盐类在水中微观溶解机制的理论研究
Creators: name: Chengwen Liu
Publisher: name: figshare
Year: 2019
Subjects: subject: 30799 Theoretical and Computational Chemistry not elsewhere classified
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences

description: Inorganic salts play critical roles in various aspects of human life. For example, they are involved in many biological functions, including signal transduction, enzymatic reactions, and acting as non-specific salt buffers for biomolecules. The solvation of salts is a very fundamental physiochemical process and it can profoundly alter chemical and physical interactions in systems. To understand the ion-water interactions, Hofmeister proposed the famous “Hofmeister Series” in 1888. However,lots of controversies and unclearness on the molecular mechanisms behind this series exist. Therefore, it is crucial to explore the solvation process of inorganic salts in water and to understand the molecular interactions among cations, anions and water for various complex chemical and biological processes.In this thesis, we choose three alkali-halide ion pairs, LiI, CsI and NaCl, which locate in three different positions of Hofmeister series, as model systems. These three ion pairs are in combination of a number of water molecules to form ion-water clusters, to study the gradual microsolvation of the ion pairs. While cannot be expected to quantitatively predict the effects of liquid-phase solvation on these salts, these microsolvation models provide valuable insights into the solvent around the M+ and X- ions and the effect of solvation on the MX bond. The cluster method is apromising approach for studying the solution behavior in viarous studies. For small systems, we combined quantum chemical calculations with negative ion photoelectron spectroscopy (NIPES) measurements to investigate the electronic states, energetics and structural properties of the clusters. While for the relatively large-size (consisting of more water molecules) clusters, due to their remarkably complex potential energysurface, we utilize the well-established Integrated Tempering Sampling (ITS) method to overcome the difficulties in sampling the configuration space. Then we perform calculations by using a series of low-to-high level computational methods, i.e., from force fields to semi-empirical, Density Functional Theory (DFT) and MP2 methods. In summary, we use ITS(MM)-QM framework to obtain the systematic sampling of the configuration space and the thermodynamic properties of the clusters, as well asthe accurate calculations on representative low-energy cluster structures.The results indicate that the microsolvation mechanisms of LiI, CsI and NaCl in water are very different from each other, representing on: 1. interactions between cations and water follow the order Li+ &gt; Na+ &gt; Cs+. When the number of water molecules is larger than 4, lithium ion adopts its well-believed tetra-coordinated form; while the coordination number of sodium ion can vary from 4 to 6; the coordination number can be more flexible (ranging from 3 to 6) for cesium ion. For the anions, chloride-water interaction is stronger than iodide-water interaction. The commonfeature between them is that they all prefer residing on the surface of the clusters. 2. The solvation of the three ion pairs follows the order of LiI &gt; NaCl &gt; CsI. For the Li-I ion pair, when five water molecules exist, the first hydration shell of Li+ is well-formed and Li-I is an SIP form ion pair; when more than ten water molecules exist, the Li-I distance is elongated further. It needs at least ten water molecules to fully separate Na-Cl ion pair. As for Cs-I ion pair, even the number of water molecules reaches 20, the Cs-I distance only changes a little. These results show that it is much easier for CsI to be in pair than NaCl and LiI. This finding is in accordance with the law of matching water affinities. 3.Through the analysis on the effects ofions on water hydrogen bond networks we found that, compared to pure-water clusters, LiI makes the hydrogen bonds formed by the waters in the first hydration shell much stronger, but makes the water molecules outside the first hydration shell form less hydrogen bonds. CsI only slightly perturbs the water hydrogen bonds. From the study on NaCl monomer, dimer and trimer we found that, the strength of Na∙∙∙O and Cl∙∙∙H interactions are quite similar, which is represented by the formation of cuboid structures in clusters. This finding well reflects the fact that these two ions locate on the borderline of the Hofmeister Series. These results indicate that the ion cooperativity plays significant role in the formation of salt-water clusters.The results from these studies provide the molecular level information for the norganic salts dissolution process, Hofmeister Series and ion-specific pairing. The ITS(MM)-QM framework has been validated in studying pure-water and salt-water clusters. In the future work, it will be used in systems composed of complex anions, such as SO42− , HSO4− and NO3− , and high volatile organic compounds, such as aldehydes, alcohols and acids. It is of great importance to exploring the detailed interactions among these species, as their significance in atmospheric aerosols related to air pollution in China. The potential development of this method lies in thecombination of ITS method with ab initio molecular dynamics (AIMD) simulations. Considering of the high cost of AIMD, semi-empirical methods, such as SCC-DFTB, will be more practical. This developed version is helpful in studying various chemical reactions happening at interface and in complex systems.