Accommodation coefficients for water vapor at the air/water interface
Introduction
The process of gas uptake at a liquid surface can be characterized by two probabilities of accommodation. The thermal accommodation coefficient, S, is a measure of the number of gas molecules that equilibrate to the temperature of the liquidThe mass accommodation coefficient, α, describes the probability of the gas molecule being incorporated into the liquid
An accurate determination of these probabilities is crucial to understanding the nucleation and growth kinetics of cloud droplets and, subsequently, to cloud albedo and climate change [1], [2]. The accommodation process is treated commonly as a two-step mechanism consisting of gas adsorption at the liquid surface followed by desorption into the gas phase or solvation in the bulk liquid [3]. The rate constant for gas adsorption at the liquid surface is proportional to S, while α is related to the rate constants for desorption into the gas phase, kdesorb, and solvation in the bulk liquid, ksolv, by
Unfortunately, various experimental determinations of S and α for water vapor on liquid water are inconsistent. Reported values of S range from 0.7 [4] to 1.0 [5], [6]. The range of experimentally determined α values is even larger than that for S. A review article on this subject shows that α values in the range from 0.03 to 1.0 have been reported [7]. Subsequent to that review article, the temperature dependence of α has been studied using a droplet train apparatus [6]. A negative temperature dependence is observed with α=0.32 at 258 K and α=0.17 at 280 K.
Molecular dynamics (MD) computer simulations provide another method for the determination of accommodation coefficients. This approach has been used previously for the determination of α, but a determination of S from MD simulations is not available to our knowledge. A recent study by Morita et al. [8] reports α>0.99 at 273 K. While the MD simulations are most consistent with the experimental conditions in the droplet train apparatus, the values of α obtained from these methods are not in agreement. Another MD simulation that focused on the temperature dependence of α obtained values in the range from 0.961 (T=330 K) to 0.286 (T=550 K) [9]. It is evident from the lack of agreement between experiments and between experiments and simulations that further work is necessary on the accommodation of water vapor on liquid water. Reconciliation of experimental and simulation results is especially compelling in light of the importance of accurate S and α values to atmospheric chemistry [1], [2].
In this Letter, results from MD simulations for the thermal and mass accommodation coefficients of water vapor on liquid water at 300 K are presented. The simulation temperature is selected to be intermediate to previous MD simulations at 273 K [8] and 330 K [9]. The objectives of this study are to develop an understanding of the accommodation process from a molecular perspective on a picosecond time scale and to determine values for S and α at 300 K.
S and α are calculated according to Eqs. , , respectively, by preparing 250 initial conditions of a gas phase water molecule with a thermal velocity approaching the surface of liquid water and monitoring the incoming molecule's velocity and position. In addition to water [8], [9], this approach has been used previously to study the mass accommodation of ethanol [10], methanol [11] and HO2[12] on liquid water and hydroxyl and ozone on aqueous salt solution surfaces [13]. Using an alternative approach, Taylor and Garrett [14] calculate the potential of mean force for the transfer of a solute from the gas phase to the bulk liquid and use transition state theory to determine α according to Eq. (3) for ethanol and ethylene glycol on liquid water. Both approaches yield a consistent result for the mass accommodation of ethanol at the air/liquid interface of water and should be used in conjunction so that the short and long time aspects of the accommodation process are studied [10].
From the simulation results presented in this Letter, it is determined that S=1.0 and α=0.99 at 300 K. Although a couple of water molecules directly scatter upon impact with the surface, the water molecules are equilibrated to the liquid temperature. As the incoming molecule approaches, the translational kinetic energy in the direction normal to the surface increases. Upon striking the surface, the rotational kinetic energy is slightly excited and the total excess kinetic energy is dissipated within 4 ps. The structure in the first and second solvent shells of the adsorbed water molecule is equilibrated within 4 ps of striking the surface also. Desorption from the surface is not observed within 90 ps, which is consistent with the potential of mean force for the transfer of a water molecule across the air/liquid interface of water [14], [15].
The remainder of this Letter is organized as follows. In Section 2, the MD methodology is described. The results and discussion are presented in Section 3, followed by conclusions in Section 4.
Section snippets
MD methodology
Configurations of the liquid to be used for the accommodation study are generated from a simulation of 864 water molecules in a slab geometry with box dimensions of 30 Å (x) × 30 Å (y) × 100 Å (z). The z dimension is perpendicular to the air/liquid interface. Following a 500 ps equilibration, a 250 ps equilibrium simulation is performed. Configurations from the equilibrium simulation, stored every 50 ps, are used to generate five initial configurations for the accommodation study. For the remainder
The overall process
For the process of a thermal gas phase molecule impinging on a liquid surface, there are several possible outcomes for the incident molecule. The molecule may directly scatter back into the gas phase or adsorb at the interface. If adsorbed, the molecule, at a later time, may desorb back into the gas phase, remain adsorbed at the interface or be absorbed into the bulk liquid. The number of trajectories that result in each of these outcomes is determined from the data shown in Fig. 1. In Fig. 1a,
Conclusions
Molecular dynamics computer simulations are used to determine the thermal and mass accommodation coefficients for water vapor on liquid water at 300 K. S and α are determined to be 1.0 and 0.99, respectively. In addition, significant molecular insight into the uptake process of a gas phase molecule with thermal impact velocity at a liquid surface is gained on a picosecond time scale. The process is characterized by an increase in the translational kinetic energy in the direction normal to the
Acknowledgements
We thank Dr. Douglas R. Worsnop for encouraging us to perform this study. A Collaborative Research in Chemistry grant from the National Science Foundation (CHE-0209719) funds this work.
References (30)
- et al.
J. Colloid Interface Sci.
(1986) Chem. Phys. Lett.
(2003)- et al.
Chem. Phys. Lett.
(2004) J. Comput. Phys.
(1977)- et al.
J. Comput. Chem.
(1977) Comput. Phys. Commun.
(1995)- et al.
Nature
(1997) - et al.
Tellus
(2001) - et al.
Prog. React. Kinet. Mech.
(2002) - et al.
J. Chem. Phys.
(1999)
J. Phys. Chem. A
Aerosol Sci. Technol.
J. Phys. Chem. B
J. Phys. Chem. B
Cited by (49)
Analysis of the evaporation coefficients of water, heavy water, and methanol in a high vacuum environment
2023, International Journal of Heat and Mass TransferMembrane desalination performance governed by molecular reflection at the liquid-vapor interface
2019, International Journal of Heat and Mass TransferMolecular dynamics study of octane condensation coefficient at room temperature
2017, International Journal of Heat and Mass TransferCitation Excerpt :Molecular dynamics simulations [13] have proven to be a powerful tool for providing atomic-scale insight into the structure, dynamics, and kinetics underlying interfacial properties and processes. Molecular dynamics simulations have been applied to study the condensation coefficient of Lennard-Jones liquids [14–16], water [17–21,6,22,10,23,24] and, more recently, to the interface transfer properties of octane [14,25–28,7]. Nevertheless, utilization of molecular modeling is still a developing area in engineering thermal management applications [3].
Gas-adsorption dynamics at the water-air interface, revealed by resonant droplet tensiometry
2016, Chemical Engineering ScienceCitation Excerpt :Both theoretical and experimental approaches have been used recently to examine the adsorption of volatile organic compounds into the liquid–vapor interface. Most often the theoretical approach is realized by numerical simulations with the help of molecular dynamics (Canneaux et al., 2006; Vieceli et al., 2004; Morita, 2003). The experimental approach employs surface tension measurements at the water–vapor interface.
Molecular dynamics simulations of the condensation coefficient of water
2013, Fluid Phase EquilibriaCitation Excerpt :Thus, when the molecule is in the gas phase, it has a smaller dipole moment than in the bulk. However, these models predict a condensation coefficient of approximately unity [29], leading to the conclusion that the variation of the dipole moment as the molecule moves from gas to surface to liquid is insufficient to give the experimentally observed condensation coefficients. As such, we also adjusted the Lennard-Jones parameters of the gas phase molecule in order to match the experimentally determined condensation coefficient.
Desorption lifetimes and activation energies influencing gas-surface interactions and multiphase chemical kinetics
2024, Atmospheric Chemistry and Physics