Bulk and interfacial aqueous fluoride: an investigation via first principles molecular dynamics.

Using first principles molecular dynamics simulation, we have studied a fluoride anion embedded in a periodically replicated water slab composed of 215 water molecules to mimic both bulk and interfacial solvation. In contrast to some recent experiments, our findings suggest that there are only small structural changes for fluoride and its first solvation shell in the bulk. Moreover, the presence of fluoride does not significantly alter the rotational dynamics of nearby water. In addition, we have computed the molecular dipole moments using Wannier centers. At the interface, the presence of fluoride increases the molecular dipole moments of nearby water molecules, whereas in the bulk, the dipole moments for water appear to be essentially invariant to the presence of fluoride in the vicinity. Previous studies of the air-water interface have showed interfacial water to have higher average HOMO energies and, thus, likely to be more prone to electrophilic attack. With the addition of fluoride, the most likely reactive site for electrophilic reactions shifts to the anion. This finding could explain the known large increase in reaction rates for heterogeneous process of interest in atmospheric science. The reactive properties of other anions near the air-water interface are of general interest in heterogeneous chemistry and can be elucidated using a similar type of analysis, as performed here for the fluoride anion.