Theoretical Study The Structural Characteristics Of Hydration Of Chloride Brine Solution

The chloride brine solution exists widely in seawater, but it is difficult to gain ion hydrated structure and ion association characteristics from conventional experimental methods. In this paper, the effects of different concentration s, temperatures and pressures on the ion cluster characteristics in MgCl₂ and CaCl₂ concentrated solutions have been investigated by the molecular dynamics method. We also investigated the influence of the addition of little LiCl on the microstructure characteristics in MgCl₂ solution and that of MgCl₂ and CaCl₂ on that in LiCl concentrated solution under various conditions. Moreover, the CPMD method has also been used to explore the ion association characteristics in MgCl₂ supersaturated solution, LiCl concentrated solution and LiCl concentrated solution containing a small amount of MgCl₂. The results of this dissertation are as follows.Dynamic simulation results show that there exist the solvent shared ion pair with single dissociated ion?SSIP/s? and the contact ion pair?CIP? conformers in MgCl₂ solution. SSIP/s represents that the associated structure of Mg²⁺ and a singleCl-, and CIP represents that the associated structure of Mg²⁺ and twoCl-. With the increase of concentration, ionic cluster size all becomes larger in MgCl₂ or CaCl₂ solution. Somewhat differently from the situation in the MgCl₂ dilute solution, there are only SSIP/s conformers in CaCl₂ dilute solution. It can be attributed to the hydration characteristics of Mg²⁺ and Ca²⁺. The contraint of Mg²⁺ is located in the first hydration shell, while the constraints of Ca²⁺ on the first and the second hydration shell are both strong. Though Mg²⁺ prefers to the six hydrated geometry, it also has the five and the four hydrated geometry. The energy difference among these isomers is small, and thus the ion association can occur easily. So the CIP and the SSIP/s conformers are easy to be found in MgCl₂ aqueous solution. The ion association in CaCl₂ aqueous solution is weaker, and only some SSIP/s conformers were found. With the temperature increasing, the ion cluster size gets smaller and especially in the 483 K and 573 K, the number of the SSIP/s conformer increases obviously, which indicates that the high temperature is not good for the ion association in the MgCl₂ solution. However, MD results indicate that with the temperature increasing, the ion association enhances in CaCl₂ aqueous solution, and the ion cluster size becomes larger. With the pressure increasing, the size of the largest ion cluster in the MgCl₂ solution is almost the same. When the pressure rises to 20.0 bar, the ion cluster size becomes smaller, which show that ion association only slightly changes at relatively high pressure in MgCl₂ aqueous solution. With the pressure increasing, the ion association enhance s in CaCl₂ concentrated solution. In addition, though there is only SSIP/s conformers in CaCl₂ dilute solution, but the ion cluster analyses show that the lifetime of CIP conformers is longer than that of SSIP/s conformers. It can be attributed to the hydration of CIP conformers which enhance the stability of CIP conformers in CaCl₂ dilute solution.When adding small amount of LiCl, the ion cluster size becomes smaller in MgCl₂ concentrated solution. Compared with the MgCl₂ concentrated solution, the number of the SSIP/s and that of CIP conformers both increase in the mixed solution. At the same time, the averaged lifetime of the SSIP/s conformers become shorter, and that of the CIP conformers get longer, sinceCl- prefers to contact with the Li⁺. We also consider the sameCl- between the MgCl₂ solution and the LiCl-MgCl₂ mixed solution. In contrast to the MgCl₂ solution, ion association becomes more obvious in such mixed solution. When adding little LiCl in MgCl₂ supersaturated solution, the size of ion clusters gets larger and the number of ion clusters also increases obviously. It indicates that the tendency of ion association becomes stronger with the increasing ofClconcentration.With the temperature increasing, the ion cluster size becomes larger in LiCl concentrated solution. At the ambient temperature, when put ting little MgCl₂ into the LiCl concentrated solution, ion cluster gets larger, but the addition of the Mg²⁺ andCldoes not result in obvious change on ion association in LiCl concentrated solution. At 323 K, the size of the largest ion cluster becomes s maller but the ion cluster size becomes more uniform. It indicates that the growth of ion clusters is inhibited at high temperature, which is in favor for metastable supersaturated solution or dissolution. At 298 K, with the addition of Ca²⁺ andCl-, the size of the largest ion cluster becomes larger in LiCl concentrated solution, and thus ion association was enhanced. At 323 K, the size of the ion cluster in the CaCl₂-LiCl mixed solution becomes smaller but uniform, and the lifetime of the ion cluster in such binary system does not change obviously. It can be concluded that the addition of CaCl₂ almost has no effect on the ion association in LiCl solution, but to some extent it really benefits for the dissolution or the formation of the metastable supersaturation in such binary solution.