The air-water interface is ubiquitous in the natural environment, at the ocean surface as well as the surface of aqueous aerosols on Earth. Organic molecules aggregate to the air-water interface, even soluble or volatile organics tend to favor partitioning to the interface over gas phase or bulk aqueous phase. While there are several ways to selectively probe the molecules at the interface, external reflection absorption spectroscopy remains a rather underutilized technique. This surface sensitive technique can be used to identify and characterize molecules at the air-water interface in monolayers or even sub-monolayer concentrations. This presentation will discuss the Infrared surface reflection absorption spectroscopy experiment to run exclusively either parallel (P) or perpendicular (S) polarization relative to the surface normal, which allows information about molecular orientation at the surface along with the typical geometry and conformational information that infrared spectroscopy obtains. 2-oxo-octanoic acid (2OOA), a molecule with limited solubility in water, which provides a surrogate for organic molecules in the natural environment, has been studied using external reflection absorption spectroscopy as well as conventional gas phase infrared spectroscopy and solid phase ATR. The information that can be gathered from the use of external reflection absorption spectroscopy includes identifying hydrogen bonding at the interface, molecular orientation and conformation like alkyl chain tilt angle relative to the surface and gauche defects. Reflection absorption spectroscopy can also be used to follow photochemical and dark reactions occurring at the air-water interface which is highly relevant for chemical processing of organics in the natural environment.

Reflection absorption spectroscopy at the air-water interface

ACS Spring 2022

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