A Theoretical Study Of The Association Characteristics And Nucleation Mechanisms In Two Kinds Of Sulfate Solutions

Magnesium sulfate?MgSO₄?and calcium sulfate?CaSO₄?are common inorganic substances in nature,where they play important roles in a wide range of industrial fields.Among them,due to seawater and salt lakes contained more Mg²⁺and SO₄^2-ions,the extracted MgSO₄ crystals in industry are mainly by drying the brine.As a common mineral,CaSO₄ has extensive commercial value in building materials,biomineralization,desiccant and medical applications,etc.However,the understanding of the microscopic properties of ions and the mechanism of nucleation and crystallization for MgSO₄ and CaSO₄ aqueous solutions is still unadequate at molecular level,and there are still many controversies.If the microscopic understanding of these aspects can be improved,it is very likely to effectively control their crystallization process,which can achieve more efficient economic value.Therefore,the ion association characteristics and the hydration dynamics of MgSO ₄and CaSO₄ aqueous solution at various concentrations were systematically investigated using classical molecular dynamics?MD?simulations and density functional theory?DFT?.And the results reveal the nucleation and crystallization mechanism of MgSO₄and CaSO₄ aqueous solutions at microscopic level.The main results of this paper are as follows:The results from MD and DFT methods show that contact associated?CA?and solvent separated?SS?structures of Mg²⁺and SO₄^2-ions are the dominant species and play different roles in the crystallization and gelation processes of MgSO₄ aqueous solution due to their distinctive association tendency and the distinctive dynamics of their second hydration shells.The CA structures are more stable and the further aggregation of these clusters is sluggish in MgSO₄ aqueous solution.The supersaturation or the formation of gel state can be partially attributed to the abundance of CA structures in supersaturated MgSO₄ aqueous solution.In contrast,although the ion exchange in SS structures occurs frequently,the aggregation of SS structures is active and can be greatly promoted by increasing of the concentration.The dynamics of the second hydration shells around Mg²⁺ions of SS structures also becomes more active as the concentration increases.The dynamic evolution characteristics of SS structures demonstrate their aggregation tendency for nucleation in the oversaturated MgSO₄ aqueous solution.In addition,based on the constraint MD simulation,it was found that the first shell of Mg²⁺ions from the extracted SS structure in supersaturated MgSO₄ aqueous solution is directly associated with six water molecules,and the six SO ₄^2-ions are distributed in its second shell.And some long-resident water molecules in the second hydration shell around Mg²⁺ions of SS structures can be gradually incorporated into heptahydrates as its seventh water molecule during the nucleation process.This indicates that the configurations of some Mg²⁺ions of SS structures extracted from our MD simulations are similar to those of the Mg²⁺ions in the crystal.Therefore,the dynamic evolution characteristics of SS structures demonstrate their aggregation tendency for nucleation in the oversaturated MgSO₄ aqueous solution,which helps to understand the nucleation mechanism in MgSO₄ aqueous solution and the role of ion hydration dynamics in the nucleation process.In this work,the ionic association characteristics,hydration characteristics,pre-nucleation cluster characteristics,the early nucleation pathway and crystallization mechanism in aqueous CaSO₄ solution were also studied in detail.MD results show that neutral clusters,positively and negatively charged clusters may be involved in the nucleation in supersaturated CaSO₄ aqueous solution,while pre-nucleation clusters are more likely to be neutral CaSO₄ clusters.The analyses of the water residence around these CaSO₄ clusters show that the exchange of some water molecules around Ca²⁺in these CaSO₄ clusters is more active compared with that of Ca²⁺?aq?,which will favor the further aggregation.The analyses of the water residence also show that the residence of some water molecules is enhanced in CaSO₄ clusters.That is,the residence of water molecules around Ca²⁺of CaSO₄ clusters or upon the clustering of Ca²⁺and SO₄^2-ions are polarized as the size of CaSO₄ cluster increases.For the residence of water molecules in the first shell of Ca²⁺around CaSO₄ clusters,there is no difference for neutral clusters,positively and negatively charged clusters.Whereas,the residence of some water molecules around CaSO₄ clusters can be enhanced markedly as the size of CaSO₄ cluster increase.For neutral?CaSO₄?_n cluster with large size?n=28?in supersaturated CaSO₄ aqueous solution,the residence of some water molecules around this cluster enhances increases,and the active exchange of some water molecules around?CaSO₄?_n cluster becomes less obvious.The characteristics of?CaSO₄?_n cluster are still quite different with the block unit of[CaSO₄·2H₂O],the averaged number of long resident water molecule does not obviously increase as the cluster size increases,and the averaged number of long resident water molecule is less than 1.It can be inferred that the neutral?CaSO₄?_n cluster and its aggregation are more likely to be correlated with an amorphous hydrous CaSO₄ structures.Therefore,the aggregation of neutral?CaSO₄?_n cluster or prenucleation clusters in the supersaturated CaSO₄ aqueous solution will lead to the formation of morphous hydrous CaSO ₄structures while not CaSO₄ crystal.The research in this paper is of great significance for explaining the early nucleation characteristics and non-classical nucleation mechanism of CaSO₄ aqueous solution.