Computational and solubility equilibrium experimental insight into Ca2+-fluoride complexation and their dissociation behaviors in aqueous solutions: implication for the association constant measured using fluoride ion selective electrodes.

Although the Ca2+-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 CaFx2-x (x = 1, 2) and the association/dissociation behavior of Ca2+-F- in aqueous CaF2 solutions. The primary shell of Ca2+ is found to be very flexible in the association of Ca2+-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(H2O)14+, the contact ion pair (CIP) is more favorable and occurs with no energy barrier, whereas the formation of CaF2(aq.) must overcome a ∼3.6 kJ mol-1 energy barrier; moreover, the CIP and solvent shared ion pair (SSIP) dynamically coexist for CaF2(H2O)14 in aqueous CaF2 solutions. Calculations for the dissociation process of CaF(H2O)6+ show a dramatic energy increase going from SSIP to free Ca2+ and F-, ascribed to the surprisingly long-range electrostatic attraction between Ca2+ 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 Ca2+ and F- to form the Ca2+-F- SSIP. The combination of these results with CaF2(s) solubility measurements suggests that the higher-order Ca2+-F- complexes are absent in aqueous CaF2 solutions.