The Interface Between Ionic Crystals And Solutions:A Molecular Dynamics Simulation

The interface between ionic crystal and solution is common in nature and human life,and is very important to understand crystallization,dissolution and adsorption and so on.Duo to the existence of multi-components,long-range Coulomb interaction,the water molecules,the structure and physical properities in this kind of interface are very complex and are difficult to be understood,which is rather different from that in simple interfaces.Up to now,there are two long-standing questions to be answered in physics:Firstly,are there some general feature in the interfacial structure?Secondly,a universal model to explain interface charge is still lack.In this thesis,the basic structure,hydration shell of ions,clusters in the interface between the alkali halide and its solution,have been investigated by the molecular dynamics simulation.The major results and conclusions include:(1).The general feature in the structure of alkali halide-solution interface.Compared to the structure of simple solid-liquid interface,this kind of interface is more complex.Firstly,the fluctuation of density for the ions and water molecules near the interface is not with regular periodicity and is not synchronous with each other.Secondly,there are two kinds of polarization for the water molecules in the interface.Thirdly,the cations prefer to coordinate with water molecules,while the anions prefer to coordinate with cation.(2).Hydration shell of ions and extended electric double layer model.In the interface,the hydration shell is pinned,and the pinning strength gradually decreases from crystal to solution;The dipole of water molecules in the hydrated shell shows an abnormal behavior,namely the degree of the order for the water molecule dipole increases from crystal to solution.The ions deviation from the ideal lattice in the first layer is positively related to the ion coordination number of water molecules.Based on the hydration shell of ions,an extended electric double layer model has been established.The interface charge predicted by this model is consistent with that measured by experiment.(3).The odd-even alternation in stability of clusters in the interface.The cluster at alkali halide-solution interface were investigated by molecular dynamics simulations.We have found that,firstly,the clusters at interface is two dimentional,and for each size of two-dimensional clusters(2DCs),a specific stoichiometric ratio is dominant.Secondly,2DCs show a remarkable feature of odd-even alternation in stability,and there are two patterns for the odd-even alternation.Thirdly,the stability of water molecules in the first hydration shell of 2DCs also shows the feature of odd-even alternation,which is consistent with that of the associated 2DCs.The pattern of the odd-even alternation in the stability of 2DCs is determined by the strength of the interaction between the dipole of water molecules and ions.(4).The effects of crystal surface morphology on the interfaceThe structure of NaCl(110)-solution interface have been studies,and compared with that in NaCl(001)-solution interface.The results show that,firstly,the degree of layering order in the(110)interface is larger than that in the(001)interface,which is contrary to the case in the simple solid-liquid interface.Secondly,one-dimensional clusters are formed in the(110)interface,and some ions are adsorbed on the top of the crystal surface,which is not the epitaxial position.Thirdly,the expansion and contraction for the hydration shell of Cl~-is not related to the position of Cl~-at the interface,but only related to whether it coordinate with Na~+.This may be the reason why the hydration shell of Cl~-have been changed before and after the dissolution of NaCl crystal.