Abstract
We present the results of ab initio molecular dynamics simulations of the solution–air interface of aqueous lithium bromide (LiBr). We find that, in agreement with the experimental data and previous simulation results with empirical polarizable force field models, Br– anions prefer to accumulate just below the first molecular water layer near the interface, whereas Li+ cations remain deeply buried several molecular layers from the interface, even at very high concentration. The separation of ions has a profound effect on the average orientation of water molecules in the vicinity of the interface. We also find that the hydration number of Li+ cations in the center of the slab Nc,Li+–H2O ≈ 4.7 ± 0.3, regardless of the salt concentration. This estimate is consistent with the recent experimental neutron scattering data, confirming that results from nonpolarizable empirical models, which consistently predict tetrahedral coordination of Li+ to four solvent molecules, are incorrect. Consequently, disruption of the hydrogen bond network caused by Li+ may be overestimated in nonpolarizable empirical models. Overall, our results suggest that empirical models, in particular nonpolarizable models, may not capture all of the properties of the solution–air interface necessary to fully understand the interfacial chemistry.
Highlights
From seawater to the cellular environment and in many industrial processes, ionic salts are a major component of aqueous systems
Two recent studies have investigated the air−liquid interface of aqueous sodium chloride using Car−Parrinello molecular dynamics (CPMD).[25,26]
We examine the effect of addition of lithium bromide (LiBr) salt to water using density functional theory (DFT)-based simulations, in particular ab initio molecular dynamics (AIMD), and Born−Oppenheimer molecular dynamics (BOMD)
Summary
From seawater to the cellular environment and in many industrial processes, ionic salts are a major component of aqueous systems. All simulations with nonpolarizable empirical models predict a stable tetrahedral coordination shell with four hydrating water molecules, and no otherwise reliable nonpolarizable empirical model for the Li+−water interaction has been shown to predict much deviation from this tetrahedral hydration.[35] One interpretation of this very stable tetrahedral coordination is that the Li+ ion acts in some ways as a larger ion which includes the four hydrating water molecules.[17,36] Available simulation data using AIMD or hybrid QM-MM methods indicate a tetrahedral coordination for Li+;37−39 these studies did not include dispersion effects in their computations of the quantum mechanical potential energy surface, and because inclusion of dispersion has been shown to be critical to describe the structure of pure water accurately,[23,24,40−43] it is likely that dispersion will be important in ionic solutions as well. We take advantage of the symmetry of the slab to average results on either side of the center of mass and show only one half of the slab in our figures
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