Revealing the Molecular Origin of Anisotropy around Chloride Ions in Bulk Water

A clear picture of the local solvation structure around halide anions in liquid water remains elusive. This discussion has been stimulated by pioneering simulation results that proposed a "hydrophobic cavity" around anions in the bulk, which is analogous to air at the air-water interface. However, there is also sound experimental and theoretical evidence that halide ions are rather symmetrically solvated in the bulk, leading to a different viewpoint. Using extensive ab initio molecular dynamics simulations of an aqueous Cl^- solution, we indeed find an anisotropic arrangement of H-bonded versus interstitial water molecules. The latter are not H-bonded to the anions and thus do not couple much electronically to Cl^-. The resulting purely electronic anisotropy of the local solvation environment correlates with that structural anisotropy, which however should not be understood as an empty cavity─as it would be at the air-water interface─but rather contains interstitial water molecules.