Effect of pH on pyruvic acid in bulk and at the air/liquid interface

Effect of pH on pyruvic acid in bulk and at the air/liquid interface
Veronika Wank and Ellen H. G. Backus
University of Vienna, Faculty of Chemistry, Department of Physical Chemistry,Währinger Straße 42, 1090 Vienna, Austria
Abstract:
Pyruvic acid, known as a relevant carboxylic acid for plant metabolic processes is also a contributor to the formation of secondary organic aerosols (SOA) and thus of great importance for the atmospheric cycle, especially in aerosols. At the interface, the behavior of relevant atmospheric molecules in aerosols is a major topic, as the surface is one of the first areas where chemical processes take place and thus determines the main reactivity of the aerosol. For acidic molecules, the acid/base behavior, especially at the interface, is relevant for understanding the chemical interaction of organic matter in atmospheric aerosols, where reaction rates and product distributions change due to different pH conditions, e.g. chemical processing and molecule transport.
Since the pH of aqueous aerosols can vary widely, it is particularly important to understand changes in the surface structure due to the resulting (de)protonation reactions.
This encourages us to take a closer look at the interfacial region of pyruvic acid in aqueous solution by using a complex surface-specific spectroscopic technique, so called sum frequency generation spectroscopy (SFG). For comparison, infrared bulk measurements utilizing ATR spectroscopy were completed. By combining ATR and SFG, the protonation state of pyruvic acid for bulk and interface was determined by probing the vibrational signatures of the carboxylic acid groups.
Our results show that pyruvic acid at the water interface is more alkaline than in the bulk, which indicates that the carboxylic acid group deprotonates at a higher pH value at the surface than in the bulk. It is also evident from the SFG spectra that at lower pH the water molecules on the surface are displaced by PA molecules, whereas at higher pH the water molecules return to the surface and the PA molecules tend to go into the bulk.
This implies that the protonation state of carboxylic acids like PA can thus affect the molecular orientation, conformation and function of molecules on aqueous surfaces, which likely has a significant impact on the chemical processes taking place at the aerosol surface in the atmosphere.