Control of Interfacial Cl2 and N2O5 Reactivity by a Zwitterionic Phospholipid in Comparison with Ionic and Uncharged Surfactants

Probing the interfacial structure of aqueous surfactants through helium atom evaporation

Dissolved helium atoms evaporate from liquids in super-Maxwellian speed distributions because their interactions are too weak to enforce full thermal equilibration at the surface as they are "squeezed" out of solution. The excess speeds of these He atoms reflect their final interactions with solvent and solute molecules at the surfaces of water and other liquids. We extend this observation by monitoring He atom evaporation from salty water solutions coated with surfactants. These surface-active molecules span neutral, anionic, and cationic amphiphiles: butanol, 3-methyl-1-butanol, pentanol, pentanoic acid, pentanoate, tetrabutylammonium, benzyltrimethylammonium, hexyltrimethylammonium, and dodecyltrimethylammonium, each characterized by surface tension measurements. The helium energy distributions, recorded in vacuum using a salty water microjet, reveal a sharp distinction between neutral and ionic surfactant films. Helium atoms evaporate through neutral surfactant monolayers in speed distributions that are similar to a pure hydrocarbon, reflecting the common alkyl chains of both. In contrast, He atoms appear to evaporate through ionic surfactant layers in distributions that are closer to pure salty water. We speculate that the ionic surfactants distribute themselves more loosely and deeply through the top layers of the aqueous solution than do neutral surfactants, with gaps between the surfactants that may be filled with salty water. This difference is supported by prior molecular dynamics simulations and ion scattering measurements of surfactant solutions.

Molecular Insights into Chemical Reactions at Aqueous Aerosol Interfaces

Atmospheric aerosols facilitate reactions between ambient gases and dissolved species. Here, we review our efforts to interrogate the uptake of these gases and the mechanisms of their reactions both theoretically and experimentally. We highlight the fascinating behavior of N2O5 in solutions ranging from pure water to complex mixtures, chosen because its aerosol-mediated reactions significantly impact global ozone, hydroxyl, and methane concentrations. As a hydrophobic, weakly soluble, and highly reactive species, N2O5 is a sensitive probe of the chemical and physical properties of aerosol interfaces. We employ contemporary theory to disentangle the fate of N2O5 as it approaches pure and salty water, starting with adsorption and ending with hydrolysis to HNO3, chlorination to ClNO2, or evaporation. Flow reactor and gas-liquid scattering experiments probe even greater complexity as added ions, organic molecules, and surfactants alter the interfacial composition and reaction rates. Together, we reveal a new perspective on multiphase chemistry in the atmosphere.

Revealing the Impacts of Chemical Complexity on Submicrometer Sea Spray Aerosol Morphology

Sea spray aerosol (SSA) ejected through bursting bubbles at the ocean surface is a complex mixture of salts and organic species. Submicrometer SSA particles have long atmospheric lifetimes and play a critical role in the climate system. Composition impacts their ability to form marine clouds, yet their cloud-forming potential is difficult to study due to their small size. Here, we use large-scale molecular dynamics (MD) simulations as a "computational microscope" to provide never-before-seen views of 40 nm model aerosol particles and their molecular morphologies. We investigate how increasing chemical complexity impacts the distribution of organic material throughout individual particles for a range of organic constituents with varying chemical properties. Our simulations show that common organic marine surfactants readily partition between both the surface and interior of the aerosol, indicating that nascent SSA may be more heterogeneous than traditional morphological models suggest. We support our computational observations of SSA surface heterogeneity with Brewster angle microscopy on model interfaces. These observations indicate that increased chemical complexity in submicrometer SSA leads to a reduced surface coverage by marine organics, which may facilitate water uptake in the atmosphere. Our work thus establishes large-scale MD simulations as a novel technique for interrogating aerosols at the single-particle level.

Effects of Microhydration on the Mechanisms of Hydrolysis and Cl- Substitution in Reactions of N2 O5 and Seawater

The reaction of N₂ O₅ at atmospheric interfaces has recently received considerable attention due to its importance in atmospheric chemistry. N₂ O₅ reacts preferentially with Cl^- to form ClNO₂ /NO₃ ^- (Cl^- substitution), but can also react with H₂ O to form 2HNO₃ (hydrolysis). In this paper, we explore these competing reactions in a theoretical study of the clusters N₂ O₅ /Cl^- /nH₂ O (n=2-5), resulting in the identification of three reaction motifs. First, we uncovered an S_N 2-type Cl^- substitution reaction of N₂ O₅ that occurs very quickly due to low barriers to reaction. Second, we found a low-lying pathway to hydrolysis via a ClNO₂ intermediate (two-step hydrolysis). Finally, we found a direct hydrolysis pathway where H₂ O attacks N₂ O₅ (one-step hydrolysis). We find that Cl^- substitution is the fastest reaction in every cluster. Between one-step and two-step hydrolysis, we find that one-step hydrolysis barriers are lower, making two-step hydrolysis (via ClNO₂ intermediate) likely only when concentrations of Cl^- are high.

Evaporation and Molecular Beam Scattering from a Flat Liquid Jet

An experimental setup for molecular beam scattering from flat liquid sheets has been developed, with the goal of studying reactions at gas-liquid interfaces for volatile liquids. Specifically, a crossed molecular beam instrument that can measure angular and translational energy distributions of scattered products has been adapted for liquid jet scattering. A microfluidic chip is used to create a stable flat liquid sheet inside vacuum from which scattering occurs, and both evaporation and scattering from this sheet are characterized using a rotatable mass spectrometer that can measure product time-of-flight distributions. This article describes the instrument and reports on the first measurements of evaporation of dodecane and Ne from a Ne-doped dodecane flat jet, as well as scattering of Ne from a flat jet of pure dodecane.

Catalytic Coupling of Free Radical Oxidation and Electrophilic Chlorine Addition by Phase-Transfer Intermediates in Liquid Aerosols

While examining the heterogeneous reaction of chlorine atoms with alkenes, in the presence of Cl₂, we have observed an unexpectedly large enhancement of reactivity and the predominance of chlorinated reaction products even under high O₂ conditions, where Cl atom recycling is expected to be minimal. These observations cannot be explained by known free radical oxidation or cycling mechanisms, but rather we find evidence for the multiphase catalytic coupling of free radical oxidation with electrophilic Cl₂ addition. The mechanism entails the production of oxygenated reaction intermediates, which act as gas-liquid phase-transfer catalysts (gl-PTCs) by promoting the accommodation of gas-phase Cl₂ by the aerosol, thereby enhancing electrophilic addition. Although the majority of PTCs typically couple chemistry between two immiscible liquid phases (aqueous/organic), there are few examples of PTCs that couple gas-liquid reactions. This work shows how multiphase reaction schemes of aerosols can be reimagined for understanding catalytic reaction mechanisms.

Competing Segregation of Br- and Cl- to a Surface Coated with a Cationic Surfactant: Direct Measurements of Ion and Solvent Depth Profiles

Ion-surface scattering experiments can be used to measure elemental depth profiles on the angstrom scale in complex liquid mixtures. We employ NICISS (neutral impact collision ion scattering spectroscopy) to measure depth profiles of dissolved ions and solvent in liquid glycerol containing the cationic surfactant tetrahexylammonium bromide (THA⁺/Br^-) at 0.013 M and mixtures of NaBr + NaCl at 0.4 M total concentration. The experiments reveal that Br^- outcompetes Cl^- in its attraction to surface THA⁺, and that THA⁺ segregates more extensively when more Br^- ions are present. Intriguingly, the depths spanned by THA⁺, Br^-, and Cl^- ions generally increase with Br^- bulk concentration, expanding from ∼10 to ∼25 Å for both Br^- and Cl^- depth profiles. This broadening likely occurs because of an increasing pileup of THA⁺ ions in a multilayer region that spreads the halide ions over a wider depth. The experiments indicate that cationic surfactants enhance Br^- and Cl^- concentrations in the surface region far beyond their bulk-phase values, making solutions coated with these surfactants potentially more reactive toward gases that can oxidize the halide ions.

Experimental Depth Profiles of Surfactants, Ions, and Solvent at the Angstrom Scale: Studies of Cationic and Anionic Surfactants and Their Salting Out

Neutral impact ion scattering spectroscopy (NICISS) is used to measure the depth profiles of ionic surfactants, counterions, and solvent molecules on the angstrom scale. The chosen surfactants are 0.010 m tetrahexylammonium bromide (THA⁺/Br^-) and 0.0050 m sodium dodecyl sulfate (Na⁺/DS^-) in the absence and presence of 0.30 m NaBr in liquid glycerol. NICISS determines the depth profiles of the elements C, O, Na, S, and Br through the loss in energy of 5 keV He atoms that travel into and out of the liquid, which is then converted into depth. In the absence of NaBr, we find that THA⁺ and its Br^- counterion segregate together because of charge attraction, forming a narrow double layer that is 10 Å wide and 150 times more concentrated than in the bulk. With the addition of NaBr, THA⁺ is "salted out" to the surface, increasing the interfacial Br^- concentration by 3-fold and spreading the anions over a ∼30 Å depth. Added NaBr similarly increases the interfacial concentration of DS^- ions and broadens their positions. Conversely, the dissolved Br^- ions are significantly depleted over a depth of 0-40 Å from the surface because of charge repulsion from DS^- ions within the interfacial region. These different interfacial Br^- propensities correlate with previously measured gas-liquid reactivities: gaseous Cl₂ readily reacts with Br^- ions in the presence of THA⁺ but drops 70-fold in the presence of DS^-, demonstrating that surfactant headgroup charge controls the reactivity of Br^- through changes in its depth profile.

Wintertime Arctic Sea Spray Aerosol Composition Controlled by Sea Ice Lead Microbiology

The Arctic is experiencing the greatest warming on Earth, as most evident by rapid sea ice loss. Delayed sea ice freeze-up in the Alaskan Arctic is decreasing wintertime sea ice extent and changing marine biological activity. However, the impacts of newly open water on wintertime sea spray aerosol (SSA) production and atmospheric composition are unknown. Herein, we identify SSA, produced locally from open sea ice fractures (leads), as the dominant aerosol source in the coastal Alaskan Arctic during winter, highlighting the year-round nature of Arctic SSA emissions. Nearly all of the individual SSA featured thick organic coatings, consisting of marine saccharides, amino acids, fatty acids, and divalent cations, consistent with exopolymeric secretions produced as cryoprotectants by sea ice algae and bacteria. In contrast, local summertime SSA lacked these organic carbon coatings, or featured thin coatings, with only open water nearby. The individual SSA composition was not consistent with frost flowers or surface snow above sea ice, suggesting that neither hypothesized frost flower aerosolization nor blowing snow sublimation resulted in the observed SSA. These results further demonstrate the need for inclusion of lead-based SSA production in modeling of Arctic atmospheric composition. The identified connections between changing sea ice, microbiology, and SSA point to the significance of sea ice lead biogeochemistry in altering Arctic atmospheric composition, clouds, and climate feedbacks during winter.

Surfactant Charge Modulates Structure and Stability of Lipase-Embedded Monolayers at Marine-Relevant Aerosol Surfaces

Lipases, as well as other enzymes, are present and active within the sea surface microlayer (SSML). Upon bubble bursting, lipases partition into sea spray aerosol (SSA) along with surface-active molecules such as lipids. Lipases are likely to be embedded in the lipid monolayer at the SSA surface and thus have the potential to influence SSA interfacial structure and chemistry. Elucidating the structure of the lipid monolayer at SSA interfaces and how this structure is altered upon interaction with a protein system like lipase is of interest, given the importance of how aerosols interact with sunlight, influence cloud formation, and provide surfaces for chemical reactions. Herein, we report an integrated experimental and computational study of Burkholderia cepacia lipase (BCL) embedded in a lipid monolayer and highlight the important role of electrostatic, rather than hydrophobic, interactions as a driver for monolayer stability. Specifically, we combine Langmuir film experiments and molecular dynamics (MD) simulations to examine the detailed interactions between the zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayer and BCL. Upon insertion of BCL from the underlying subphase into the lipid monolayer, it is shown that BCL permeates and largely disorders the monolayer while strongly interacting with zwitterionic DPPC molecules, as experimentally observed by Langmuir adsorption curves and infrared reflectance absorbance spectroscopy. Explicitly solvated, all-atom MD is then used to provide insights into inter- and intramolecular interactions that drive these observations, with specific attention to the formation of salt bridges or ionic-bonding interactions. We show that after insertion into the DPPC monolayer, lipase is maintained at high surface pressures and in large BCL concentrations by forming a salt-bridge-stabilized lipase-DPPC complex. In comparison, when embedded in an anionic monolayer at low surface pressures, BCL preferentially forms intramolecular salt bridges, reducing its total favorable interactions with the surfactant and partitioning out of the monolayer shortly after injection. Overall, this study shows that the structure and dynamics of lipase-embedded SSA surfaces vary based on surface charge and pressure and that these variations have the potential to differentially modulate the properties of marine aerosols.