Computational Investigation of the First Solvation Shell Structure of Interfacial and Bulk Aqueous Chloride and Iodide Ions

Molecular dynamics simulations with polarizable interaction potentials were carried out to understand the solvation structure of chloride and iodide anions in bulk and interfacial water, showing qualitative similarities between the first solvation shell Structures at the interface and bulk. For the more polarizable iodide, its solvation structure was found to be more anisotropic than chloride, and this trend persisted at both the interface and in the bulk. The anisotropy of the solvation structure correlated with polarizability, but it was also found to inversely correlate with anion size. When polarizability was reduced to near zero, a very small anisotropy in the water solvation structure around the ion still persisted. Polarizable anions were found to have on average an induced dipole in the bulk that was significantly larger than zero. This induced dipole resulted in the water hydrogen atoms having stronger interactions with the anions on one side of them, in which the dipole was pointing. In contrast, the other side of the anions, in which the induced dipole was pointing away from, had fewer water molecules present and, for the case of iodide, was rather devoid of water molecules all together at both the interface and in the bulk. This region formed a small cavity in the bulk, whereas at the air-water interface it was simply part of the air interface. In the bulk, this small cavity may be viewed as somewhat hydrophobic, and the need for the extinction of this cavity may be one of the major driving forces for the more polarizable anions to reside at the air-water interface.