Free energy study of H2O, N2O5, SO2, and O3 gas sorption by water droplets/slabs

Abstract

Understanding gas sorption by water in the atmosphere is an active research area because the gases can significantly alter the radiation and chemical properties of the atmosphere. We attempt to elucidate the molecular details of the gas sorption of water and three common atmospheric gases (NO, SO, and O) by water droplets/slabs in molecular dynamics simulations. The system size effects are investigated, and we show that the calculated solvation free energy decreases linearly as a function of the reciprocal of the number of water molecules from 1/215 to 1/1000 in both the slab and the droplet systems. By analyzing the infinitely large system size limit by extrapolation, we find that all our droplet results are more accurate than the slab results when compared to the experimental values. We also show how the choice of restraints in umbrella sampling can affect the sampling efficiency for the droplet systems. The free energy changes were decomposed into the energetic ΔU and entropic -TΔS contributions to reveal the molecular details of the gas sorption processes. By further decomposing ΔU into Lennard-Jones and Coulombic interactions, we observe that the ΔU trends are primarily determined by local effects due to the size of the gas molecule, charge distribution, and solvation structure around the gas molecule. Moreover, we find that there is a strong correlation between the change in the entropic contribution and the mean residence time of water, which is spatially nonlocal and related to the mobility of water.

Elucidating the mechanism of reactive uptake of N$_2$O$_5$ in aqueous aerosol

“…3. This indicates that N 2 O 5 is preferentially solvated at the interface, which is consistent with previous studies using empirical potentials 18,19 and the weak hydration observed in our bulk simulations. From this, we estimate an evaporation rate of k e = 12.5 ns −1 .…”

“…Molecular simulations can in principle be used to gain microscopic insight into the reactive uptake of atmospheric gases into solution, but traditional theoretical methods are insufficient to reach the broad range of length and time scales required. 12 Classical force fields have been used to study the physical solvation of N 2 O 5 , 18,19 where it is computationally tractable to employ enhanced sampling methods and represent large inhomogeneous systems. However, existing potentials are not suitable to model chemical reactions, precluding a study of the hydrolysis reaction.…”

Highly Accurate Many-Body Potentials for Simulations of N2O5 in Water: Benchmarks, Development, and Validation

“…Classical force fields, which are widely used in computer simulations because of their computational efficiency, have been employed in molecular dynamics (MD) simulations to study the hydration properties of N2O5. 5,23 However, due to their inherent approximations and limited accuracy, classical force fields might not be relied upon for quantitative predictions. In ab initio MD (AIMD), the configurations of a system are explored at a given thermodynamic condition with energies and forces computed on-the-fly from first-principles electronic structure calculations 24,25 .…”

Thermodynamic mechanism of selective cocrystallization explored by MD simulation and phase diagram analysis

“…Thanks to the rapid development of the supercomputers and computation chemistry, many researchers have investigated the strength of solute‐solute, solute‐solvent, and solvent‐solvent interactions in solution, to explain the solubility behavior by comparing the free energies. Various of theoretical models for the calculation of SFE have been proposed, such as the implicit solvent efficient, density functional theory (DFT), reference‐modified density functional theory (RMDFT), integral equation theory, energy representation method based on molecular dynamics (MD) simulation, and DFT interaction potentials with MD simulation (DFT‐MD) . Compared with all above‐mentioned methods, the standard approaches for calculating the SFE based on MD, either the free energy perturbation method or the thermodynamic integration method, are more widely applied, in virtue of theoretically higher accuracy .…”