Insight into the molecular mechanism of water evaporation via the finite temperature string method
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Insight into the molecular mechanism of water evaporation via the finite temperature.pdf
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4.68 MB
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Author(s)
Musolino, Nicholas
Trout, Bernhardt L.
Date Issued
April 2013
Journal
The Journal of Chemical Physics
Publisher
American Institute of Physics (AIP)
Citation
Musolino, Nicholas, and Bernhardt L. Trout. “Insight into the Molecular Mechanism of Water Evaporation via the Finite Temperature String Method.” The Journal of Chemical Physics 138, no. 13 (2013): 134707. © 2013 American Institute of Physics
Version
Final published version
Abstract
The process of water's evaporation at its liquid/air interface has proven challenging to study experimentally and, because it constitutes a rare event on molecular time scales, presents a challenge for computer simulations as well. In this work, we simulated water's evaporation using the classical extended simple point charge model water model, and identified a minimum free energy path for this process in terms of 10 descriptive order parameters. The measured free energy change was 7.4 kcal/mol at 298 K, in reasonable agreement with the experimental value of 6.3 kcal/mol, and the mean first-passage time was 1375 ns for a single molecule, corresponding to an evaporation coefficient of 0.25. In the observed minimum free energy process, the water molecule diffuses to the surface, and tends to rotate so that its dipole and one O–H bond are oriented outward as it crosses the Gibbs dividing surface. As the water molecule moves further outward through the interfacial region, its local density is higher than the time-averaged density, indicating a local solvation shell that protrudes from the interface. The water molecule loses donor and acceptor hydrogen bonds, and then, with its dipole nearly normal to the interface, stops donating its remaining hydrogen bond. At that point, when the final, accepted hydrogen bond is broken, the water molecule is free. We also analyzed which order parameters are most important in the process and in reactive trajectories, and found that the relative orientation of water molecules near the evaporating molecule, and the number of accepted hydrogen bonds, were important variables in reactive trajectories and in kinetic descriptions of the process.
MIT Department
Massachusetts Institute of Technology. Department of Chemical Engineering
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DOI of Published Version
https://doi.org/10.1063/1.4798458
