Energetic description of the liquid–vapor interface of water with organic coating molecules

Abstract

In situ measurements revealed the presence of a myriad of organic chemical compounds in cloud droplets. Among them, a significant fraction is composed of polar or amphiphilic compounds and these compounds have been detected in various contrasting environments (marine, biogenic, urban areas). This raises a simple question of their spatial positioning in a cloud droplet. Previous study suggested they can form an organic surface coating at the air/water interface, thus potentially perturbing the exchanges of molecules between the gaseous and aqueous phases. The present work aims at investigating the properties of the water surface at the molecular scale by selecting 4 organic compounds representative of the molecular diversity observed in clouds. Two fatty acids (cis-pinonic acid and nonanoic acid) commonly detected in the atmosphere (aerosol particles and cloud waters) were chosen. Levoglucosan is an anhydro sugar ubiquitous in the air and well-known as a tracer of biomass burning. Finally, oxalic acid, one of the most abundant di-acids in the atmosphere, has been chosen. Its ability to form a coating film was analyzed through the calculation of a free energy profile along the direction normal to the surface of water. Nonanoic and cis-pinonic acids have been shown to partition to the surface, in contrast to levoglucosan and oxalic acid that remain in water. The surface tension of the liquid–vapor (LV) interface of water was calculated as a function of surface excess of nonanoic and cis-pinonic acids. We completed this study by examining how the presence of a hydrophobic monolayer of nonanoic molecules at the surface of water can change the behavior of a hydrophilic molecules such as levoglucosan at the interface. Our results clearly indicate that levoglucosan molecules in the gas phase can be adsorbed on this organic layer. Results from this study are of particular interest for atmospheric research since by confirming the concept of organic film formation at the air/droplet interface, they lead to questions about possible heterogeneous reactivity phenomena and potential modification of compound exchanges between the gas and aqueous phases.