Modelling the effects of salt solutions on the hydration of calcium ions

Abstract

Classical molecular dynamics simulations of several aqueous alkali halide salt solutions have been used to determine the effect of electrolytes on the structure of water and the hydration properties of calcium ions. Compared with the simulations of Ca(2+) ions in pure liquid water, the frequency of water exchange in the first hydration shell of calcium, which is a fundamental process in controlling the reactivity of calcium(ii) aqua-ions, is drastically reduced in the presence of other electrolytes in solution. The strong stabilization of the hydration shell of Ca(2+) occurs not only when the halide anions are directly coordinated to calcium, but also when the alkali and halide ions are placed at or outside the second coordination shell of Ca(2+), suggesting that the reactivity of the first solvation shell of the calcium ion can be influenced by the specific affinity of other ions in solution for the water molecules coordinated to Ca(2+). Analysis of the hydrogen-bonded structure of water in the vicinity of the calcium ion shows that the average number of hydrogen bonds per water molecules, which is 1.8 in pure liquid water, decreases as the concentration of alkali-halide salts in solution increases, and that the temporal fluctuations of hydrogen bonds are significantly larger than those obtained for Ca(2+) in pure liquid water. This effect has been explained in terms of the dynamics of reorganization of the O-H···X(-) (X = F, Cl and Br) hydrogen bond. This work shows the importance of solution composition in determining the hydrogen-bonding network and ligand-exchange dynamics around metal ions, both in solution and at the mineral-water interfaces, which in turn has implications for interactions occurring at the mineral-water interface, ultimately controlling the mobilization of ions in the environment as well as in industrial processes.

Molecular Dynamics Simulations of Hydroxyapatite Nanopores in Contact with Electrolyte Solutions: The Effect of Nanoconfinement and Solvated Ions on the Surface Reactivity and the Structural, Dynamical, and Vibrational Properties of Water

“…The value of D W for bulk SPC/E water is 25.8 × 10 −10 m 2 ·s −1 , in good agreement with previous work [60]. In bulk CaCl 2 and CaF 2 solutions there is a marked decrease in water diffusivity, which is substantially higher than in aqueous alkali halide solutions [61]. For example, in 1.9 mol·kg −1 NaCl (aq) the value of D W is 22.0 × 10 −10 m 2 ·s −1 , which is higher than the value of D W (16.3 × 10 −10 m 2 ·s −1 ) in the 1.3 mol·kg −1 CaCl 2 (aq).…”

“…This result can be rationalized in terms of the stronger coordination of water molecules to Ca 2+ compared with Na + . In fact, the mean residence time (MRT) of water molecules in the first hydration shell of Ca 2+ is in the range of 23-105 ps (depending on the interatomic potential model used to describe the calcium-water interaction [57,62,63]), whereas the MRT of Na + is only 8 ps [61]. This implies that the hydration shell of the sodium ion is more labile than that of calcium ions.…”

“…Chloride and fluoride ions impact on the reactivity of calcium by directly coordinating to the metal ion [72] but also when they are in the second coordination shell of Ca 2+ [61]. In Figure 6, the Ca-O(water), Ca-Cl, and Ca-F RDFs in the HAP (H = 60 Å) nanopores show that surface calcium ions are tightly coordinated to water molecules whereas chloride and fluoride ions are mainly located in the second or higher hydration layers.…”

On the selectivity of the NaChBac channel: an integrated computational and experimental analysis of sodium and calcium permeation

“…The fact that the free energy of solvation of calcium is about three times and a half higher than that of sodium might explain why calcium keeps its whole solvation shell upon entering into the SF. This behaviour is also consistent with the dynamics of exchange of water molecules between different coordination shells around metal ions studied by Di Tommaso et al 41 In particular, these authors reported that the frequency of water exchange in the first shell of calcium decreases as the halide concentration increases. Therefore, electrolytes in solution strongly stabilize the first hydration shell of Ca 2+ .…”

Molecular insight into the nanoconfined calcite–solution interface

“…Further, the increase in surface concentration of Ca 2+ ions with bulk concentration obtained through the analysis of the FTTs is consistent with the less-negative surface potentials of silica and calcite obtained by fitting the DLVO forces (SI Appendix, Tables S2 and S3). Although the concentration is expected to influence both the water structure (27) and the size of ion hydration shells (28), we do not attempt to assign a different composition to the multiple peaks shown in Fig. 3 due to the overlapped associated errors.…”