Hydroxyl Radical at the Air−Water Interface

Abstract

Interaction of the hydroxyl radical with the liquid water surface was studied using classical molecular dynamics computer simulations. From a series of scattering trajectories, the thermal and mass accommodation coefficients of OH on liquid water at 300 K were determined to be 0.95 and 0.83, respectively. The calculated free energy profile for transfer of OH across the air-water interface at 300 K exhibits a minimum in the interfacial region, with the free energy of adsorbtion (DeltaGa) being about 1 kcal/mol more negative than the hydration free energy (DeltaGs). The propensity of the hydroxyl radical for the air-water interface manifests itself in partitioning of OH radicals between the bulk water and the surface. The enhancement of the surface concentration of OH relative to its concentration in the aqueous phase suggests that important OH chemistry may be occurring in the interfacial layer of water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modeling heterogeneous atmospheric chemical processes.

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

“…41 The free energy of hydration can be calculated in MD at the infinite dilution limit, i.e., computing the free energy cost of moving one MA molecule from the gas phase toward the bulk water. The MD-computed free energy of hydration, ∆G * , differs from that calculated at standard conditions, ∆G 0 ; these values are related through 38,41,42 …”

“…We note that in this case the residence time can range broadly from 24 ps to 866 ps; this finding comes in spite of the fact that we employed both a larger number of simulations and longer timed simulations compared to similar studies of gas molecule uptakes present in the literature. [42][43][44] The statistics obtained for the residence time are insufficient to properly sketch a population distribution, and are therefore insufficient to draw any reliable conclusions concerning the average time of surface residence. Nevertheless, these results provide a range of values that can be used to better constrain nonreactive uptake coefficient in chemical transport models.…”

A new approach to studying aqueous reactions using diffuse reflectance infrared Fourier transform spectrometry: application to the uptake and oxidation of SO2 on OH-processed model sea salt aerosol

“…Cl 2 þ 2 Cl À ð10Þ However, given the predicted propensity for OH to reside at the interface, [114][115][116] some of the OH generated in solution by reactions (5) and (6) will diffuse to the interface, enhancing the interface oxidation and generation of base beyond that expected due to the uptake and reaction of gas phase OH alone. The relatively rapid disappearance of sulfite and formation of sulfate (Fig.…”

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“…However, chaotropic ions with low surface charge density and/or high polarizability (such as Cl -, Br -, I -, HO 2 -and O 2 -) will favor the gas-liquid interfaces (Garrett, 2004;Jungwirth and Winter, 2008) as they only interact weakly with water but are influenced favorably by the highly polarized surface. Aqueous radicals also prefer to reside at such interfaces (Roeselová al, 2004), as do some molecular species that prefer to hydrogen bond on the outside of clathrate-like structures, like superoxide (Shi et al 2003). Small cations are found away from the interface towards the bulk where their requirement for efficient hydration may be satisfied.…”

“…Thus, the main driving forces for the entry of such ions into the anisotropic environment of the interface are the stabilizing polarization interactions. Similarly to chaotropic ions, hydroxyl radicals also prefer to reside at air-water interfaces (Roeselová et al 2004); the radicals donating one hydrogen bond but accepting less than two (VandeVondele and Sprik, 2005).…”