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 2+ -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/ ab initio molecular dynamics simulations and density functional theory calculations was used to investigate the structure and hydration of CaF x 2− x ( x = 1, 2) and the association/dissociation behavior of Ca 2+ -F − in aqueous CaF 2 solutions. The primary shell of Ca 2+ is found to be very flexible in the association of Ca 2+ -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(H 2 O) 14 + , the contact ion pair (CIP) is more favorable and occurs with no energy barrier, whereas the formation of CaF 2 (aq.) must overcome a ∼3.6 kJ mol −1 energy barrier; moreover, the CIP and solvent shared ion pair (SSIP) dynamically coexist for CaF 2 (H 2 O) 14 in aqueous CaF 2 solutions. Calculations for the dissociation process of CaF(H 2 O) 6 + show a dramatic energy increase going from SSIP to free Ca 2+ and F − , ascribed to the surprisingly long-range electrostatic attraction between Ca 2+ 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 2+ and F − to form the Ca 2+ -F − SSIP. The combination of these results with CaF 2(s) solubility measurements suggests that the higher-order Ca 2+ -F − complexes are absent in aqueous CaF 2 solutions. Not the special F H bond interactions but the residual influence of the electrostatic attraction of Ca 2+ -F − governs the dissociation behavior of Ca 2+ -F − going from SSIP to completely free Ca 2+ and F − despite the long distance between the two ions.