Mapping out the aqueous surface chemistry of metal oxide nanocrystals; carboxylate, phosphonate and catecholate ligands

Iron oxide and hafnium oxide nanocrystals are two of the few successful examples of inorganic nanocrystals used in a clinical setting. Although crucial to their application, their aqueous surface chemistry is not fully understood. The literature contains conflicting reports regarding the optimum binding group. To alleviate these inconsistencies, we set out to systematically investigate the interaction of carboxylic acids, phosphonic acids and catechols to metal oxide nanocrystals in polar media. Using Nuclear Magnetic Resonance spectroscopy and Dynamic Light Scattering, we map out the pH-dependent binding affinity of the ligands towards hafnium oxide nanocrystals (an NMR compatible model system). Carboxylic acids easily desorb in water from the surface and only provide limited colloidal stability from pH 2 – 6. Phosphonic acids on the other hand provide colloidal stability over a broader pH range but also feature a pH-dependent desorption from the surface. They are most suited for acidic to neutral environments (pH < 8). Finally, nitrocatechol derivatives provide a tightly bound ligand shell and colloidal stability at physiological and basic pH (6-10). While dynamically bound ligands (carboxylates and phosphonates) do not provide colloidal stability in phosphate buffered saline, the tightly bound nitrocatechols provide long term stability. We thus shed light on the complex ligand binding dynamics on metal oxide nanocrystals in aqueous environments. Finally, we provide a practical colloidal stability map, guiding researchers to rationally design ligands for their desired application.

Compound jetting from bubble bursting at an air-oil-water interface

Bursting of bubbles at a liquid surface is ubiquitous in a wide range of physical, biological, and geological phenomena, as a key source of aerosol droplets for mass transport across the interface. However, how a structurally complex interface, widely present in nature, mediates the bursting process remains largely unknown. Here, we document the bubble-bursting jet dynamics at an oil-covered aqueous surface, which typifies the sea surface microlayer as well as an oil spill on the ocean. The jet tip radius and velocity are altered with even a thin oil layer, and oily aerosol droplets are produced. We provide evidence that the coupling of oil spreading and cavity collapse dynamics results in a multi-phase jet and the follow-up droplet size change. The oil spreading influences the effective viscous damping, and scaling laws are proposed to quantify the jetting dynamics. Our study not only advances the fundamental understanding of bubble bursting dynamics, but also may shed light on the airborne transmission of organic matters in nature related to aerosol production.

Heterogeneous uptake of NH3 on ambient PM2.5 in Beijing and Shijiazhuang: Possible influence of aerosol acidity

&lt;p&gt;Ammonium salts (NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;) is the important component of PM&lt;sub&gt;2.5&lt;/sub&gt; and has a significant impact on air quality, climate, human health, and natural ecosystems. The contribution of NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; to PM&lt;sub&gt;2.5&lt;/sub&gt; is increasing at urban sites. Ammonia (NH&lt;sub&gt;3&lt;/sub&gt;) with global emissions estimated at greater than 33 Tg(N) Yr&lt;sup&gt;-1&lt;/sup&gt; is the only precursor of particulate NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; in the atmosphere. Thus, it is important to understand the conversion kinetics from NH&lt;sub&gt;3&lt;/sub&gt; to NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; in the atmosphere. However, the uptake coefficient of NH&lt;sub&gt;3&lt;/sub&gt; (&amp;#947;&lt;sub&gt;NH3&lt;/sub&gt;) on aerosol particles are scarce at the present time. In this work, we reported the &amp;#947;&lt;sub&gt;NH3&lt;/sub&gt; on ambient PM&lt;sub&gt;2.5&lt;/sub&gt; in Beijing and Shijiazhuang in China. The &amp;#947;&lt;sub&gt;NH3&lt;/sub&gt; values on ambient PM&lt;sub&gt;2.5&lt;/sub&gt; are (1.13&amp;#177;12.4)&amp;#215;10&lt;sup&gt;-4&lt;/sup&gt; and (6.88&amp;#177;40.7)&amp;#215;10&lt;sup&gt;-4&lt;/sup&gt; in Shijiazhuang and Beijing, respectively. They are significantly lower than those on sulfuric acid droplet (0.1-1), aqueous surface (~5&amp;#215;10&lt;sup&gt;-3&lt;/sup&gt;-0.1) and acidified secondary organic aerosol (~10&lt;sup&gt;-3&lt;/sup&gt;-~10&lt;sup&gt;-2&lt;/sup&gt;), while are comparable with that on ice surface (5.3&amp;#177;2.2 &amp;#215;10&lt;sup&gt;-4&lt;/sup&gt;) and on sulfuric acid in the presence of organic gases (2&amp;#215;10&lt;sup&gt;-4&lt;/sup&gt;-4&amp;#215;10&lt;sup&gt;-3&lt;/sup&gt;). An annual increase of &amp;#947;&lt;sub&gt;NH3&lt;/sub&gt; in the statistic sense is observed and the possible reason related to the aerosol acidity has also been discussed.&lt;/p&gt;

Surface streaming on nonbreaking wind waves

&lt;p&gt;The energetic, coherent vortical motions in the aqueous surface layer beneath the wind waves dominate the liquid-phase controlled transport processes across the air-water interface. Through interacting with the interface, these coherent vortices manifest themselves by forming quasi-streamwise, high-speed streaks on the wind waves. The density of these streamwise streaks, which can be quantified by the transverse spacing of streaks, thus characterizes the interfacial transfer contributed by the coherent vortices. The formation of surface streaming on the wind waves is geometrically similar to the low-speed streaks observed in the turbulent wall layers. It is generally accepted that the mean spanwise spacing between these low-speed streaks, when scaled by the viscous length, would exhibit a universal value of 100. Observations in wind-wave flumes, however, show that the transverse scale between high-speed streaming on nonbreaking wind waves is narrower than that between low-speed streaks next to no-slip wall. Comparative numerical simulations of shear flow bounded by flat and wavy surfaces are conducted to explain the variation. Analysis of the vorticity transport in the simulated flows bounded by a wavy surface reveals that the presence of surface waves enhances the production of streamwise enstrophy and, consequently, intensifies the generation of quasi-streamwise vortices that form the elongated streaks.&lt;br&gt;This work is supported by the Taiwan Ministry of Science and Technology (107-2611-M-002 -014 -MY3 and 110-2923-M-002 -014 -MY3).&lt;/p&gt;

Proton egress pathway during the S1–S2 transition of the Oxygen Evolving Complex of Photosystem II

Photosystem II uses water as the ultimate electron source of the photosynthetic electron transfer chain. Water is oxidized to dioxygen at the Oxygen Evolving Complex (OEC), a Mn4CaO5 inorganic core embedded in the lumenal side of PSII. Water-filled channels are thought to bring in substrate water molecules to the OEC, remove the substrate protons to the lumen, and may transport the product oxygen. Three water-filled channels, denoted large, narrow, and broad, that extend from the OEC towards the aqueous surface more than 15 Å away are seen. However, the actual mechanisms of water supply to the OEC, the removal of protons to the lumen and diffusion of oxygen away from the OEC have yet to be established. Here, we combine Molecular Dynamics (MD), Multi Conformation Continuum Electrostatics (MCCE) and Network Analysis to compare and contrast the three potential proton transfer paths during the S1 to S2 transition of the OEC. Hydrogen bond network analysis shows that the three channels are highly interconnected with similar energetics for hydronium as calculated for all paths near the OEC. The channels diverge as they approach the lumen, with the water chain in the broad channel better interconnected that in the narrow and large channels, where disruptions in the network are observed at about 10 Å from the OEC. In addition, the barrier for hydronium translocation is lower in the broad channel, suggesting that a proton from the OEC could access the paths near the OEC, and likely exit to the lumen via the broad channel, passing through PsbO.

Solution chemistry in the surface region of aqueous solutions

Solution chemistry is commonly regarded as the physical chemistry of reactions and chemical equilibria taking place in the bulk of a solvent, and between solutes in solution, and solids or gases in contact with the solution. Our knowledge about such reactions and equilibria in aqueous solution is very detailed such as their physico–chemical constants at varying temperature, pressure, ionic medium and strength. In this paper the solution chemistry in the surface region of aqueous solutions, down to ca. 10 Å below the water–air interface, will be discussed. In this region, the density and relative permittivity are significantly smaller than in the aqueous bulk strongly affecting the chemical behaviour of solutes. Surface sensitive X-ray spectroscopic methods have recently been applicable on liquids and solutions by use of liquid jets. This allows the investigation of the speciation of compounds present in the water–air interface and the surface region, a region hardly studied before. Speciation studies show overwhelmingly that neutral molecules are accumulated in the surface region, while charged species are depleted from it. It has been shown that the equilibria between aqueous bulk, surface region, solids and/or air are very fast allowing effective transport of chemicals over the aqueous surface region.