulk Environments by Changing Solvent Polarity: Isomerization of Glyoxal in Different Solvent Environments.

Conformational isomerism plays a central role in organic synthesis and biological processes; however, effective control of isomerization processes still remains challenging and elusive. Here, we propose a novel paradigm for conformational control of isomerization in the condensed phase, in which the polarity of the solvent determines the relative concentration of conformers at the interfacial and bulk regions. By the use of state-of-the-art molecular dynamics simulations of glyoxal in different solvents, we demonstrate that the isomerization process is dipole driven: the solvent favors conformational changes toward conformers having molecular dipoles that better match its polar character. Thus, the solvent polarity modulates the conformational change, stabilizing and selectively segregating in the bulk vs the interface one conformer with respect to the others. The findings in this paper have broader implications affecting systems involving compounds with conformers of different polarity. This work suggests novel mechanisms for tuning the catalytic activity of surfaces in conformationally controlled (photo)chemical reactions and for designing a new class of molecular switches that are active in different solvent environments.

Interfaces Select Specific Stereochemical Conformations: The Isomerization of Glyoxal at the Liquid Water Interface.

Interfacial chemistry involving glyoxal at aerosol surfaces is postulated to catalyze aerosol growth. This chemistry remains speculative due to a lack of detailed information concerning the physicochemical behavior of glyoxal at the interface of atmospheric aerosols. Here, we report results from high-level electronic structure calculations as well as both classical and Born-Oppenheimer ab initio molecular dynamics simulations of glyoxal solvation at the air/liquid water interface. When compared to the gas phase, the trans to cis isomerization of glyoxal at the liquid water interface is found to be catalyzed; additionally, the trans conformation is selectively solvated within the bulk to a greater degree than is the cis conformation. These two processes, i.e., the catalytic effect at the water interface and the differentially selective solvation, act to enhance the concentration of the cis isomer of glyoxal at the water interface. This has important consequences for the interpretation of experiments and for the modeling of glyoxal chemistry both at the interface of water clouds and at aerosols. Broader implications of this work relate to describing the role of interfaces in selecting specific stereo molecular structures at interfacial environments.

Protein-protein structure prediction by scoring molecular dynamics trajectories of putative poses.

The prediction of protein-protein interactions and their structural configuration remains a largely unsolved problem. Most of the algorithms aimed at finding the native conformation of a protein complex starting from the structure of its monomers are based on searching the structure corresponding to the global minimum of a suitable scoring function. However, protein complexes are often highly flexible, with mobile side chains and transient contacts due to thermal fluctuations. Flexibility can be neglected if one aims at finding quickly the approximate structure of the native complex, but may play a role in structure refinement, and in discriminating solutions characterized by similar scores. We here benchmark the capability of some state-of-the-art scoring functions (BACH-SixthSense, PIE/PISA and Rosetta) in discriminating finite-temperature ensembles of structures corresponding to the native state and to non-native configurations. We produce the ensembles by running thousands of molecular dynamics simulations in explicit solvent starting from poses generated by rigid docking and optimized in vacuum. We find that while Rosetta outperformed the other two scoring functions in scoring the structures in vacuum, BACH-SixthSense and PIE/PISA perform better in distinguishing near-native ensembles of structures generated by molecular dynamics in explicit solvent. Proteins 2016; 84:1312-1320. © 2016 Wiley Periodicals, Inc.

Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface.

Methylamine is an abundant amine compound detected in the atmosphere which can affect the nature of atmospheric aerosol surfaces, changing their chemical and optical properties. Molecular dynamics simulation results show that methylamine accommodation on water is close to unity with the hydrophilic head group solvated in the interfacial environment and the methyl group pointing into the air phase. A detailed analysis of the hydrogen bond network indicates stronger hydrogen bonds between water and the primary amine group at the interface, suggesting that atmospheric trace gases will likely react with the methyl group instead of the solvated amine site. These findings suggest new chemical pathways for methylamine acting on atmospheric aerosols in which the methyl group is the site of orientation specific chemistry involving its conversion into a carbonyl site providing hydrophilic groups for uptake of additional water. This conversion may explain the tendency of aged organic aerosols to form cloud condensation nuclei. At the same time, formation of NH2 radical and formaldehyde is suggested to be a new source for NH2 radicals at aerosol surfaces, other than by reaction of absorbed NH3. The results have general implications for the chemistry of other amphiphilic organics, amines in particular, at the surface of atmospherically relevant aerosols.

Mechanism of anisotropic surface self-diffusivity at the prismatic ice-vapor interface.

Predictive theoretical models for mesoscopic roughening of ice require improved understanding of attachment kinetics occurring at the ice-vapor interface. Here, we use classical molecular dynamics to explore the generality and mechanics of a transition from anisotropic to isotropic self-diffusivity on exposed prismatic surfaces. We find that self-diffusion parallel to the crystallographic a-axis is favored over the c-axis at sub-melt temperatures below about -35 °C, for three different representations of the water-water intermolecular potential. In the low-temperature anisotropic regime, diffusion results from interstitial admolecules encountering entropically distinct barriers to diffusion in the two in-plane directions. At higher temperatures, isotropic self-diffusion occurring deeper within the quasi-liquid layer becomes the dominant mechanism, owing to its larger energy of activation.

Ab initio study of the reaction of ozone with bromide ion.

Surface level ozone destruction in polar environments may be initiated by oxidation of bromide ions by ozone, ultimately leading to Br2 production. Ab initio calculations are used to support the development of atmospheric chemistry models, but errors can occur in study of the bromide-ozone reaction due to inappropriate treatment of the many-electron species and the ionic nature of the reaction. In this work, a high level ab initio study is used to take into account the electronic correlation and the polarization effects. Our results show three possible pathways for the reaction. In particular, we find that this process, though endothermic on the singlet spin state surface, can be energetically feasible on the triplet surface. The triplet surface can be reached through photoexcitation of ozone or by the spin crossing of the potential energy surface. Because this process is known to occur in the dark, it may be that it occurs after intersystem crossing to a triplet surface. This paper also provides a starting point calibration for any future ab initio calculation studies of the bromide-ozone reaction, from the gas to the condensed phase.

Adsorption, mobility, and self-association of naphthalene and 1-methylnaphthalene at the water-vapor interface.

The adsorption, mobility, and self-association of naphthalene (NPH) and 1-methylnaphthalene (1MN), two of the simplest polycyclic aromatic hydrocarbons (PAHs), at the surface of liquid water at 289 K were investigated using classical molecular dynamics (MD) simulations and free energy profile calculations across the water-vapor interface. Both NPH and 1MN, which exhibit a strong preference to be adsorbed at the water-vapor interface, are found to readily self-associate at the water surface, adopting mostly configurations with distinctly nonparallel arrangement of the two monomers. The additional methyl group of 1MN represents only a minor perturbation in terms of the hydration properties, interfacial orientation, and self-association with respect to NPH. Implications of the observed self-association behavior for fluorescence spectroscopy of NPH and 1MN in aqueous interfacial environment are discussed.

Spectroscopic properties of benzene at the air-ice interface: a combined experimental-computational approach.

A combined experimental and computational approach was used to study the spectroscopic properties of benzene at the ice-air interface at 253 and 77 K in comparison with its spectroscopic behavior in aqueous solutions. Benzene-contaminated ice samples were prepared either by shock-freezing of benzene aqueous solutions or by benzene vapor-deposition on pure ice grains and examined using UV diffuse reflectance and emission spectroscopies. Neither the absorption nor excitation nor emission spectra provided unambiguous evidence of benzene associates on the ice surface even at a higher surface coverage. Only a small increase in the fluorescence intensity in the region above 290 nm found experimentally might be associated with formation of benzene excimers perturbed by the interaction with the ice surface as shown by ADC(2) excited-state calculations. The benzene associates were found by MD simulations and ground-state DFT calculations, although not in the arrangement that corresponds to the excimer structures. Our experimental results clearly demonstrated that the energy of the S0 → S1 electronic transition of benzene is not markedly affected by the phase change or the microenvironment at the ice-air interface and its absorption is limited to the wavelengths below 268 nm. Neither benzene interactions with the water molecules of ice nor the formation of dimers and microcrystals at the air-ice interface thus causes any substantial bathochromic shift in its absorption spectrum. Such a critical evaluation of the photophysical properties of organic contaminants of snow and ice is essential for predictions and modeling of chemical processes occurring in polar regions.

Addressing the Effectiveness and Molecular Mechanism of the Catalytic CO2 Hydration in Aqueous Solutions by Nickel Nanoparticles.

Hydration of carbon dioxide in water solution is the rate limiting step for the CO2 mineralization process, a process which is at the base of many carbon capture and utilization (CCU) technologies aiming to convert carbon dioxide to added-value products and mitigate climate change. Here, we present a combined experimental and computational study to clarify the effectiveness and molecular mechanism by which nickel nanoparticles, NiNPs, may enhance CO2 hydration in aqueous solutions. Contrary to previous literature, our kinetic experiments recording changes of pHs, conductivity, and dissolved carbon dioxide in solution reveal a minimal effect of the NiNPs in catalyzing CO2 hydration. Our atomistic simulations indicate that the Ni metal surface can coordinate only a limited number of water molecules, leaving uncoordinated metal sites for the binding of carbon dioxide or other cations in solution. This deactivates the catalyst and limits the continuous re-formation of a hydroxyl-decorated surface, which was a key chemical step in the previously suggested Ni-catalyzed hydration mechanism of carbon dioxide in aqueous solutions. At our experimental conditions, which expand the investigation of NiNP applicability toward a wider range of scenarios for CCU, NiNPs show a limited catalytic effect on the rate of CO2 hydration. Our study also highlights the importance of the solvation regime: while Ni surfaces may accelerate carbon dioxide hydration in water restricted environments, it may not be the case in fully hydrated conditions.

Adsorbed Water Promotes Chemically Active Environments on the Surface of Sodium Chloride.

Gas-particle interfaces are chemically active environments. This study investigates the reactivity of SO2 on NaCl surfaces using advanced experimental and theoretical methods with a NH4Cl substrate also examined for cation effects. Results show that NaCl surfaces rapidly convert to Na2SO4 with a new chlorine component when exposed to SO2 under low humidity. In contrast, NH4Cl surfaces have limited SO2 uptake and do not change significantly. Depth profiles reveal transformed layers and elemental ratios at the crystal surfaces. The chlorine species detected originates from Cl- expelled from the NaCl crystal structure, as determined by atomistic density functional theory calculations. Molecular dynamics simulations highlight the chemically active NaCl surface environment, driven by a strong interfacial electric field and the presence of sub-monolayer water coverage. These findings underscore the chemical activity of salt surfaces and the unexpected chemistry that arises from their interaction with interfacial water, even under very dry conditions.

Comparison of selected polarizable and nonpolarizable water models in molecular dynamics simulations of ice I(h).

We present a molecular dynamics simulation study in which we determined the melting point of ice I(h) for the polarizable SWM4-NDP water model (Lamoureux et al., Chem. Phys. Lett., 2006, 418, 245-249) and compared the performance of several popular water force fields, both polarizable and nonpolarizable, in terms of melting temperature, stability and orientational structuring of ice. The simulations yield the melting temperature of SWM4-NDP ice as low as T(m) = 185 ± 10 K, despite the quadrupole moment of a SWM4-NDP water molecule being close to the experimental gas phase value. The results thus show that the dependence of T(m) on the molecular quadrupole, observed for the three- and four-site water models, is generally lost if polarization is explicitly included. The study also shows that adding polarizability to a planar three-charge water model increases orientational disorder in hexagonal ice. In addition, analysis of the tetrahedral order in bulk ice reveals a correlation between the pre-existing degree of orientational disorder in ice simulated using different polarizable and nonpolarizable models and the melting temperature of the models. Our findings thus suggest some new considerations regarding the role of polarization forces in a crystalline solid that may guide future development of reliable polarizable water models for ice.

Orcinol and resorcinol induce local ordering of water molecules near the liquid-vapor interface.

Resorcinol and orcinol are simple members of the family of phenolic compounds present in particulate matter in the atmosphere; they are amphiphilic in nature and thus surface active in aqueous solution. Here, we used X-ray photoelectron spectroscopy to probe the concentration of resorcinol (benzene-1,3-diol) and orcinol (5-methylbenzene-1,3-diol) at the liquid-vapor interface of aqueous solutions. Qualitatively consistent surface propensity and preferential orientation was obtained by molecular dynamics simulations. Auger electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to probe the hydrogen bonding (HB) structure, indicating that the local structure of water molecules near the surface of the resorcinol and orcinol solutions tends towards a larger fraction of tetrahedrally coordinated molecules than observed at the liquid-vapor interface of pure water. The order parameter obtained from the molecular dynamics simulations confirm these observations. This effect is being discussed in terms of the formation of an ordered structure of these molecules at the surface leading to patterns of hydrated OH groups with distances among them that are relatively close to those in ice. These results suggest that the self-assembly of phenolic species at the aqueous solution-air interface could induce freezing similar to the case of fatty alcohol monolayers and, thus, be of relevance for ice nucleation in the atmosphere. We also attempted at looking at the changes of the O 1b1, 3a2 and 1b2 molecular orbitals of liquid water, which are known to be sensitive to the HB structure as well, in response to the presence of resorcinol and orcinol. However, these changes remained negligible within uncertainty for both experimentally obtained valence spectra and theoretically calculated density of states.

XPS, structural and antimicrobial studies of novel functionalized halloysite nanotubes.

A novel robust preparation method based on thermal salt decomposition has been elaborated for synthesis of halloysite nanotubes (HNTs) impregnated with silver and iron oxide nanoparticles. The developed method is simple, time-effective, and can be employed for large scale material fabrication. Different characterization techniques, including X-ray diffraction (XRD), scanning and transmission electron spectroscopy (SEM and TEM) and energy dispersive X-ray spectroscopy (EDS) have been used to characterize the functionalized HNTs composite materials. Surface elemental and chemical state analysis was conducted using X-ray photoelectron spectrometer (XPS). The functionalized HNTs exhibit enhanced total surface area (by 17.5%) and pore volume (by 11%) compare to the raw HNTs calculated by using the Brunauer-Emmett-Teller (BET) method. It was shown that functionalized HNTs possess high antimicrobial properties towards both gram- positive and gram-negative bacteria species. The enhanced surface area and bactericidal properties of functionalized HNTs could be beneficial for employing of the prepared material as low cost filtration media for water treatment applications. Molecular dynamics (FPMD) were performed to obtain insights about possible physiochemical mechanisms for chemical adsorption and on the HNT thermal stability.

Reply to "Comment on 'Liquid-Gas Interface of Iron Aqueous Solutions and Fenton Reagents'".

A Viewpoint regarding our recently published Letter in this Journal (Gladich et al. J. Phys. Chem. Lett. 13, 2994-3001) criticizes some of our conclusions. While a sentence in the abstract and one in the conclusion of the Letter might seem too conclusive, in the body text we objectively discussed experimental results obtained by means of three different surface-/interface-sensitive spectroscopies. Such results were supported by theoretical calculations. The aim of our work was not to criticize past results. On the contrary, we critically discussed our data taking into account those obtained in the past.