On the Rise: Experimental and Computational Vibrational Sum Frequency Spectroscopy Studies of Pyruvic Acid and Its Surface-Active Oligomer Species at the Air-Water Interface.

Abstract

It is well known that atmospheric aerosol play important roles in the environment. However, there is still much to learn about the processes that form aerosols, particularly aqueous secondary organic aerosols. While pyruvic acid (PA) is often better known for its biological significance, it is also an abundant atmospheric secondary organic ketoacid. It has been shown that, in bulk aqueous environments, PA exists in equilibrium between unhydrated α-keto carboxylic acid (PYA) and singly hydrated geminal diol carboxylic acid (PYT), favoring the diol. These studies have also identified oligomer products in the bulk, including zymonic acid (ZYA) and parapyruvic acid (PPA). The surface behavior of these oligomers has not been studied, and their contributions (if any) to the interface are unknown. Here, we address this knowledge gap by examining the molecular species present at the interface of aqueous PA systems using vibrational sum frequency spectroscopy (VSFS), a surface-sensitive technique. VSFS provides information about interfacial molecular populations, orientations, and behaviors. Computational studies using classical molecular dynamics and quantum mechanical density functional theory are employed in combination to afford further insights into these systems. Our studies indicate populations of at least two intensely surface-active oligomeric species at the interface. Computational results demonstrate that, along with PYA and PYT, both PPA and ZYA are surface-active with strong VSF responses that can account for features in the experimental spectra.