Anion-water interactions of weakly hydrated anions: molecular dynamics simulations of aqueous NaBF4 and NaPF6

In aqueous ionic solutions, both the structure and the dynamics of water are altered dramatically with respect to the pure solvent. The emergence of novel experimental techniques makes these changes accessible to detailed investigations. At the same time, computational studies deliver unique possibilities for the interpretation of the experimental data at the molecular level. Here, using molecular dynamics simulations, we demonstrate how competing mechanisms can explain the seemingly contradictory statements about the structure and dynamics of ion-coordinated solvent in aqueous solutions of two interesting and technologically important electrolytes, NaBF4 and NaPF5. While the static structural data (i.e. radial, radial-angular and spatial distribution functions, as well as hydrogen bonding statistics) unequivocally point at very weak anion-water hydrogen bonding in both salts, dynamic analyses (in particular, orientational anisotropy decay and solvent residence times) reveal quite significant retardation of water rotation and mobility due to solute coordination. Additionally, rotational immobilisation of coordinated solvent molecules is clearly unrelated to the hydrogen bond strength between them, as demonstrated by the interatomic oxygen-oxygen distance distributions for coordinated and bulk water.