Experimental and Computational Study of Molecular Water Interactions with Condensed Nopinone Surfaces Under Atmospherically Relevant Conditions

Abstract

Water and organics are omnipresent in the atmosphere, and their interactions influence the properties and lifetime of both aerosols and clouds. Nopinone is one of the major reaction products formed from β-pinene oxidation, a compound emitted by coniferous trees, and it has been found in both gas and particle phases in the atmosphere. Here, we investigate the interactions between water molecules and nopinone surfaces by combining environmental molecular beam (EMB) experiments and molecular dynamics (MD) simulations. The EMB method enables detailed studies of the dynamics and kinetics of water interacting with solid nopinone at 170−240 K and graphite coated with a molecularly thin nopinone layer at 200−270 K. MD simulations that mimic the experimental conditions have been performed to add insights into the molecular-level processes. Water molecules impinging on nopinone surfaces are efficiently trapped (≥97%), and only a minor fraction scatters inelastically while maintaining 35−65% of their incident kinetic energy (23.2 ± 1.0 kJ mol −1 ). A large fraction (60−80%) of the trapped molecules desorbs rapidly, whereas a small fraction (20−40%) remains on the surface for more than 10 ms. The MD calculations confirm both rapid water desorption and the occurrence of strongly bound surface states. A comparison of the experimental and computational results suggests that the formation of surface-bound water clusters enhances water uptake on the investigated surfaces.

Methanol Interactions with Nopinone Surfaces during Phase Evolution

“…The thickness of the multilayer is monitored by the laser, whereas the thin nopinone coating cannot be seen by the laser but are detected by helium scattering. 11 The ToF distributions are fitted to resolve the kinetics and dynamics of the interactions between the impacting methanol molecules/clusters with the nopinone surfaces. Typically, impinging molecules can be scattered inelastically or thermally trapped, thus these components are sought for in our analysis.…”

“…9 On nopinone surface, the uptake of water molecules and the detailed molecular dynamics and kinetics were studied by an environmental molecular beam (EMB) technique. 10,11 The thin coating and multilayer of nopinone show different water uptake ability, where more water was taken up by the multilayer.…”

“…Molecular dynamics (MD) simulations show that bulk diffusion is very limited, and that the enhanced water uptake is instead mainly caused by water molecules that more easily find strong binding states on the multilayer surface than on the thin coating. 11 Apart from water vapor-surface interactions, the organics-surface interactions are also important because of the abundance of organic vapors in the atmosphere. A recent EMB-MD study shows that methanol monomers and clusters are efficiently trapped on a graphite surface, and the trapped molecules rapidly diffuse along the surface to find other clusters or form new clusters at low surface coverages.…”

“…Herrera et al 13 reported that small methanol clusters are preferably formed on graphite because of the interaction of hydrogen bonds. Apart from the graphite surface, various organic surfaces have also been studied by EMB, 14 including alcohols, [15][16][17][18][19] ketones 10,11 and carboxylic acids, 20 but the interactions of organic surfaces with another organic gas species have never been studied by the EMB.…”

“…The combination of EMB and MD has been shown to be an effective approach to investigate the gas uptake and sorption kinetics of gas interactions with various surfaces. 11,12,16,17 In this study, we look into the kinetics of methanol monomers and clusters on nopinone surfaces (thin coating and multilayer) by the EMB experiments and the MD simulations. The inter-species interactions are compared to the internal interactions within phases, which reveals the important parameters determining the thermodynamics and kinetics of mixed organic systems.…”