Molecular dynamics simulation study on distinctive hydration characteristics of highly coordinated calcium chloride complexes

In this study, molecular dynamics simulations were used to investigate the role of hydration shells on the stabilization of ion clusters. The stabilization of [CaClx]2−x (x = 4–6) species can be enhanced by their hydration shells. The shrinkage of the third hydration shell can be observed for the [CaClx]2−x complex while not for the [CuCl]+ or [CaCl2]0 species in an aqueous solution. The shrinking third shell together with the second shell of Ca2+ form a compact inner hydration shell around those clusters promoting their existence in an aqueous solution. Their inner hydration shells, forming a distinctive hydrogen bond network around the ion cluster, act as a “cage” structure, confining multi-Cl− ions around Ca2+, and promote the occurrence of [CaClx]2−x species in the aqueous solution. Our results demonstrate that the structural characteristics of some ion clusters combined with their distinctive hydration characteristics have a huge impact on ion association in an aqueous phase.