Although important for atmospheric processes and gas-phase catalysis, very little is known about the hydration state of ions in the vapor phase. Due to the high solvation free energy of ions of more than 100 kBT, the vapor phase density of ions is extremely low, which hampers experimental as well as simulation studies. As a consequence, the equilibrium hydration state of ions is not known and the question whether ions in the vapor phase are hydrated, i.e., surrounded by a hydration shell of adsorbed water molecules or not, is not settled.
Combining four different equilibrium and non-equilibrium molecular dynamics simulation protocols, we overcome technical simulation obstacles and study the evaporation energetics and kinetics of a chloride ion from liquid water. We find that as chloride permeates the interface, a water finger forms that breaks at a chloride separation of ≈ 2.8 nm from the Gibbs dividing surface. The video in Fig. 1 shows a simulation of a chloride ion pulled with a constant velocity from the liquid water phase into the vapor phase.
After breakage of the water finger, about 7 water molecules stay bound to chloride in saturated water vapor, as we corroborate by continuum dielectrics and statistical mechanics models. This vapor hydration layer significantly lowers the solvation free energy of chloride by about 26 kBT. However, for a description of the barrier crossing not only the height of the potential (which appears in the exponent of the Arrhenius law) is relevant but also the friction coefficient or diffusivity in the transition state.
By additional simulations, we extract the effective chloride diffusivity in the transition state. Figure 2 shows a simulation where a constant force acts on the ion. This force reduces the barrier height and allows to capture transitions over the barrier in times that are accessible in atomistic simulations.
By running several of these non-equilibrium simulations at different applied forces, we find that the transition state diffusivity is about 6 times higher than in bulk, which reflects the highly mobile hydration dynamics as the water finger breaks. Both modifications of the solvation free energy and the transition state diffusivity significantly increase the chloride evaporation flux from the quiescent interface of an electrolyte solution, which we predict from reaction kinetic theory. In addition, we predict the total chloride evaporation flux from the earth’s ocean, which is still too low to account for the significant chloride concentration one finds in the earth´s atmosphere. This in turn suggests that non-equilibrium spray formation due to wind and oceanic waves must be dominant.
The simulation framework introduced in our paper allows for the study and the understanding of ionic evaporation kinetics and the hydration state of ions in the vapor phase and therefore will advance our understanding of gas phase reactions involving ions in general. Our simulation techniques are not limited to chloride but will be used for all kinds of rare evaporation phenomena.
For more information, you can read more about our work in Nature Chemistry Communications following the link: https://www.nature.com/articles/s42004-022-00669-5
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