Theoretical Study The Structural Characteristics Of Hydration Of Chloride Brine Solution

Chloride brine solution exists widely in seawater and saline lake， butmicrostructure information is not directly obtained in experiment, such as ionhydration structure. Great attention has been paid to theoretical study of chloridehydration. In order to get accurate description of the thermodynamic properties fromdilute solution to the molten salt brine solution system, it is necessary to further studyon the species and distribution of each species salt in brine solution system. Based onthe hydrated cluster model, LiCl（H₂O）n， MgCl₂（H₂O）n， CaCl₂（H₂O）n，[CuCl₃^--Li+-（H₂O）n] and [CuCl₂-LiCl-（H₂O）n] hydrates were investigated using B3LYPmethod. The stability of LiCl，MgCl₂， CaCl₂species in dilute solution was analysedusing cluster/polarized continuum model combined with the approximate saturatedhydration of central ions. The characteristics of LiCl（H₂O）nclusters was investigatedusing ab initio molecular dynamics （AIMD）, transition state theory and classicalmolecular dynamics simulation （MD） method, and the effects of temperature werediscussed as well. Characteristics of the first hydration shell, and the stability of ionpair for MgCl₂（H₂O）nand CaCl₂（H₂O）nclusters were investigated using AIMDsimulations, and the effects of temperature were also discussed. In addition, theelectronic absorption spectra of some stable structure of [CuCl₃^--Li+-（H₂O）n] and[CuCl₂-LiCl-（H₂O）n] clusters were obtained using LRC-TDDFT method.Generally, three-coordinated and four-coordinated structures of LiCl hydrates arealmost isoenergetic in the gas phase, while four-coordinated structures are slightlymore stable in the dilute aqueous solution. Solvent separated ion pair （SSIP） andcontact ion pair （CIP） isomers are almost isoenergetic both in the gas phase and in thedilute aqueous solution. This indicates that the presence of CIP structures in the LiClaqueous solution can not completely be neglected even in the dilute aqueous solution.The association of Li+and Cl-or the dissociation of CIP in the aqueous solution doesnot result in obvious charge transfer between Li+and Cl-, and the electrostaticinteraction between Li+and Cl-in the aqueous solution is very weak. In addition, thereis only a small energy barrier between CIP conformer and its SSIP isomers, and suchenergy barrier may mainly result from the reorganization of Li+hydration shell ratherthan the electrostatic interaction between Li+and Cl-.The transformation between CIPand SSIP conformer of LiCl aqueous solution can be occurred very easily and frequently. It agrees with our results of classical MD simulations. Furthermore, ourAIMD simulations and classical MD simulation all demonstrate that the possibility ofCIP conformers presented in the aqueous solution increases as temperature rising,while that of SSIP decreases. Therefore, the interaction of Li+hydrated species and Cl-hydrated species probably acts like neutral hydrated clusters in the aqueous solution,and a lot of ion pairs or ion clusters presented in the concentrated LiCl aqueoussolution do not result in nucleation of LiCl crystallization.In the gas phase, SSIP and CIP isomers of MgCl₂hydrates are still almostisoenergetic, while CIP conformers of CaCl₂hydrates are obviously stable than theirSSIP （a Cl--dissociation） isomers, and SSIP （a Cl--dissociation） conformers of CaCl₂hydrates are more stable than its SSIP （two Cl--dissociation） conformers. Mg2+ion ofMgCl₂hydrates tends to low coordination at high concentration, while tends to highcoordination at low concentration. However, six-coordinated structures are more stablefor CaCl₂hydrates. In the dilute aqueous solution, MgCl₂hydrates exists in the form ofSSIP （two Cl-dissociation） structure， while SSIP and CIP conformers of CaCl₂hydrates are still almost isoenergetic. In addition, during the dissolution process,hydrates of MgCl₂are easier to form an approximate saturated hydration shell than thatof CaCl₂.For [CuCl₃^--Li+-（H₂O）n] and [CuCl₂-LiCl-（H₂O）n]clusters, Cu2+and Li+bothprefer four coordination. Regardless [CuCl₃^--Li+-（H₂O）n] or [CuCl₂-LiCl-（H₂O）n]clusters, charge population on Cu2+or Cl-has no significant change and the distancebetween Cu2+and Cl-almost remains unchanged with increasing water molecules.Moreover, electronic absorption spectra show that [CuCl₃^--Li+-（H₂O）n] clusters havetwo obvious absorption peaks at²90nm and380nm, which is similar with UV-VISspectra of [CuCl₃^-] species.[CuCl₂-LiCl-（H₂O）n] clusters have an obvious absorptionpeak at273nm, which corresponds with the characteristic peak of [CuCl₂] speciesobtained from UV-VIS spectroscopy experiment and theoretical calculations. In a word,the presence Li+in the aqueous solution does not result in obvious influence toUV-VIS spectrum of [CuCl₃^-] species，so does the presence of LiCl ion pairs in theaqueous solution to [CuCl₂] species.