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

Abstract

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...

Thermodynamic and Kinetic Studies on the Conversion of Solvent-Shared to Contact Ion Pairs in Sparingly Soluble MF2 (M = Mg2+ and Ca2+) Aqueous Solutions: Implications for Understanding Supersaturated Behavior and Association Constant Determination

“…Subsequently, all simulations were run for 20 ps to achieve good statistics. In our previous theoretical study on the Ca 2+ -F – system, the convergence test suggests that the running times of the CMD and AIMD simulations in the present work are sufficient to achieve equilibrium …”

“…However, F – does not all exist in the form of completely free hydration ions in CMD-box-u and CMD-box-b1 . Due to the strong EA between Mg 2+ and F – , , all 20 F – ions in the former and 8 F – ions in the latter are involved in the formation of SSIPs, even 2SSIP. The statistical analysis (Figure S3 and Table S5) suggests that Mg­(H 2 O) 6 F + and MgF­(H 2 O) 6 F SSIPs predominate, except for the completely free F – , in both boxes.…”

“…The trajectory data of the last 3 ns were collected for subsequent statistical analysis. The residence time of the water molecules or F – in the second shell around MgF x 2– x ( x = 1 and 2) clusters was calculated by the method used in our previous works. , The exchange frequency ( f ex ) of water molecules in the second hydration shell of Mg 2+ was estimated using N / t c , where N is the number of water molecules entering and exiting the second hydration shell of Mg 2+ in the collected simulated time t c (i.e., 3 ns).…”

“…(ii) Although MgF 2 /CaF 2 is also sparingly soluble in pure water, the interaction of M 2+ (M = Mg 2+ and Ca 2+ ) and F – was regarded as being very weak, and thus, only MgF + /CaF + was regarded as being present in early thermodynamic studies by fluoride ion-selective electrodes (ISEs). , This seems to contradict the nucleation mechanism via the aggregation of neutral ion pairs, as observed in the CaSO 4 /CaCO 3 system above. (iii) The results of our previous computational study on CaF 2 aqueous solution demonstrated that the neutral CaF 2 (aq) pair is strongly implicated from a thermodynamic stability point of view. However, the anomalous rise of the dissociated energy profile of CaF + calculated by ab initio molecular dynamics (AIMD) suggests that the ligand reaction measured by fluoride ISEs might correspond to a conversion process of free Ca 2+ and F – to Ca 2+ -F – SSIPs rather than CIPs.…”

Evaluation for internal consistency in the thermodynamic network involving fluorite, cryolite and villiaumite solubilities and aqueous species at 25°C and 1 bar

“…For fluorite solubility, the molal concentrations of the cation and anion are about the same and having the log K 2 >> log K 1 does not seem reasonable so this formation constant has not been used in the calculations below. Recent ab initio computations by Zhang et al (2021) demonstrated the configurational feasibility of the CaF° 2 ion triplet but it had a large energy barrier which make it less likely. Higher-order ion clusters were shown not to be feasible.…”