Our research combines liquid microjet and molecular beam scattering techniques. We are currently working on three different projects aimed at developing a “blow-by-blow” description of collisions and reactions at gas-liquid interfaces: 1) super-Maxwellian helium evaporation from aqueous solutions, 2) reactive uptake of atmospheric gases into sea spray aerosol particles, 3) ion scattering methods to measure depth profiles on the Angstrom scale (in collaboration with Gunther Andersson at Flinders University, Australia) and 4) creation and reaction of solvated electrons at the surface of salty and surfactant-coated water.
To learn more about research in the Nathanson group, check out our poster, Collisions and Reactions at Gas-Liquid Interfaces and a review article, https://doi.org/10.1039/C6CS00079G For individual projects, please see the descriptions below.
High-Energy Helium Evaporation
Atomic and molecular solutes evaporate and dissolve by traversing an atomically thin boundary separating liquid and gas. Most solutes spend only short times in this interfacial region, making them difficult to observe. Experiments that monitor the velocities of evaporating species, however, can capture their final interactions with surface solvent molecules. We find that polarizable gases such as N2 and Ar evaporate from protic and hydrocarbon liquids with Maxwell-Boltzmann speed distributions. Surprisingly, the weakly interacting helium atom emerges from these liquids at high kinetic energies, exceeding the expected energy of evaporation from salty water by 70%. This super-Maxwellian evaporation implies in reverse that He atoms preferentially dissolve when they strike the surface at high energies, as if ballistically penetrating into the solvent. The excess He translational energy can be correlated with the shape of the potential of mean forces of the evaporating He atom, revealing intimate details of the He-water interactions just as the He atom launches into vacuum. We are extending these studies now to surfactant-coated jets to explore the dynamics of He atoms passing between the surfactant molecules. For evaporation of He atoms from pure and salty water, see: https://doi.org/10.1063/1.4940144
Reactive Gas Uptake into Sea Spray Aerosols
in collaboration with CAICE (Center for Aerosol Impacts on Chemistry of the Environment)
The rates of heterogeneous and multiphase reactions involving trace gases and sea spray aerosols are tied to the molecular-scale physical and chemical properties of the aerosol-gas interface. These interfacial chemical reactions are important for understanding the production and fate of natural and pollutant gases in the atmosphere. Our current research focuses on reactions of N2O5, whose nighttime uptake in sea spray and aqueous aerosol particles influences the global concentrations of NOx, HNO3, O3, OH, and CH4.
We use a molecular beam of reagent molecules directed at model aerosol solutions to analyze the gas-phase products with a mass spectrometer. These studies help clarify the mechanistic details of gas-particle heterogeneous reactions. The figure below depicts our gas-microjet scattering experiments in vacuum to explore reactions of N2O5 with salty and surfactant-coated water microjets in vacuum, including reactions with halide ions such as Br– and surfactants such as tetrabutylammonium and butanol. See, for example, https://pubs.acs.org/doi/10.1021/acs.jpca.7b02040
Ion Scattering Measurements of the Depth Profiles of Halide Ions and Surfactants at the Surfaces of Liquids
in collaboration with CAICE (Center for Aerosol Impacts on Chemistry of the Environment)
Working with Prof. Gunther Andersson in his laboratory at Flinders University in Adelaide, Australia, we use helium ion scattering to determine the interfacial depth profiles of ions and surfactants in salty and surfactant coated glycerol, a model for sugar-water solutions in organic-rich particles. The methods are illustrated in the figure below and described in https://pubs.acs.org/doi/10.1021/acs.jpcb.9b11686
The figure on the right above illustrates how the scattering of He+ ions may be used to determine the depth profile of elements on the Angstrom length scale in liquid glycerol. The deeper the ions travel, the more energy they lose through grazing collisions: a fraction of these penetrating ions are turned around by head-on collisions with Br– ions and recoil back through solution and into vacuum, where they are counted. The measured energy loss can then be converted to depth, as shown in the depth profiles at the left.
The three depth profiles correspond to He atoms recoiling from glycerol solutions of: 1) red curve: 10 mM cationic surfactant tetrahexylammonium bromide, (C6H11)4N+/Br– mixed with 0.3 M NaBr, 2) black dashed curve: pure 0.3 M NaBr, and 3) blue curve: anionic surfactant sodium dodecyl sulfate, C12H23OSO3–/Na+, mixed with 0.3 M NaBr.
The depth profiles each report on the location of Br– ions in the interfacial region. The Br– signal from the cationic surfactant (red curve) is enormous and localized within the top 10 Angstroms, implying that Br– ions collect near the surface, as depicted in the cartoon in the middle top. In contrast, the blue curve (anionic surfactant) is small and shallow, indicating that Br– are depleted near the surface. The intermediate black signal corresponds to 0.3 M Br– in the absence of a surfactant. The relative strengths of the depth profiles demonstrate that the cationic surfactant draws Br– to the surface, while the anionic surfactant pushes Br– into solution. These observations correlate well with our intuition of charge-charge attraction and repulsion, and reveal that highly concentrated layers of Br– can be created by cationic surfactants.
Reactions of Solvated Electrons at the Surface of Water
Solvated electrons are among the most reactive species that can exist in water, and they have spurred intense investigations over many decades. What happens when these electrons react at the surface of water instead of the bulk? We can create these electrons by directing a beam of sodium atoms at the surface of a flowing microjet. The Na atoms immediately ionize into a sodium cation and electron, which can then react with solvent water and dissolved solutes. Our studies are focusing on reactions of these electrons with surface-active molecules to isolate reactions in the interfacial region. Stay tuned for our first results in water! For experiments using glycerol as a solvent, see https://science.sciencemag.org/content/335/6072/1072.abstract
