Computational and solubility equilibrium experimental insight into Ca-fluoride complexation and their dissociation behaviors in aqueous solutions: implication for the association constant measured using fluoride ion selective electrodes.

Although the Ca-F association is of great importance for aqueous environments and industrial systems containing F, as well as for defluorination processes, many details of the association solvation structures and behavior remain unclear. Herein, a combination of classical/ molecular dynamics simulations and density functional theory calculations was used to investigate the structure and hydration of CaF ( = 1, 2) and the association/dissociation behavior of Ca-F in aqueous CaF solutions. The primary shell of Ca is found to be very flexible in the association of Ca-F, with coordination numbers dynamically oscillating in the range of 6-9, with 6 and 7 being the most favorable. The calculations show that for CaF(HO), the contact ion pair (CIP) is more favorable and occurs with no energy barrier, whereas the formation of CaF(aq.) must overcome a ∼3.6 kJ mol energy barrier; moreover, the CIP and solvent shared ion pair (SSIP) dynamically coexist for CaF(HO) in aqueous CaF solutions. Calculations for the dissociation process of CaF(HO) show a dramatic energy increase going from SSIP to free Ca and F, ascribed to the surprisingly long-range electrostatic attraction between Ca and F rather than to special F⋯H interactions. The energy increase results in the estimated association constant of CaF being larger than that previously measured using fluoride ion selective electrodes. This is attributed to the fact that the latter value might correspond to the ligand reaction of free Ca and F to form the Ca-F SSIP. The combination of these results with CaF solubility measurements suggests that the higher-order Ca-F complexes are absent in aqueous CaF solutions.