Theoretical Study Of The Structural Characteristics And Hydration Shells Of Ca-Cl Ion-associated Clusters In The Salt Aqueous Solution

The structural characteristics and properties of CaCl2 aqueous solution is of great significance to several industrial processes such as inorganic materials of energy storage and saline lake exploration. In this work, first-principle calculations were carried out to investigate structural characteristics of the hydrated [CaClx]2-x (x=3～6) clusters, the hydration of ion clusters and the shielding role of ionic hydration on the inner clusters in the aqueous solution. The structural characteristics of the [CaClx]2-x (x=3～6) clusters and the influence of temperatures on those highly coordinated Ca-Cl associated species in concentrated CaCl2 solution were investigated by MD simulations. Unbiased MD and biased MD simulations were employed to investigate the structural characteristics of the [CaClx]2-x (x=3～6) clusters in the concentrated LiCl brine and also the influence of temperature on those highly coordinated Ca-Cl associated species. The hydration shells of [CaClx]2-x (x=3～6) clusters in concentrated LiCl aqueous solution and the shielding role of ionic hydration on the inner clusters in concentrated LiCl brine were studied at various temperatures. The main results are as follows:Quantum chemistry calculations shows that the [CaClx]2-x (x=3～6) clusters are unstable in the gas phase. However, the stabilities of the highly coordinated Ca-Cl ion-associated species can be enhanced to some extent when ion hydration is considered, indicating that ion hydration can enhance the stabilizing abilities of the [CaClx]2-x (x=3～6) clusters although those ion clusters may be are still unstable. Population analyses of natural bonding orbital shows such enhancing effect may also contribute the charge transfer from ion cluster to hydration shell, especially for [CaCl5]3- and [CaCl6]4-.MD simulations of concentrated CaCl2 solutions showed that the highly coordinated [CaClx]2-x (x=3～6) care unstable species, generally converting into smaller structures or aggregating into larger clusters. There are a great number of Ca2+, with strong hydration ability and flexible hydration shell in concentrated CaCl2 solution, which could destroy the hydration shell of the high-coordinated Ca-Cl clusters and pave the way for disassociation or aggregation of high-coordinated Ca-Cl clusters. The analysis of the hydration shell of [CaClx]2-x (x=3～6) species shows that the hydration shells of the high-coordinated Ca-Cl complexesare relatively stable, which can be enhanced by ionic association in solution. And thus the relatively stable and symmetric hydration layers of the highly coordinated [CaClx]2-x (x=3～6) clusters can protect the inner high-coordinated clusters from being disturbed. As temperature increases, the hydration shells become more flexible, which could pave the way for the ionic exchange between the first and second complexation shell of central Ca2+ and result in the disassociation of highly coordinated [CaClx]2-x (x=3～6) clusters. In this work, the existence of high-coordinated Ca-Cl complexes in nonequilibrium conditions can be attributed to the hydration characteristics of those complexes and to inadequate hydration of ions in mixed CaCl2-LiCl aqueous solution. This study of high-coordinated metal halide species providesa guide for solving the problem in geothermal exploration and energy storage, such supercooling of CaCl2 hydrates.