Reactivity of Volatile Organic Compounds at the Surface of a Water Droplet

Abstract

Knowledge of the role of water droplets and aerosols in atmospheric chemistry is crucial to significantly improve our understanding of global warming and air quality. Chemistry at the air/water interface, in particular, is still poorly understood. There is a great need to understand how clouds and aerosols process chemistry of organics prevalent in the atmosphere. We report in this study the first computer simulation of a volatile organic compound (formaldehyde) at the air/water interface with explicit description of its ground and excited states electronic properties. We use an elaborated technique that combines molecular dynamics simulations together with a quantum/classical description of the formaldehyde-water system. We show that in spite of a large affinity for water, formaldehyde exhibits a preference for the air/water interface with respect to the bulk, roughly by 1.5 kcal/mol. Another important finding in our simulations is that frontier orbitals HOMO and LUMO undergo substantial stabilization at the interface due to surface water reorientation, which induces a local positive electrostatic potential. Such a potential is significantly larger than the one estimated in bulk water suggesting that the reactivity of formaldehyde could change with respect to both gas phase and bulk water. The conclusions presented in this work are expected to help/guide future experiments studying the chemical reactivity of volatile organic compounds at the air/water interface.

Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface

“…Interestingly, this proposed mechanism for the chemistry of MA at the air/liquid water interface also suggests a new source for NH 2 radicals at aerosol surfaces, other than by reaction of absorbed NH 3 . This provides important support for the recent work of Martin-Costa et al 77 and Zhong et al 68 on the interaction of NH 2 radical and volatile organic compounds at the surface of water droplet. Future work is planned to examine the influence of both temperature and humidity on the position/orientation of such by-products at aerosol droplets.…”

“…Methylamine belongs to a class of organics referred as amphiphilic organics, which are known to be partitioned at the interface creating a hydrophobic film on an aqueous aerosol 73,74 and determining many chemical properties of aerosols. Recent experimental and theoretical research [75][76][77] has shown that atmospheric reactions of organic molecules can be enhanced on a surface compared to the gas phase. As will any surface active molecule, amphiphilic organics will reduce the surface tension of water, affecting the droplet growth and trace-gas uptake, finally determining the reactivity towards oxidative gases and the ability of the aerosol to absorb or scatter radiation.…”

“…Nevertheless, Figure 13 points also to the possibility of another channel with the formation of NH 2 radical and formaldehyde, which are also known to be preferentially located at the surface of liquid water. 68,77 It is known from studies 83 of substituted alkoxy radicals that C-X bonds are weaker than C-H ones because of the lower activation energy barrier for bond cleavage, suggesting that the release of NH 2 radical is also possible. Interestingly, this proposed mechanism for the chemistry of MA at the air/liquid water interface also suggests a new source for NH 2 radicals at aerosol surfaces, other than by reaction of absorbed NH 3 .…”

Reactivity of aldehydes at the air–water interface. Insights from molecular dynamics simulations and ab initio calculations

“…This molecule exhibits a preference for the interface that is about 3 kcal mol −1 with respect to bulk water, not far from the value 2.8 kcal mol −1 reported earlier by Canneaux et al 15 The result for formaldehyde (1.5 kcal mol −1 ) reproduces the value reported before using QM/MM simulations. 2 It is interesting to point out that the affinity for the air water interface increases in the order formaldehyde < acetaldehyde < benzaldehyde (compare the stabilisation at the interface with respect to the gas phase), which parallels the increasing hydrogen-bond acceptor character of the compounds. It also parallels the increasing polarity of the carbonyl group in the three aldehydes (see below).…”

A surface second harmonic generation investigation of volatile organic compound adsorption on a liquid mercury surface

“…[7][8][9] For instance, the rate and mechanism by which VOCs adsorb and react onto water droplets or surface of aerosol particles will dictate how far and fast these pollutants can be transported in the atmosphere. This capability of monitoring adsorption mechanisms and adsorption kinetics under ambient condition will be particularly useful in characterizing the adsorption of atmospheric pollutants, such as VOCs, to the air/aqueous interface.…”