Catalyst-Free Decarboxylative Amination of Carboxylic Acids in Water Microdroplets

Heavy Water Microdroplet Surface Enriches the Lighter Isotopologue Impurities

Water microdroplets promote unusual chemical reactions at the air-water interface. However, the interfacial structure of water microdroplets and its potential influence on chemical processes are still enigmatic. Here, we present evidence of in-droplet fractionation of water isotopologues. Employing a sonic spray, we atomized the heavy water (D2O, 99.9 atom % D) solution of three classes of organic compounds (basic, acidic, and neutral). The analytes were predominantly desorbed from the resulting droplet surface in protonated form rather than deuterated form, as detected by mass spectrometry. This result remained unaltered upon adding formic acid-d2 (DCOOD) to the droplet. Monitoring Dakin oxidation of benzaldehyde at the surface of binary microdroplets composed of 1:1 (v/v) D2O/H218O revealed the preferred formation of phenolate-16O over phenolate-18O. Atmospheric pressure chemical ionization mass spectrometric analysis of the vapor composition in the sprayed aerosol revealed the preferential evaporation of lighter water isotopologue impurities from the surface of heavy water microdroplets. These results indicate the enrichment of lighter water isotopologue impurities (HOD/H2O) on the surface of heavy water microdroplets, implying possible future developments for water isotopologue fractionation using microdroplets.

Water Microdroplets in Air: A Hitherto Unnoticed Natural Source of Nitrogen Oxides

Water microdroplets are widespread in the atmosphere. We report a striking observation that micron-sized water droplets obtained from zero-volt spray sources (sonic spray, humidifier, spray bottle, steamer, etc.) spontaneously generate nitrogen oxides. The mechanistic investigation through the development of custom-designed sampling sources combined with mass spectrometry and isotope labeling experiments confirmed that air nitrogen reacts with the water at the air-water interface, fixing molecular nitrogen to its oxides (NO, NO2, and N2O) and acids (HNO2 and HNO3) at trace levels without any catalyst. These reactions are attributed to the consequence of an experimentally detected feeble corona discharge (breakdown of air) at the air-water interface, likely driven by the high intrinsic electric field at the surface of water microdroplets. The extent of this corona discharge effect varies depending on the pH, salinity/impurity, size, speed, and lifetime of microdroplets in the air. Thus, this study discloses that the air-water interface of microdroplets breaks the strong chemical bond of nitrogen (N2), producing nitrogen oxides in the environment, while lightning strikes and microbial processes in soil are considered their dominant natural sources. As nitrogen oxides are toxic air pollutants, their spontaneous formation at the air-water interface should have important implications in atmospheric reactions, requiring further investigations.

Spontaneous Oxidation in Aqueous Microdroplets: Water Radical Cation as Primary Oxidizing Agent

Exploration of the unique chemical properties of interfaces can unlock new understanding. A striking example is the finding of accelerated reactions, particularly spontaneous oxidation reactions, that occur without assistance of catalysts or external oxidants at the air interface of both aqueous and organic solutions (provided they contain some water). This finding opened a new area of interfacial chemistry but also caused heated debate regarding the primary chemical species responsible for the observed oxidation. An overview of the literature covering oxidation in microdroplets with air interfaces is provided, together with a critical examination of previous findings and hypotheses. The water radical cation/radical anion pair, formed spontaneously and responsible for the electric field at or near the droplet/air interface, is suggested to constitute the primary redox species. Mechanisms of accelerated microdroplet reactions are critically discussed and it is shown that hydroxyl radical/hydrogen peroxide formation in microdroplets does not require that these species be the primary oxidant. Instead, we suggest that hydroxyl radical and hydrogen peroxide are the products of water radical cation decay in water. The importance of microdroplet chemistry in the prebiotic environment is sketched briefly and the role of partial solvation in reaction acceleration is noted.

Pyrogenic Carbon Degradation by Galvanic Coupling with Sprayed Seawater Microdroplets

Surface waves are known for their mechanical role in coastal processes that influence the weather and climate. However, their chemical impact, particularly on the transformation of pyrogenic carbon, is poorly understood. Pyrogenic carbon is generally assumed to show negligible postformational alteration of its stable carbon isotope composition. Here we present an electrochemical interaction of pyrogenic carbon with the sprayed seawater microdroplets resulting from wave breaking, driven by the galvanic coupling between the microdroplet water-carbon interfaces and the microdroplet water-vapor interfaces. This enables refractory pyrogenic carbon to rapidly degrade via the oxygenation and mineralization reaction, which makes it ∼2.6‰ enriched in 13C, far exceeding the generally assumed postformation alteration values (<0.5‰) of pyrogenic carbon. The unique chemical dynamics of seawater microdroplets provide new insights into the discrepancy in carbon isotope signatures between riverine and marine black carbon, emphasizing the potential of coastal oceans for carbon sequestration in the global carbon cycle.

Interfacial chemical reactivity enhancement

Interfacial enhancements of chemical reaction equilibria and rates in liquid droplets are predicted using a combined theoretical and experimental analysis strategy. Self-consistent solutions of reaction and adsorption equilibria indicate that interfacial reactivity enhancement is driven primarily by the adsorption free energy of the product (or activated complex). Reactant surface activity has a smaller indirect influence on reactivity due to compensating reactant interfacial concentration and adsorption free energy changes, as well as adsorption-induced depletion of the droplet core. Experimental air-water interfacial adsorption free energies and critical micelle concentration correlations provide quantitative surface activity estimates as a function of molecular structure, predicting an increase in interfacial reactivity with increasing product size and decreasing product polarity, aromaticity, and charge (but less so for anions than cations). Reactions with small, neutral, or charged products are predicted to have little reactivity enhancement at an air-water interface unless the product is rendered sufficiently surface active by, for example, interactions with interfacial water dangling OH groups, charge transfer, or voltage fluctuations.

Superfast Formation of C(sp2 )-N, C(sp2 )-P, and C(sp2 )-S Vinylic Bonds in Water Microdroplets

We report examples of C(sp2 )-N, C(sp2 )-S, and C(sp2 )-P bond-forming transformations in water microdroplets at room temperature and atmospheric pressure using N2 as a nebulizing gas. When an aqueous solution of vinylic acid and amine is electrosprayed (+3 kV), the corresponding C(sp2 )-N product is formed in a single step, which was characterized using mass spectrometry (MS) and tandem mass spectrometry (MS2 ). The scope of this reaction was extended to other amines and other unsaturated acids, including acrylic (CH2 =CHCOOH) and crotonic (CH3 CH=CHCOOH) acids. We also found that thiols and phosphines are viable nucleophiles, and the corresponding C(sp2 )-S and C(sp2 )-P products are observed in positive ion mode using MS and MS2 .

Deciphering the Kinetics of Spontaneous Generation of H2O2 in Individual Water Microdroplets

Spontaneous generation of H2O2 in sub-10 μm-sized water microdroplets has received increasing interest since its first discovery in 2019. On the other hand, due to the short lifetime of these microdroplets (rapid evaporation) and lack of suitable tools to real-time monitor the generation of H2O2 in individual microdroplets, such a seemingly thermodynamically unfavorable process has also raised vigorous debates on the origin of H2O2 and the underlying mechanism. Herein, we prepared water microdroplets with a long lifetime (>1 h) by virtue of microwell confinement and dynamically monitored the spontaneous generation of H2O2 in individual microdroplets via time-lapsed fluorescence imaging. It was unveiled that H2O2 was continuously generated in the as-prepared water microdroplets and an apparent equilibrium concentration of ∼3 μM of H2O2 in the presence of a H2O2-consuming reaction can be obtained. Through engineering the geometry of these microdroplets, we further revealed that the generation rates of H2O2 in individual microdroplets were positively proportional to their surface-to-volume ratios. This also allowed us to extract a maximal H2O2 generation rate of 7.7 nmol m-2 min-1 in the presence of a H2O2-consuming reaction and derive the corresponding probability of spontaneous conversion of interfacial H2O into H2O2 for the first time, that is, ∼1 of 65,000 water molecules in 1 s. These findings delivered strong evidence that the spontaneous generation of H2O2 indeed occurs at the surface of microdroplets and provided us with an important starting point to further enhance the yield of H2O2 in water microdroplets for future applications.

Charged Microdroplets as Microelectrochemical Cells for CO2 Reduction and C-C Coupling

Charged microdroplets offer novel electrochemical environments, distinct from traditional solid-liquid or solid-liquid-gas interfaces, due to the intense electric fields at liquid-gas interfaces. In this study, we propose that charged microdroplets serve as microelectrochemical cells (MECs), enabling unique electrochemical reactions at the gas-liquid interface. Using electrospray-generated microdroplets, we achieved multielectron CO2 reduction and C-C coupling to synthesize ethanol using molecular catalysts. These catalysts effectively harness and relay electrons, enhancing the longevity of solvated electrons and enabling multielectron reactions. Importantly, we revealed the intrinsic relationship between the size and charge density of a MEC and its reaction selectivity. Employing in situ mass spectrometry, we identified reaction intermediates (molecular catalyst adducts with HCOO) and oxidation products, elucidating the CO2 reduction mechanism and the comprehensive reaction procedure. Our research underscores the promising role of charged microdroplets in pioneering new electrochemical systems.

Arc-Induced Electrospray Ionization Mass Spectrometry

Arc-induced electrospray ionization mass spectrometry (AESI-MS) was developed during which alternating current electrospray is simply achieved through the arc plasma. The AESI source exploits the arc's temperature and charge properties to generate aerosols consisting of charged microdroplets. The electrospray region, in which organic molecules are contained within microdroplets, partially overlaps with the arc plasma region. Guided by the electric field, these molecules undergo ionization, yielding ionic target analytes. AESI represents a soft ionization method that combines the mechanisms of atmospheric pressure chemical ionization and electrospray ionization, facilitating the ionization of analytes with wide ranging polarities. The precisely targeted spraying area enhances ion entry into the mass analyzer, thereby enabling excellent ionization efficiency. The AESI source exhibits several notable advantages over the electrospray ionization source, including an elevated but comparable level of active species concentrations and types, simplified mass spectra for direct amino acid analysis, high salt tolerance, versatile analysis of compounds with varying polarities, and reliable quantitative analysis of amino acids in complex matrices. Overall, AESI broadens the methodologies employed to generate microdroplets, providing a technological and scientific framework for creating distinctive electrospray ionization techniques.

Is Reaction Acceleration of Microdroplet Chemistry Favorable to Controlling the Enantioselectivity?

Microdroplet chemistry has been proven to amazingly accelerate many chemical and biological reactions in the past 2 decades. Current microdroplet accelerated reactions are predominantly symmetric synthetic but minorly asymmetric synthetic reactions, where stereoselectivity is scarcely concerned. This study selected unimolecular and bimolecular reactions, multicomponent Passerini reactions, and enzymatic ketone reduction as the model reactions to illustrate whether reaction acceleration of microdroplet chemistry is favorable to retaining a chiral center and controlling the enantioselectivity or not. The results illustrated that microdroplet chemistry did not disrupt pre-existing stereogenic centers in chiral starting materials during reactions but did harm to stereospecificity in asymmetric catalysis by chiral catalysts and chiral organic ligands with the exclusion of enzymatic reactions. Our preliminary study reminds us of more cautions to the product enantioselectivity when conducting asymmetric catalysis in microdroplets. We also hope this study may promote more valuable further research on the stereoselectivity of microdroplet chemistry.

Prebiotic synthesis of mineral-bearing microdroplet from inorganic carbon photoreduction at air-water interface

The origin of life on Earth is an enigmatic and intricate conundrum that has yet to be comprehensively resolved despite recent significant developments within the discipline of archaeology and geology. Chemically, metal-sulfide minerals are speculated to serve as an important medium for giving birth in early life, while yet so far direct evidence to support the hypothesis for the highly efficient conversion of inorganic carbon into praxiological biomolecules remains scarce. In this work, we provide an initial indication that sphalerite, employed as a typical mineral, shows its enormous capability for promoting the conversion of inorganic carbon into elementary biomolecule formic acid (HCOOH) in airborne mineral-bearing aerosol microdroplet, which is over two orders of magnitude higher than that of the corresponding conventional bulk-like aqueous phase medium in the environment (e.g. river, lake, sea, etc.). This significant enhancement was further validated by a wide range of minerals and clays, including CuS, NiS, CoS, CdS, MnS, elemental sulfur, Arizona Test Dust, loess, nontronite, and montmorillonite. We reveal that the abundant interface of unique physical-chemical features instinct for aerosol or cloud microdroplets reduces the reaction energy barrier for the reaction, thus leading to extremely high HCOOH production (2.52 × 1014 kg year-1). This study unfolds unrecognized remarkable contributions of the considered scheme in the accumulation of prebiotic biomolecules in the ancient period of the Earth.

Size-Dependent Sigmoidal Reaction Kinetics for Pyruvic Acid Condensation at the Air-Water Interface in Aqueous Microdroplets

The chemistry of pyruvic acid (PA) under thermal dark conditions is limited in bulk solutions, but in microdroplets it is shown to readily occur. Utilizing in situ micro-Raman spectroscopy as a probe, we investigated the chemistry of PA within aqueous microdroplets in a relative humidity- and temperature-controlled environmental cell. We found that PA undergoes a condensation reaction to yield mostly zymonic acid. Interestingly, the reaction follows a size-dependent sigmoidal kinetic profile, i.e., an induction period followed by reaction and then completion. The induction time is linearly proportional to the surface area (R2), and the maximum apparent reaction rate is proportional to the surface-to-volume ratio (1/R), showing that both the induction and reaction occur at the air-water interface. Furthermore, the droplet size is shown to be dynamic due to changes in droplet composition and re-equilibration with the relative humidity within the environmental cell as the reaction proceeds. Overall, the size-dependent sigmoidal kinetics, shown for the first time in microdroplets, demonstrates the complexity of the reaction mechanism and the importance of the air-water interface in the pyruvic acid condensation reaction.

Hydrocarbon Degradation by Contact with Anoxic Water Microdroplets

Oils are hydrophobic, but their degradation is frequently found to be accelerated in the presence of water microdroplets. The direct chemical consequences of water-oil contact have long been overlooked. We show that aqueous microdroplets in emulsified water-hexadecane (C16H34) mixtures can spontaneously produce CO2, •H, H2, and short-chain hydrocarbons (mainly C1 and C2) as detected by gas chromatography, electron paramagnetic resonance spectroscopy, and mass spectrometry. This reaction results from contact electrification at the water-oil microdroplet interface, in which reactive oxygen species are produced, such as hydrated hydroxyl radicals and hydrogen peroxide. We also find that the H2 originates from the water microdroplet and not the hydrocarbon it contacts. These observations highlight the potential of interfacial contact electrification to produce new chemistry.

Dual Tunability for Uncatalyzed N-Alkylation of Primary Amines Enabled by Plasma-Microdroplet Fusion

The fusion of non-thermal plasma with charged microdroplets facilitates catalyst-free N-alkylation for a variety of primary amines, without halide salt biproduct generation. Significant reaction enhancement (up to >200×) is observed over microdroplet reactions generated from electrospray. This enhancement for the plasma-microdroplet system is attributed to the combined effects of energetic collisions and the presence of reactive oxygen species (ROS). The ROS (e.g., O2 ⋅- ) act as a proton sink to increase abundance of free neutral amines in the charged microdroplet environment. The effect of ROS on N-alkylation is confirmed through three unique experiments: (i) utilization of radical scavenging reagent, (ii) characterization of internal energy distribution, and (iii) controls performed without plasma, which lacked reaction acceleration. Establishing plasma discharge in the wake of charged microdroplets as a green synthetic methodology overcomes two major challenges within conventional gas-phase plasma chemistry, including the lack of selectivity and product scale-up. Both limitations are overcome here, where dual tunability is achieved by controlling reagent concentration and residence time in the microdroplet environment, affording single or double N-alkylated products. Products are readily collected yielding milligram quantities in eight hours. These results showcase a novel synthetic strategy that represents a straightforward and sustainable C-N bond-forming process.

Detecting Early-Stage Intermediates of Free-Radical Oxidative Degradation in Charged Aqueous Microdroplets

We report the detection of early-stage intermediates of spontaneous free-radical oxidation of organic pollutants such as aliphatic amino alcohols and diamines in charged aqueous microdroplets in the ambient atmosphere. We propose that the intrinsic formation of reactive oxygen species at the air-water interface is responsible for the radical oxidation of the sp3 carbon. We suggest that our work will aid the understanding of the degradation mechanisms of organic molecules in the environment.

One-Step, Catalyst-Free Formation of Phenol from Benzoic Acid Using Water Microdroplets

Benzoic acid dissolved in water is electrosprayed (-4 kV) by using nitrogen gas at a pressure of 120 psi to form ∼10 μm diameter microdroplets. Analysis with mass spectrometry (MS) and tandem mass spectrometry (MS2) of the resulting microdroplets shows the direct formation of phenol via decarboxylation without any catalyst or added reagents. This process represents an ecofriendly, environmentally benign method for producing phenol and related aromatic alcohols from their corresponding aromatic acids. The mechanism of this transformation was unambiguously characterized using mass spectrometry, radical trapping, and 18O labeling.

Water Reactivity in Electrified Interfaces: The Simultaneous Production of Electricity, Hydrogen, and Hydrogen Peroxide at Room Temperature

Hygroelectric cells deliver hydrogen, hydrogen peroxide, and electric current simultaneously at room temperature from liquid water or vapor. Different cell arrangements allowed the electrical measurements and the detection and measurement of the reaction products by two methods each. Thermodynamic analysis shows that water dehydrogenation is a non-spontaneous reaction under standard conditions, but it can occur within an open, non-electroneutral system, thus supporting the experimental results. That is a new example of chemical reactivity modification in charged interfaces, analogous to the hydrogen peroxide formation in charged aqueous aerosol droplets. Extension of the experimental methods and the thermodynamic analysis used in this work may allow the prediction of interesting new chemical reactions that are otherwise unexpected. On the other hand, this adds a new facet to the complex behavior of interfaces. Hygroelectric cells shown in this work are built from commodity materials, using standard laboratory or industrial processes that are easily scaled up. Thus, hygroelectricity may eventually become a source of energy and valuable chemicals.

One-Step Formation of Pharmaceuticals Having a Phenylacetic Acid Core Using Water Microdroplets

The properties of water microdroplets strikingly differ from bulk water. Using room-temperature water microdroplets, we find that toluene can react with CO₂ to form phenylacetic acid in one step without any catalyst with negative high voltage applied at the sprayer source. The chemical components of these microdroplets are identified by mass spectrometry, and product structures are confirmed by tandem mass spectrometry. In this manner, we generate three drug molecules in a single step: 4-aminophenylacetic acid (epithelial peptide transporter PepT1 inhibitor), 3,4-dihydroxyphenylacetic acid (dopamine metabolite neurotransmitter), and phenylacetic acid (sodium salt form; treatment of urea cycle disorder). Mechanistic studies show that benzyl radicals formed from hydroxyl radicals at the water microdroplet interface drive these carboxylation reactions. This water microdroplet chemistry is general, allowing activation and subsequent carboxylation of aryl α-C-H groups.