| AVS 60th International Symposium and Exhibition | |
| Surface Science | Wednesday Sessions |
| Session SS+AS-WeM |
| Session: | Catalysis in Prebiotic Chemistry (8:00-10:00 am)/Environmental Interfaces (10:40 am-12:00 pm) |
| Presenter: | M.H.C. Van Spyk, University of California, Irvine |
| Authors: | M.H.C. Van Spyk, University of California, Irvine A.M. Margarella, University of California, Irvine K.A. Perrine, University of California, Irvine B. Winter, Helmholtz-Zentrum Berlin für Materialien und Energie/Elektronenspeicherring BESSY II, Germany J.C. Hemminger, University of California, Irvine |
| Correspondent: | Click to Email |
The distribution of solutes near the air/water interface is known to affect chemical reactions occurring at the surface of aqueous atmospheric aerosols. Ammonia is a basic gas which is introduced into the air both naturally and biogenically, and is important for atmospheric reactions. Solvated ammonia (NH3) is in equilibrium with the protonated cation ammonium (NH4+), and variation of the NH4+/NH3 ratio with pH for bulk solutions is reasonably well known. In contrast, little is known about the NH4+/NH3 ratio at the liquid/vapor interface, or how it varies with bulk solution pH. Understanding the interfacial chemistry of ammonia solutions is important because the NH4+/NH3 ratio may differ at the air/water interface leading to changes in reactivity that can influence air quality.
Liquid jet X-ray photoelectron spectroscopy (LJ-XPS) was used to investigate the interfacial behavior of aqueous ammonia in relation to pH at a solution temperature of 5 °C. For these studies, aqueous ammonium solutions were generated by dissolving ammonium hydroxide or ammonium chloride in water, and the pH was varied using hydrochloric acid or sodium hydroxide, respectively. A continuously renewed jet of solution was probed using photoelectron spectroscopy with synchrotron radiation which was tuned to produce various photoelectron kinetic energies corresponding to the desired sampling depth. Spectra were collected from the bulk solution by generating photoelectrons with high kinetic energies (600 eV) in which case the electron mean free path in solution is sufficiently large so that the experiment probes the bulk solution. Spectra were collected from the near-surface region by exciting photoelectrons with low kinetic energy (150 eV) so that signal from the bulk is attenuated. Nitrogen (N1s) and oxygen (O1s) photoelectron spectra were collected at two kinetic energies to study the relative amounts of each species at the surface versus the bulk for different bulk pH values. The N1s spectra were deconvolved into three peaks from aqueous NH4+, and NH3, and gaseous NH3, and since the spectra were recorded at the magic angle, the ratios of the integrated peak areas provide a comparison of the relative amounts of each species. It is observed that the NH4+/NH3 ratio decreases with increasing bulk solution pH for the surface and bulk of solution, and there is a reduction of the NH4+/NH3 ratio at the surface, compared to the bulk. The titrations of ammonium chloride and ammonium hydroxide may be contrasted to understand the impact of chloride concentration. This study illustrates that interfacial aqueous ammonia experiences a different equilibrium than that in the bulk.