First-Principles Molecular Dynamics Simulation of Atmospherically Relevant Anion Solvation in Supercooled Water Droplet

We present a comprehensive first-principles Born–Oppenheimer molecular dynamics (BOMD) simulation study of halide anion solvation in a deeply supercooled water droplet (with diameter ∼1.8 nm). We show that larger halide anions Br^– and I^– show “outer-layer surface preference”, whereas F^– exhibits bulk preference. Contrary to behavior of other halide anions, Cl^– in the water droplet appears to exhibit no strong tendency of surface or bulk preference at either the supercooled or ambient condition, a phenomenon not previously reported in the literature. BOMD simulation indicates that fully hydrated complex of F^– is mainly five-fold coordinated (showing square pyramid structure), whereas Cl^–, Br^– and I^– hydrated complexes are either five- or six-fold coordinated (showing sandwich-like structure). Among Cl^–, Br^– and I^– anions, BOMD simulation indicates that I^– exhibits the largest diffusion coefficient despite its largest size. However, computed resident time of the four halide ions suggests that Br^– can approach from the interior to the surface of the water droplet at a much faster rate than I^– and Cl^–.