Could you explain the significance of your article to the non-specialist?

Many chemical processes in the atmosphere and environment are driven by the interaction of gases with liquid surfaces. To date, little is known about the chemical composition of liquid surfaces, which can significantly differ from that of the bulk solution. This is important since the concentration of solution species at the vapor/liquid interface could influence the chemical reactivity of aerosol particles and cloud droplets with atmospheric trace gases. By combining synchrotron-based photoemission spectroscopy with a liquid droplet train, we are able to observe how the concentration of a bulk solution of two miscible liquids, methanol and water, is enriched in the organic over a distance of ~1 to 2 nm from the liquid droplet surface. This apparatus significantly extends previous liquid phase photoemission measurements due to the broad range of solution types that can be studied and its ability to measure gas uptake by the liquid surface on the millisecond timescale. In the future this apparatus will be used to better understand the molecular details that control the reactive uptake of gas phase molecules by liquid surfaces.

What has motivated you to conduct this work?

Our goal is to develop an experimental method to quantify how the chemical composition of a liquid interface changes due to the heterogeneous uptake or reaction of gas molecules. Droplet trains have been used for a number of years by the atmospheric science community to investigate the uptake and reaction of gases by liquid surfaces. In those experiments the gas phase species is monitored as a function of exposure time to the droplet surfaces, thereby measuring uptake kinetics. To further understand the molecular details of this process, one would ideally like to examine both the gas and liquid surface during a chemical reaction. To do this we combine synchrotron-based ambient pressure photoelectron spectroscopy (APXPS) and a droplet train, which is a way to probe the liquid-vapor interface directly. This allows, for instance, a more detailed molecular picture of how gas molecules react at a liquid interface or how surfactants might stabilize the formation of reaction intermediates.

Where do you see this work developing in the future?

A dedicated droplet train/APXPS instrument is currently being constructed at Lawrence Berkeley National Laboratory. This instrument will allow us to study a wide range of liquids of varying compositions and concentrations in the presence of gases at pressures in the Torr range.