An official website of the United States government
Abstract Formation of energetic species at the surface of aqueous microdroplets, including abundant hydroxyl radicals, oxidation products, and ionized N2and O2gas, has been previously attributed to the high electric field at the droplet surface. Here, evidence for a new mechanism for electronic excitation involving electron emission from negatively charged water droplets is shown. Droplet evaporation can lead to the emission of ions and droplet fission, but unlike positively charged droplets, negatively charged droplets can also shed charge by electron emission. With nanoelectrospray, no anions or negatively charged droplets are produced with a positive electrospray potential. In contrast, abundant O2+•and H3O+(H2O) are formed with negative electrospray. When toluene vapor is introduced with negative electrospray, abundant toluene radical cations and fragments are produced. Both O2+•and toluene radical cations are produced with pneumatic nebulization. The electrons produced from evaporating negatively charged droplets can be accelerated by an external electric field in electrospray, or by the field generated between droplets with opposite polarities produced by pneumatic nebulization. This electron emission/ionization mechanism leads to electronic excitation >10 eV, and it may explain some of the surprising chemistries that were previously attributed to the high intrinsic electric field at the surface of aqueous droplets.
more »
« less
The role of charge in microdroplet redox chemistry
Abstract In charged water microdroplets, which occur in nature or in the lab upon ultrasonication or in electrospray processes, the thermodynamics for reactive chemistry can be dramatically altered relative to the bulk phase. Here, we provide a theoretical basis for the observation of accelerated chemistry by simulating water droplets of increasing charge imbalance to create redox agents such as hydroxyl and hydrogen radicals and solvated electrons. We compute the hydration enthalpy of OH−and H+that controls the electron transfer process, and the corresponding changes in vertical ionization energy and vertical electron affinity of the ions, to create OH•and H•reactive species. We find that at ~ 20 − 50% of the Rayleigh limit of droplet charge the hydration enthalpy of both OH−and H+have decreased by >50 kcal/mol such that electron transfer becomes thermodynamically favorable, in correspondence with the more favorable vertical electron affinity of H+and the lowered vertical ionization energy of OH−. We provide scaling arguments that show that the nanoscale calculations and conclusions extend to the experimental microdroplet length scale. The relevance of the droplet charge for chemical reactivity is illustrated for the formation of H2O2, and has clear implications for other redox reactions observed to occur with enhanced rates in microdroplets.
more »
« less
- Award ID(s):
- 2313791
- PAR ID:
- 10503971
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 15
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Plasmas in contact with liquids can degrade organic molecules in a solution, as reactive oxygen and nitrogen species produced in the plasma solvate into the liquid. Immersing small droplets (tens of microns in diameter) in the plasma can more rapidly activate the liquid compared to treating a large volume of liquid with a smaller surface-to-volume ratio. The interactions between a radio frequency glow discharge sustained in He/H2O and a water droplet containing formate (HCOO−aq) immersed in and flowing through the plasma were modeled using a zero-dimensional global plasma chemistry model to investigate these activation processes. HCOO−aqinteracts with OHaq, which is produced from the solvation of OH from the gas phase. The resulting HCOO−aqconcentrations were benchmarked with previously reported experimental measurements. The diameter of the droplet, initial HCOO−aqconcentration, and gas flow rate affect only the HCOO−aqconcentration and OHaqdensity, leaving the OH density in the gas phase unaffected. Power deposition and gas mixture (e.g. percentage of H2O) change both the gas and liquid phase chemistry. A general trend was observed: during the first portion of droplet exposure to the plasma, OHaqprimarily consumes HCOO−aq. However, O2−aq, a byproduct of HCOO−aqconsumption, consumes OHaqonce O2−aqreaches a critically large density. Using HCOO−aqas a surrogate for OHaq-sensitive contaminants, combinations of residence time, droplet diameter, water vapor density, and power will determine the optimum remediation strategy.more » « less
-
Abstract Plasmas interacting with liquid microdroplets are gaining momentum due to their ability to significantly enhance the reactivity transfer from the gas phase plasma to the liquid. This is, for example, critically important for efficiently decomposing organic pollutants in water. In this contribution, the role of ⋅ OH as well as non- ⋅ OH-driven chemistry initiated by the activation of small water microdroplets in a controlled environment by diffuse RF glow discharge in He with different gas admixtures (Ar, O 2 and humidified He) at atmospheric pressure is quantified. The effect of short-lived radicals such as O ⋅ and H ⋅ atoms, singlet delta oxygen (O 2 ( a 1 Δ g )), O 3 and metastable atoms of He and Ar, besides ⋅ OH radicals, on the decomposition of formate dissolved in droplets was analyzed using detailed plasma diagnostics, droplet characterization and ex situ chemical analysis of the treated droplets. The formate decomposition increased with increasing droplet residence time in the plasma, with ∼70% decomposition occurring within ∼15 ms of the plasma treatment time. The formate oxidation in the droplets is shown to be limited by the gas phase ⋅ OH flux at lower H 2 O concentrations with a significant enhancement in the formate decomposition at the lowest water concentration, attributed to e − /ion-induced reactions. However, the oxidation is diffusion limited in the liquid phase at higher gaseous ⋅ OH concentrations. The formate decomposition in He/O 2 plasma was similar, although with an order of magnitude higher O ⋅ radical density than the ⋅ OH density in the corresponding He/H 2 O plasma. Using a one-dimensional reaction–diffusion model, we showed that O 2 ( a 1 Δ g ) and O 3 did not play a significant role and the decomposition was due to O ⋅ , and possibly ⋅ OH generated in the vapor containing droplet-plasma boundary layer.more » « less
-
Abstract Spontaneous ionization/breakup of water at the surface of aqueous droplets has been reported with evidence ranging from formation of hydrogen peroxide and hydroxyl radicals, indicated by ions atm/z36 attributed to OH⋅‐H3O+or (H2O‐OH2)+⋅ as well as oxidation products of radical scavengers in mass spectra of water droplets formed by pneumatic nebulization. Here, aqueous droplets are formed both by nanoelectrospray, which produces highly charged nanodrops with initial diameters ~100 nm, and a vibrating mesh nebulizer, which produces 2–20 μm droplets that are initially less highly charged. The lifetimes of these droplets range from 10s of μs to 560 ms and the surface‐to‐volume ratios span ~100‐fold range. No ions atm/z36 are detected with pure water, nor are significant oxidation products for the two radical scavengers that were previously reported to be formed in high abundance. These and other results indicate that prior conclusions about spontaneous hydroxyl radical formation in unactivated water droplets are not supported by the evidence and that water appears to be stable at droplet surfaces over a wide range of droplet size, charge and lifetime.more » « less
-
null (Ed.)Elementary processes associated with ionization of liquid water provide a framework for understanding radiation-matter interactions in chemistry and biology. Although numerous studies have been conducted on the dynamics of the hydrated electron, its partner arising from ionization of liquid water, H 2 O + , remains elusive. We used tunable femtosecond soft x-ray pulses from an x-ray free electron laser to reveal the dynamics of the valence hole created by strong-field ionization and to track the primary proton transfer reaction giving rise to the formation of OH. The isolated resonance associated with the valence hole (H 2 O + /OH) enabled straightforward detection. Molecular dynamics simulations revealed that the x-ray spectra are sensitive to structural dynamics at the ionization site. We found signatures of hydrated-electron dynamics in the x-ray spectrum.more » « less
