Development and Application of Experimental Methodology for Characterizing the Chemical Composition and Reactivity of Air-Water Interfaces

California Institute Of Technology, Pasadena CA

Investigators

Abstract

With support from the Chemical Measurement and Imaging Program in the Division of Chemistry, Professor Beauchamp at the California Institute of Technology is developing and applying new experimental methodologies for the sensitive detection and characterization of a wide range of molecular species that preferentially occupy the discrete interface between gases and liquids. Although water occupies about 70% of the earth's surface, relatively little is known about the complex reactions that take place at the air-water interface. This boundary, only a few molecules in extent, represents one of the most common chemical environments in nature, present on the surface of oceans and lakes, on atmospheric aerosols, and even in the human respiratory system. Tools developed by the Beauchamp research group, including field induced droplet ionization and acoustic droplet ejection, are coupled with mass spectrometry to provide unprecedented insights into complex interfacial processes ranging from the oxidation of anthropogenic organics to form secondary organic aerosols in the atmosphere to the oxidation of key proteins in the pulmonary surfactant layer when ozone is inhaled. The conduct of these investigations provide training for a new generation of scientists, providing them with tools for new discoveries and the solution of scientific problems. Students are engaged at every level, with presentations to school groups, hosting high school student interns in the laboratory, and training graduate students and postdoctoral fellows. The technique of field-induced droplet ionization mass spectrometry (FIDI-MS), in which a pulsed electric field is applied to stretch microliter droplets to form opposing dual positive and negative electrosprays, is employed for many of these investigations. FIDI-MS has proven highly capable for selective detection and temporal monitoring of complex molecular species undergoing chemical reactions at the air-water interface. To complement studies of droplets, a second approach, demonstrated in Professor Beauchamp's laboratory, examines the composition of planar air-water interfaces using a focused acoustic pulse to eject picoliter droplets from the surface for subsequent mass spectrometric analysis. The latter experiment is performed in a Langmuir trough, where variation in surface pressure is expected to be an important parameter affecting both the structure and reactivity of interfacial layers. The investigator employs these techniques to conduct fundamental studies of the chemical reactivity of amphiphilic surface active molecules, ranging from helical peptides embedded in lipid layers to organics present in atmospheric aerosols, with gas phase species including ozone, hydroxyl radicals, oxides of nitrogen and sulfur, and acids and bases. Theoretical studies involving the development of detailed quantitative models for analyzing experimental kinetic data combined with state of the art molecular dynamics and quantum chemical calculations provides further insights in these investigations. Broad impacts of this work include the training of a new generation of research scientists who will be making contributions to our understanding of problems ranging from environmental science to human health.