Mechanistic Insights into the Reactive Uptake of Chlorine Nitrate at the Air-Water Interface

Atmospheric Hydroxyl Radical Route Revealed: Interface-Mediated Effects of Mineral-Bearing Microdroplet Aerosol

Hydroxyl radical (·OH) plays a crucial role in atmospheric chemistry, regulating the oxidative potential and aerosol composition. This study reveals an unprecedented source of ·OH in the atmosphere: mineral dust-bearing microdroplet aerosols. We demonstrate that Kaolin clay particles in microdroplet aerosols trigger rapid ·OH production upon solar irradiation, with rates reaching an order of at least 10-3 M s-1. This production rate is several orders of magnitude higher than that of the bulk phase (2.4 × 10-11 M s-1) and previously known pathways. On this basis, the surface-based interfacial ·OH production rate is estimated to be 8.9 × 10-5 mol m-2 s-1 at the air-water-solid interface of 1 μm sized aerosol particles. The enhanced ·OH formation is attributed to the unique features of air-water-solid interfaces, where the lifespan of photoinduced holes was significantly increased due to the presence of strong electric fields at the air-water interface. We further investigated the impacts of various environmental factors and aerosol properties on ·OH production, including light intensity, relative humidity, particle size, and pH. Our findings provide new insights into atmospheric photochemical processes mediated by mineral dust-bearing microdroplet aerosols, which are important contributors to ·OH source in the atmosphere. This work advances our understanding of atmospheric interfacial chemistry and its profound and lasting implications for air quality and climate.

Promoting Cl2O Generation from the HOCl + HOCl Reaction on Aqueous/Frozen Air-Water Interfaces

Hypochlorous acid (HOCl) is considered a temporary reservoir of dichlorine monoxide (Cl2O). Previous studies have suggested that Cl2O is difficult to generate from the reaction of HOCl + HOCl in the gas phase. Here, we demonstrate that Cl2O can be generated from the HOCl + HOCl reaction at aqueous/frozen air-water interfaces, which is confirmed by ab initio molecular dynamic calculations. Distinct from the one-step reaction in the gas phase, our results show that Cl2O generation from HOCl + HOCl on aqueous/frozen interfaces involves two elementary steps, namely, one HOCl deprotonation and one Cl-abstraction from the other HOCl. Specifically, the mechanisms of neutral/acidic catalysis from interfacial water/nitric acid and base catalysis from ammonia, methylamine and dimethylamine have been examined. For the former, HOCl deprotonation is the rate-limiting step, and the total k of Cl2O generation increases to 9.23 × 10-9-9.10 × 10-1 M-1 s-1 at the aqueous interface and 3.20 × 10-7-4.10 × 10-3 M-1 s-1 at the frozen interface, which is at least 23 and 25 orders of magnitude greater than that of gaseous k (3.31 × 10-32 M-1 s-1). For the latter, the rate-limiting step is changed to Cl-abstraction, whose total k dramatically increases to 1.40-8.97 × 107 M-1 s-1 at the aqueous interface and 7.12-9.99 × 106 M-1 s-1 at the frozen interface. Interestingly, the Cl2O production rates ranked in the order of dimethylamine < methylamine < ammonia and decreased with increasing catalytic alkalinity. These findings provide new insights for understanding other Cl2O sources beyond the ClONO2 + HOCl reaction.

Molecular dynamics investigation of IEPOX chemical behavior at the interface and in the bulk phase of acidic aerosols

Isoprene epoxydiol (IEPOX) is an important reactive gas-phase intermediate produced by the photooxidation of isoprene under low NOx conditions, playing a key role in the formation of secondary organic aerosols (SOA). Previous studies have mostly focused on the liquid-phase reactions of IEPOX within aerosols; however, interfacial heterogeneous chemical reactions are equally important in SOA formation. This study systematically explores the reaction mechanisms of IEPOX at the acidic aerosol interface and in the bulk phase using classical molecular dynamics (MD) and ab initio molecular dynamics simulations (AIMD). The study found that the free energy of IEPOX at the aerosol interface significantly decreases, indicating that interfacial heterogeneous chemical reactions are indispensable for the formation of IEPOX-derived SOA. The research reveals the formation pathways of 2-methyltetrols (2-MTO) and 1,3,4-trihydroxy-3-methylbutan-2-yl sulfates (2-MTOOS), finding that the protonation of the epoxy O atom and the cleavage of the C-O bond are the rate-controlling steps, while the nucleophilic addition is a spontaneous process. Through multiple sets of simulations, it was observed that the formation frequency of 2-MTO at the acidic aerosol interface and in the bulk phase reached 53.8%, significantly higher than the 30.8% of 2-MTOOS, which is consistent with field observation data. Additionally, through metadynamics (MTD) simulations, it was suggested that IEPOX could undergoes acid-catalyzed ring-opening reactions at the interface, potentially followed by the transfer of H atoms from primary alcohols into the aerosol, leading to the possible formation of the intermediate product 3-methylbut-3-ene-1,2,4-triol (one of the proposed structures of C5-alkene triols). These findings provide new insights into the formation mechanism of IEPOX-derived SOA and offer a scientific basis for future studies on their physicochemical properties and atmospheric fate.

Two-Step Noncatalyzed Hydrolysis Mechanism of Imines at the Air-Water Interface

The hydrolysis of imines has long been assumed to be their main atmospheric fate, based on early studies in the field of organic chemistry. However, the hydrolysis mechanism and kinetics of atmospheric imines remain unclear. Here, an advanced Born-Oppenheimer molecular dynamics method was employed to investigate the noncatalyzed hydrolysis mechanism and kinetics at the air-water interface by selecting CH2NH as a model molecule. The results indicate that CH2NH exhibits a pronounced surface preference. The noncatalyzed hydrolysis of CH2NH follows a unique two-step reaction mechanism involving first proton transfer and then OH- transfer through the water bridge at the air-water interface, in contrast to the traditional one-step mechanism. The calculated reaction rate for the rate-determining step is 3.32 × 105 s-1, which is 2 orders of magnitude greater than that of the bulk phase. In addition, the involvement of the interfacial electric field further enhances the reaction rate by approximately 3 orders of magnitude. The noncatalyzed hydrolysis rate at both the air-water interface and the bulk phase is higher than that of the possible acid-catalyzed one, clarifying noncatalyzed hydrolysis as the dominant mechanism for CH2NH. This study elucidates that the noncatalyzed hydrolysis of atmospheric imines is feasible at the air-water interface and that the revealed unique two-step hydrolysis mechanism has significant implications in atmospheric and water environmental chemistry.

Harnessing air-water interface to generate interfacial ROS for ultrafast environmental remediation

The air-water interface of microbubbles represents a crucial microenvironment that can dramatically accelerate reactive oxidative species (ROS) reactions. However, the dynamic nature of microbubbles presents challenges in probing ROS behaviors at the air-water interface, limiting a comprehensive understanding of their chemistry and application. Here we develop an approach to investigate the interfacial ROS via coupling microbubbles with a Fenton-like reaction. Amphiphilic single-Co-atom catalyst (Co@SCN) is employed to efficiently transport the oxidant peroxymonosulfate (PMS) from the bulk solution to the microbubble interface. This triggers an accelerated generation of interfacial sulfate radicals (SO4•-), with 20-fold higher concentration (4.48 × 10-11 M) than the bulk SO4•-. Notably, the generated SO4•- is preferentially situated at the air-water interface due to its lowest free energy and the strong hydrogen bonding interactions with H3O+. Moreover, it exhibits the highest oxidation reactivity toward gaseous pollutants like toluene, with a rate constant of 1010 M-1 s-1-over 100 times greater than bulk reactions. This work demonstrates a promising strategy to harness the air-water interface for accelerating ROS-induced reactions, highlighting the importance of interfacial ROS and its potential application.

Accelerating Toluene Oxidation over Boron-Titanium-Oxygen Interface: Steric Hindrance of the Methyl Group Induced by the Plane-Adsorption Configuration

Elimination of dilute gaseous toluene is one of the critical concerns within the field of indoor air remediation. The typical degradation route on titanium-based catalysts, "toluene-benzaldehyde-carbon dioxide", necessitates the oxidation of the methyl group as a prerequisite for photocatalytic toluene oxidation. However, the inherent planar adsorption configuration of toluene molecules, dominated by the benzene rings, leads to significant steric hindrance for the methyl group. This steric hindrance prevents the methyl group from contacting the active species on the catalyst surface, thereby limiting the removal of toluene under indoor conditions. To date, no effective strategy to control the steric hindrance of the methyl group has been identified. Herein, we showed a B-Ti-O interface that exhibits significantly enhanced toluene removal efficiency under indoor conditions. In-depth investigations revealed that, compared to typical Ti-based photocatalysts, the steric hindrance between the methyl group and the catalyst surface decreased from 3.42 to 3.03 Å on the designed interface. This reduction originates from the matching of orbital energy levels between Ti 3dz2 and C 2pz of the benzene ring. The decreased steric hindrance improved the efficiency of toluene being attacked by surface active species, allowing for rapid conversion into benzaldehyde and benzoic acid species for subsequent reactions. Our work provides novel insights into the steric hindrance effect in the elimination of aromatic volatile organic compounds.

Mechanistic Insights into N2O5-Halide Ions Chemistry at the Air-Water Interface

The activation of halogens (X = Cl, Br, I) by N2O5 is linked to NOx sources, ozone concentrations, NO3 reactivity, and the chemistry of halide-containing aerosol particles. However, a detailed chemical mechanism is still lacking. Herein, we explored the chemistry of the N2O5···X- systems at the air-water interface. Two different reaction pathways were identified for the reaction of N2O5 with X- at the air-water interface: the formation of XNO2 or XONO, along with NO3-. In the case of the Cl- system, the ClNO2 generation pathway is more favorable, while for the Br- and I- systems, the formation of BrONO and IONO is barrierless, making them the predominant products. Furthermore, the mechanisms of formation of X2 from XNO2 and XONO were also investigated. The high energy barriers of reactions and the high free energies of the products compared to those of the reactants indicate that ClNO2 is stable at the air-water interface. Contrary to the widely held belief regarding X2 producing from the reaction of XNO2 with X-, our calculations demonstrate that BrONO and IONO initially form stable BrONO···Br- and IONO···I- complexes, which then subsequently react with Br- and I- to form Br3- and I3-, respectively. Finally, Br3- and I3- decompose to form Br2 and I2. These findings have significant implications for experimental interpretation and offer new insights into halogen cycling in the atmosphere.

Building Functional Liquid-Based Interfaces: From Mechanism to Application

Functional liquid-based interfaces, with their inhomogeneous regions that emphasize the functionalized liquids, have attracted much interest as a versatile platform for a broad spectrum of applications, from chemical manufacturing to practical uses. These interfaces leverage the physicochemical characteristics of liquids, alongside dynamic behaviors induced by macroscopic wettability and microscopic molecular exchange balance, to allow for tailored properties within their functional structures. In this Minireview, we provide a foundational overview of these functional interfaces, based on the structural investigations and molecular mechanisms of interaction forces that directly modulate functionalities. Then, we discuss design strategies that have been employed in recent applications, and the crucial aspects that require focus. Finally, we highlight the current challenges in functional liquid-based interfaces and provide a perspective on future research directions.

Mechanistic Insights into Chloric Acid Production by Hydrolysis of Chlorine Trioxide at an Air-Water Interface

Chlorine oxides play crucial roles in ozone depletion, and the final oxidation steps of chlorine oxide potentially result in the formation of chloric acid (HClO3) or perchloric acid (HClO4). Herein, the solvation and reactive uptake of three stable isomers of chlorine trioxide (Cl2O3), namely, ClOCl(O)O, ClClO3, and ClOOOCl, at the air-water interface were investigated using classical and hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) coupled with advanced free energy methods. Two distinct mechanisms were revealed for the hydrolysis of ClOCl(O)O and ClClO3: molecular and ionic mechanisms. A comparison of the computed free-energy profiles for the gaseous and air-water interfacial systems indicated that the air-water interface could markedly lower the free-energy barrier for ClO 3 - or HClO3 formation while stabilizing the product state. In particular, the hydrolysis of ClClO3 at the air-water interface was barrierless. In contrast, our calculations showed that the hydrolysis of ClOOOCl was very slow, indicating that ClOOOCl was inert to water at the air-water interface. This study provides theoretical evidence for the hypothesis that HClO3 is a sink for chlorine oxides and for the widespread distributions of HClO3 recently observed in the Arctic region.

Growth of millimeter-sized 2D metal iodide crystals induced by ion-specific preference at water-air interfaces

Conventional liquid-phase methods lack precise control in synthesizing and processing materials with macroscopic sizes and atomic thicknesses. Water interfaces are ubiquitous and unique in catalyzing many chemical reactions. However, investigations on two-dimensional (2D) materials related to water interfaces remain limited. Here we report the growth of millimeter-sized 2D PbI2 single crystals at the water-air interface. The growth mechanism is based on an inherent ion-specific preference, i.e. iodine and lead ions tend to remain at the water-air interface and in bulk water, respectively. The spontaneous accumulation and in-plane arrangement within the 2D crystal of iodide ions at the water-air interface leads to the unique crystallization of PbI2 as well as other metal iodides. In particular, PbI2 crystals can be customized to specific thicknesses and further transformed into millimeter-sized mono- to few-layer perovskites. Additionally, we have developed water-based techniques, including water-soaking, spin-coating, water-etching, and water-flow-assisted transfer to recycle, thin, pattern, and position PbI2, and subsequently, perovskites. Our water-interface mediated synthesis and processing methods represents a significant advancement in achieving simple, cost-effective, and energy-efficient production of functional materials and their integrated devices.

Mechanistic insight into the competition between interfacial and bulk reactions in microdroplets through N2O5 ammonolysis and hydrolysis

Reactive uptake of dinitrogen pentaoxide (N2O5) into aqueous aerosols is a major loss channel for NOx in the troposphere; however, a quantitative understanding of the uptake mechanism is lacking. Herein, a computational chemistry strategy is developed employing high-level quantum chemical methods; the method offers detailed molecular insight into the hydrolysis and ammonolysis mechanisms of N2O5 in microdroplets. Specifically, our calculations estimate the bulk and interfacial hydrolysis rates to be (2.3 ± 1.6) × 10-3 and (6.3 ± 4.2) × 10-7 ns-1, respectively, and ammonolysis competes with hydrolysis at NH3 concentrations above 1.9 × 10-4 mol L-1. The slow interfacial hydrolysis rate suggests that interfacial processes have negligible effect on the hydrolysis of N2O5 in liquid water. In contrast, N2O5 ammonolysis in liquid water is dominated by interfacial processes due to the high interfacial ammonolysis rate. Our findings and strategy are applicable to high-chemical complexity microdroplets.

Molecular Insights into the Spontaneous Generation of Cl2O in the Reaction of ClONO2 and HOCl at the Air-Water Interface

Chemical processes involving chlorine nitrate (ClONO2) at the surface of stratospheric aerosols are crucial to ozone depletion. Herein, we show a reaction route for the formation of Cl2O, which is a source of stratospheric chlorine, in the ClONO2 + HOCl reaction at the air-water interface. Our ab initio molecular dynamics (AIMD) simulations show that the (ClONO2)Cl···O(HOCl) halogen bond plays a key role in the reaction and is the main interaction between ClONO2 and HOCl both at the air-water interface and in the bulk liquid water. Furthermore, metadynamics-based AIMD simulations reveal two pathways: (i) The OCl fragment of HOCl binds to the Cl atom in ClONO2, resulting in the formation of Cl2O and NO3-. Simultaneously, the remaining hydrogen atom is transferred to a water molecule to form H3O+. (ii) HOCl acts as a bridge for Cl atom transfer from ClONO2 to the O atom of a water molecule, and this water molecule transfers one of its H atoms to another water molecule, forming two HOCl molecules, NO3-, and H3O+. Free-energy calculations show that the former is the energetically more favorable process. More importantly, the free-energy barrier for Cl2O formation at the air-water interface is only ∼0.8 kcal/mol, and the reaction is exothermic. These findings provide insights into the importance of fundamental chlorine chemistry and the broader implications of the aerosol air-water interface for atmospheric chemistry.