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 similar to 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-.