A theoretical and experimental investigation of solubility of gypsum and anhydrite II in the complex system CaSO4 + MgSO4 + H2SO4 + H2O was conducted from 298.1 to 363.1 K. The solubility of anhydrite II in (MgSO4 + H2SO4) aqueous solutions was determined at 348.1 and 363.1 K by an isothermal method. A Pitzer thermodynamic model was selected to predict the solubility of gypsum and anhydrite II in the titled quaternary system. Good agreement between the experimental and the model values was observed, which supports the reliability of the model prediction. As predicted by the model, the transfer temperature from gypsum to anhydrite II decreases with increasing H2SO4 and MgSO4 concentrations, and the solubilities of gypsum and anhydrite II decrease monotonically with increasing MgSO4 concentration if H2SO4 concentration is higher than 0.5 mol·kg–1. On the basis of the results obtained, several suggestions for avoiding calcium sulfate scaling in the industrial process were proposed.

Solubility isotherms as well as the corresponding solid phases of the quaternary system LiCl+MgCl2+KCl+H2O and the eutectic points for the ternary systems LiCl+MgCl2+H2O, LiCl+KCl+H2O and MgCl2+KCl+H2O at 323.15 K have been elaborately determined by an isothermal equilibrium method. Five crystallization fields including two double salts (LiCl·MgCl2·7H2O(s) and KCl·MgCl2·6H2O(s)), two hydrate salts (MgCl2·6H2O(s) and LiCl·H2O(s)) and one single salt (KCl(s)) were detected in the quaternary system. The reliability of the experimental results were verified both by testing the phase diagram rule and comparing with the literature data. It was found that all of the results were accordance with the phase diagram rule, and moreover, the excellent agreement between the experimental data and the literature data was also obtained, which indicate that the solubility data obtained in this work are reliable. Based on the quaternary phase diagram, the example was provided for industrial application. The measured phase diagram reported in this work are the essential tool to guide industrial process of extracting Li from the salt lake brine containing MgCl2 and LiCl using KCl reagent.

Solubility isotherms of the sparingly soluble salts CaF2(s) and CaSO4·2H2O(s) in their mixed aqueous solutions have been measured at 298.1 K. It was found that the CaF2(s) solubility decreases with increasing CaSO4 concentration in the solution and reaches about 1/3 of the CaF2(s) solubility in pure water in the CaSO4·2H2O(S) saturated solution. A thermodynamic model was developed to predict the CaF2(s) solubility isotherm in this system, in which the short range interactions of the species in the aqueous solution are represented by ion-association constants reported in literature, and the long range interaction, i.e., the electrostatic term, is represented by the well known Davies equation. The predicted solubility isotherm reasonably agrees with the experimental results. The contributions of the long-range term and the short-range term to the calculated solubility isotherm were investigated. It was concluded that the ionic association combining with the Davies equation is sufficient to represent the excess interaction of the CaF2 + CaSO4 aqueous solution at 298.15 K. This model approach could be applicable for other dilute mixed electrolyte systems in which component activity coefficients are lacking and model parameters are difficult to determine.

The solid solution–aqueous solution (SSAS) equilibria in the KCl + RbCl + H2O system were redetermined at 298.15 K. The experimental data for (K,Rb)Cl were consistent with the formation of a continuous solid solution without miscibility gaps. The Schreinemakers’ wet residues method and an XRD quantitative analysis technique based on the Vegard approach were applied to determine the chemical composition of the solid solution phase of (K,Rb)Cl. The compositions of (K,Rb)Cl derived from the Vegard approach are in accordance with those from the wet residues method. The thermodynamic properties of mixing of the (K,Rb)Cl solid solution were theoretically predicted using atomistic simulations. From these simulations, a regular solution behavior is recognized that is consistent with the knowledge of the thermodynamic properties of mixing of (K,Rb)Cl obtained from SSAS equilibrium studies, but the predicted regular solution model parameter A0 is significantly larger than that regressed from the SSAS equilibrium data. Finally, a thermodynamic model was developed for representing the SSAS equilibria and element partitioning in the KCl + RbCl + H2O system as a function of temperature that can be used for predicting the SSAS equilibria in the studied system over the temperature range 273.15–373.15 K.

•The XAS spectra of Cu(II) in the Cl−-bearing (up to ~ 16 m) solutions are reported.•The distortion degree of [CuCl4]2 − was studied by ab initio XANES calculations.•The results favor the square pyramidal [Cu(H2O)5]2 + in dilute Cl− solution.•The squashed tetrahedral [CuCl4]2 − is dominant in the highest Cl− conc. Solution.•The obtained results using both analysis methods are in good agreement.Knowledge of the structure and speciation of aqueous Cu(II)-chloride complexes is important for understanding Cu(II) behavior in the deep removal technology of Cu impurity from nickel electrolysis anolytes containing chloride. In this paper, X-ray absorption spectroscopic measurements are reported for dissolved copper in lithium chloride (up to ~ 16 mol kg−1) solutions at room temperature. The speciation and structure of corresponding solutions has been probed by performing a combined ab initio XANES theoretical and experimental analysis. The EXAFS spectrum was analyzed as well within this approach. Our XAS data and ab initio XANES calculations favored the five-coordinated [Cu(H2O)5]2+ with square pyramidal configuration over the four-(square planar and tetrahedron, [Cu(H2O)4]2+) and six-coordinated (octahedron, [Cu(H2O)6]2+) structure in dilute Cl− solution (~ 0.55 mol kg−1). This is also supported by the EXAFS refinement with the [CuO5] model having the lowest statistical error. In the highest Cl− concentration solution, the results of both methods show that tetrahedral [CuCl4]2– complexes are predominant. Upon manually adjusting its geometric parameter to the distortion degree α of ~ 18° and average CuCl bond distance of 2.25 Å of a squashed tetrahedral model, not only does the calculated XANES spectrum well reproduce the experimental spectrum, but also the statistical error in the EXAFS refinements is lowest.

This study is part of a series of studies on the development of a multi-temperature thermodynamically consistent model for salt lake brine systems. Under the comprehensive thermodynamic framework proposed in our previous study, the thermodynamic properties of the binary systems (i.e., NaCl+H2O, KCl+H2O, MgCl2+H2O and CaCl2+H2O) are simulated by the Pitzer–Simonson–Clegg (PSC) model. Various thermodynamic properties (i.e., water activity, osmotic coefficient, mean ionic activity coefficient, enthalpy of dilution and solution, relative apparent molar enthalpy, heat capacity of aqueous phase and solid phases) are collected and fitted to the model equations. The thermodynamic properties of these systems are reproduced or predicted by the obtained model parameters. Comparison to the experimental or model values in the literature suggests that the model parameters determined in this study can describe all of the thermodynamic and phase equilibria properties over wide temperature and concentration ranges. This modeling study of binary systems provides a solid basis for property predictions of salt lake brines under complicated conditions.

The water activities of the systems MSO4 + H2O (M = Mn, Co, Ni, Cu, Zn) are essential data needed for simulating the hydrometallurgical process of these metals. In our previous work (J. Chem. Eng. Data, 2014, 59, 97–102), the experimental data of water activity for these binary systems have been reported at 323.15 K. As one part of this series of work, the experimental data of water activity for these systems are reported at 373.15 K. The reliability of the apparatus at 373.15 K was verified by measuring and comparing the water activities of the two reference systems, CaCl2 + H2O and LiCl + H2O. The results showed that the maximal relative deviation of the water activities between the reference systems was 0.054%. The water activities for the concerned five systems were found to be approximately the same at a certain salt molality below 1 M. The water activities of these systems decrease at a certain salt molality more than 1 M in the following sequence: aMnSO4 > aCuSO4(∼aZnSO4) > aCoSO4(∼aNiSO4). Furthermore, the experiment data obtained in this work were compared with the model values reported in the literature.

The water activities for the binary Li2SO4–H2O and MgSO4–H2O, as well as the ternary Li2SO4–MgSO4–H2O systems at 323.15 and 373.15 K were elaborately determined by isopiestic measurements. The relative deviation of the isopiestic molality of parallel samples in each experimental run is better than 0.2%. The experimental water activities in the binary systems were compared with literature data and good agreement was found. The results obtained show that at salt concentration below 3 m the isopiestic composition lines of the Li2SO4–MgSO4–H2O system obey the Zdanovskii rule, whereas above 3 m the composition lines positively deviate from the Zdanovskii rule.

•We simulated the thermodynamic properties of system LiCl–NaCl–KCl–H2O by PSC model.•The predicted phase diagram of the titled system agrees well with experiment data.•The predicted water activity was compared with experiment data from literature.•The accuracy of the experimental data from literature has been evaluated.The phase diagram of the quaternary system LiCl–NaCl–KCl–H2O have been predicted with a Pitzer–Simonson–Clegg thermodynamic model by combining the binary and ternary model parameters, which were determined by simulating reliable experimental data. The predicted phase diagram shows a good agreement with the available experiment data from the literature. The other thermodynamic properties (e.g. water activity) of the quaternary and its sub-ternary systems have been investigated by the model and compared with the experimental data in literature. Significant improvements have been made in comparison with assessments.

Water activity is generally considered to affect ionic association in aqueous electrolyte solutions; however, it is usually ignored when the association reactions or constants are discussed. In this work, the effect of water activity on the association reaction \( {\text{Ni}}_{{ ( {\text{aq}})}}^{2 + } + n{\text{Cl}}_{{ ( {\text{aq)}}}}^{ - } \rightleftharpoons {\text{NiCl}}_{{n{\text{ (aq)}}}}^{2 - n} \) was investigated by EXAFS and UV–Vis spectroscopy measurements on NiCl2 aqueous solutions at room temperature with constant Cl−/Ni2+ ratio (~66) and various water activities; the latter were adjusted by adding MgCl2 and Mg(ClO4)2. Both the EXAFS and the UV–Vis spectra measurements indicated that the extent of Ni–Cl association increases in decreasing water activity environments, independent on the Cl−/Ni2+ ratio. Thus, the effect of water activity on the ionic association should not be ignored and more emphasis should be paid on it, especially, when the water activity is very low.

In this work, the water activities of the NaCl–CaCl2–H2O and KCl–CaCl2–H2O ternary systems and of their sub-binary systems NaCl–H2O and KCl–H2O were measured using an isopiestic method at 323.15 K. The isopiestic composition line for the NaCl–CaCl2–H2O system was found to obey the Zdanovskii rule very well, whereas the KCl–CaCl2–H2O system deviated slightly. The addition of NaCl to an aqueous solution of CaCl2 decreased its water activity for all CaCl2 molalities. However, KCl was found to decrease and increase the water activity at low and high CaCl2 molality, respectively. The turning point appears at 8.0 mol·kg–1 CaCl2 solution. The Pitzer–Simonson–Clegg (PSC) model was applied to represent the water activity of the two ternary systems, and the calculated results are discussed.

Water activity for binary KCl–H2O, NaCl–H2O and MgCl2–H2O, as well as two ternary NaCl–MgCl2–H2O and KCl–MgCl2–H2O, systems has been measured by using an isopiestic method at 323.15 K. The isopiestic results obtained show that the isopiestic composition lines of the NaCl–MgCl2–H2O system was found to obey the Zdanovskii rule, whereas the KCl–MgCl2–H2O system was observed to deviate slightly. The experimental water activities determined were applied to regress the parameters of the Pitzer model with a good agreement. The model with new parameters is validated by comparing water activity predictions with those given in the literature and not used in the parametrization process and calculating the solubility of the NaCl–MgCl2–H2O and KCl–MgCl2–H2O systems at various temperatures with the comparison of literature values.

Solubility isotherms for the ternary system MgCl2–MgSO4–H2O and the quaternary reciprocal system Li+, Mg2+//Cl–, SO42––H2O were determined at 298.15 K by an isothermal dissolution method. In the ternary phase diagram, there are six solubility branches corresponding to the solid phases MgSO4·nH2O(s) (n = 7, 6, 5, 4, 1) and MgCl2·6H2O(s). In the quaternary equilibrium phase diagram, there are 16 solubility co-saturated lines corresponding to the solid phases MgSO4·nH2O(s) (n = 7, 6, 5, 4, 1), MgCl2·6H2O(s), Li2SO4·H2O(s), LiCl·MgCl2·7H2O(s), and LiCl·H2O(s). This report describes for the first time that the equilibrium solid phases MgSO4·H2O(s) and MgSO4·4H2O(s) have been found to exist in this quaternary system. However, the phase field of MgSO4·H2O(s) overlaps with the phase fields of MgSO4·4H2O(s) and MgSO4·5H2O(s), which indicates that MgSO4·4H2O(s) and MgSO4·5H2O(s) are metastable phases; MgSO4·H2O(s) is a relatively more stable phase in both the ternary and quaternary systems. A Pitzer–Simonson–Clegg thermodynamic model was used to simulate the properties of the sub-binary and subternary systems and to predict the solubility phase diagram of the quaternary system. The results of the modeling are in reasonable agreement with the experimental data.

Knowledge of the thermodynamic properties of aqueous copper(II) chloride complexes is important for understanding and quantitatively modeling trace copper behavior in hydrometallurgical extraction processing. In this paper, UV–Vis spectra data of Cu(II) chloride solutions with various salinities (NaCl, 0–5.57 mol·kg−1) are collected at 25 °C. The concentration distribution of Cu–Cl species is in good agreement with those calculated by a reaction model (RM). The simple hydrated ion, Cu2+, is dominant at low concentration, whereas [CuCl]+, [CuCl2]0 and [CuCl3]− become increasingly important as the chloride concentration rises. Moreover, the RM calculation suggests the present of a small amount of [CuCl4]2−. The de-convoluted molar spectrum of each species is in excellent agreement with our previous theoretical results predicted by time-dependent density functional theory treatment of aqueous Cu-containing systems. The formation constants for these copper chloride complexes have been reported and are to be preferred, except log10K2 ([CuCl2]0).

The water activities of the systems MSO4 + H2O (M = Mn, Co, Ni, Cu, Zn) at higher temperatures are important for heavy metal hydrometallurgy processing of these metals. This paper reports water activity data for these binary systems at 323.15 K from isopiestic measurements. The reliability of the apparatus was verified by determination of the water activities of the test systems at 298.15 K and also two reference systems CaCl2 + H2O and H2SO4 + H2O at 298.15 K and 323.15 K. The results obtained agree well with literature data, with maximum relative deviations of the water activities for the reference systems of less than 0.032 % at 298.15 K and 0.062 % at 323.15 K respectively. The water activities for the MSO4 + H2O (M = Co, Ni, Cu, Zn) systems at 323.15 K were found to be approximately the same up to high concentrations, but were significantly lower than the MnSO4 + H2O system. The relative deviation of isopiestic molality of parallel samples in each experimental run is better than 0.2 %. The measured water activities reported in this work can be used to parametrize thermodynamic models of heavy-metal hydrometallurgical processes.

The Ca(NO3)2–Mg(NO3)2–H2O ternary system is a prospective system for heat storage, and its polythermal phase diagram is the basis for phase change materials design. However, the existing experimental data are dispersed and inconsistent with each other. In this work, we elaborately determined solubility isotherms of the ternary system at T = (273.15, 298.15, and 323.15) K by an isothermal equilibrium method, and chose a Brunauer–Emmett–Teller model to simulate and to construct the phase diagram of the ternary system from (273.15 to 373.15) K. The determined phase diagram indicates that there are two stable solubility isotherms at (273.15 and 298.15) K corresponding to solid phase Ca(NO3)2·4H2O and Mg(NO3)2·6H2O. No stable solubility isotherm for solid phase Ca(NO3)2·3H2O at (273.15 and 298.15) K has been found, different from the results reported in several references.

Utilizing improvements in experimental equipment, analytical methods and the initial material, the solubility isotherms of the ternary system MgCl2–MgSO4–H2O were determined in detail at T = (323.15 and 348.15) K using an isothermal method of solid–liquid equilibrium. The results indicate that the solid phases MgSO4·nH2O(s) (n = 6, 1) and MgCl2·6H2O(s) are stable and MgSO4·nH2O(s) (n = 5, 4) are metastable at 323.15 K, which contradicts the results of a previous experimental study1 in which the phase MgSO4·4H2O(s) was reported as stable. The liquidus of the four solid phases MgSO4·nH2O(s) (n = 6, 4, 1) and MgCl2·6H2O(s) were measured at 348.15 K in this work. The findings indicate that the phases MgSO4·H2O(s) and MgCl2·6H2O(s) are stable and MgSO4·6H2O(s) and MgSO4·4H2O(s) are metastable at 348.15 K. Remarkable differences between this work and the literature solubility data for the phase MgSO4·H2O(s) at 348.15 K are observed. A Pitzer–Simonson–Clegg thermodynamic model was chosen to simulate the properties of the binary systems and to correlate the solubility isotherms of the ternary system at 298.15 K in our previous study and 323.15 K and 348.15 K in this work. Good agreement has been found between the calculated and experimental results. Applying the model parameters and solubility isotherms in the ternary system measured in this work, we obtained the solubility product parameters ln K and chemical potentials for the solid phases MgSO4·nH2O(s) (n = 6, 5, 4, 1) over a wider temperature range than those for the binary system MgSO4–H2O.

Solubility isotherms as well as the corresponding solid phases of the quaternary system MgCl2+LiCl+NH4Cl+H2O at 298.15 K have been elaborately measured by an isothermal equilibrium method. Four crystallization fields including two double salts (LiCl·MgCl2·7H2O(s) and NH4Cl·MgCl2·6H2O(s)), one hydrate salt (MgCl2·6H2O(s)), and one solid-solution (LiCl·H2O+NH4Cl)(ss) were detected in this system. A Pitzer-Simoson-Clegg (PSC) thermodynamic model was used to simulate and predict the thermodynamic properties of this quaternary system and its subsystems. The water activities of the ternary systems MgCl2+LiCl+H2O and LiCl+NH4Cl+H2O, as well as the solubility of the quaternary system MgCl2+LiCl+NH4Cl+H2O, were predicted by the PSC model, the results of which were compared with available literature data and the experimental results in this work. The excellent agreement between the predicted and experimental results indicates that the solubility results obtained in this work are reliable. On the basis of the quaternary phase diagram calculated by the model, several examples are provided for industrial applications.

Highlights•Water activities in the CaCl2 + SrCl2 + H2O system were measured at 298.15 K.•The Pitzer–Simonson–Clegg model was chosen to represent the properties of the system.•The predicted and experimental water activities in the system agree with each other.•The solubility isotherms of the titled system were assessed by the model calculation.•The solid solution (CaCl2 ⋅ 6H2O + SrCl2 ⋅ 6H2O)(s) is the only equilibrium solid phase in the system at 298.15 K.Water activities in the ternary system (CaCl2 + SrCl2 + H2O) and its sub-binary system (CaCl2 + H2O) at T = 298.15 K have been elaborately measured by an isopiestic method. The data of the measured water activity were used to justify the reliability of solubility isotherms reported in the literature by correlating them with a thermodynamic Pitzer–Simonson–Clegg (PSC) model. The model parameters for representing the thermodynamic properties of the (CaCl2 + H2O) system from (0 to 11) mol ⋅ kg−1 at T = 298.15 K were determined, and the experimental water activity data in the ternary system were compared with those predicted by the parameters determined in the binary systems. Their agreement indicates that the PSC model parameters can reliably represent the properties of the ternary system. Under the assumption that the equilibrium solid phases are the pure solid phases (SrCl2 ⋅ 6H2O and CaCl2 ⋅ 6H2O)(s) or the ideal solid solution consisting of CaCl2 ⋅ 6H2O(s) and SrCl2 ⋅ 6H2O(s), the solubility isotherms were predicted and compared with experimental data from the literature. It was found that the predicted solubility isotherm agrees with experimental data over the entire concentration range at T = 298.15 K under the second assumption described above; however, it does not under the first assumption. The modeling results reveal that the solid phase in equilibrium with the aqueous solution in the ternary system is an ideal solid solution consisting of SrCl2 ⋅ 6H2O(s) and CaCl2 ⋅ 6H2O(s). Based on the theoretical calculation, the possibility of the co-saturated points between SrCl2 ⋅ 6H2O(s) and the solid solution (CaCl2 ⋅ 6H2O + SrCl2 ⋅ 6H2O)(s) and between CaCl2 ⋅ 6H2O(s) and the solid solution (CaCl2 ⋅ 6H2O + SrCl2 ⋅ 6H2O)(s), which were reported by experimental researchers, has been discussed, and the Lippann diagram of this system has been presented.

In this work, a systematic investigation of the competition coordination of H2O and Cl– with Ni2+ in saturated NiCl2 aqueous solution at room temperature was conducted using density functional theory (DFT), Car–Parrinello molecular dynamics (CPMD) simulations, and extended X-ray absorption fine structure (EXAFS) spectra. The calculated results reveal that the six-coordinated structure is favorable for [NiClx(H2O)n]2–x (x = 0–2; n = 1–12) clusters in the aqueous phase. The hydration energy calculation shows that the six-coordinated solvent-shared ion pair (SSIP) ([Ni(H2O)6(H2O)n−6Cl]+) is more stable than its contact ion pair (CIP) ([NiCl(H2O)5(H2O)n−5]+) isomer as n ≥ 9 in the aqueous phase, and the six-coordinated solvent-shared ion pair with a dissociated double Cl– (SSIP/d) ([Ni(H2O)6(H2O)n−6Cl2]0) is more preferable than its CIP ([NiCl2(H2O)4(H2O)n−4]0) and solvent-shared ion pair with single dissociated Cl– (SSIP/s) ([NiCl(H2O)5(H2O)n−5Cl]0) isomers as n ≥ 11. The six-coordinated SSIP/d ([Ni(H2O)6(H2O)n−6Cl2]0) conformers are the dominant structures in a saturated NiCl2(aq) solution (NiCl2 concentration: ∼5.05 mol·kg–1, corresponding to n ≈ 11). The CPMD simulations exhibited that there are six water molecules with Ni–O distance at ∼205.0 pm on average around each Ni2+ in the first hydration sphere, even in the saturated NiCl2 aqueous solution (∼5.05 mol·kg–1) at room temperature, and no obvious Ni–Cl interaction was found. The EXAFS spectra revealed that the first solvation shell of Ni2+ is an octahedral structure with six water molecules tightly bound in the NiCl2(aq) solution with a concentration ranging from 1.00 to 5.05 mol·kg–1, and there is no obvious evidence of Ni–Cl contact ion pairs. A comprehensive conclusion from the DFT, CPMD, and EXAFS studies is that there is no obvious direct contact between Ni2+ and Cl–, even in saturated NiCl2 aqueous solution at room temperature.

The solubility isotherms for the ternary system Li2SO4–MgSO4–H2O and the quaternary system Li2SO4–K2SO4–MgSO4–H2O at T = 273.15 K were determined by the isothermal equilibrium method. A Pitzer–Simonson–Clegg (PSC) model was used to simulate the properties of the binary and ternary systems of the quaternary title system. The binary model parameters were fitted against selected reliable experimental data of water activity. The mixture model parameters were obtained by fitting them to the ternary solubility isotherms taken from the literature and determined in this work. The solubility isotherms of the quaternary title system were predicted by the PSC model and compared with the experimental results in this work. The excellent agreement between the predicted and experimental results indicates that the experimental results obtained in this work are reliable.

Solubility of gypsum in the quaternary systems CaSO4–HMSO4–H2SO4–H2O (HM = Cu, Zn, Ni, Mn) are predicted up to saturated concentrations of heavy metal sulfates and to a H2SO4 concentration of 2 m by a Pitzer thermodynamic model. Experimental solubility and water activity in the subbinary and subternary systems from the literature were used for model parametrization. Then the solubility phase diagrams for the quaternary systems were predicted directly with these obtained binary and ternary model parameters. In order to verify the reliability of the predicted results, a series of solubility measurements of gypsum in these quaternary systems have been carried out at 298.15 K and the measured results were compared with the predicted ones. It was shown that the Pitzer thermodynamic model can perfectly predict the solubilities of gypsum in the quaternary systems. Meanwhile, the newly obtained experimental data were compared with limited literature data in some of the quaternary systems; good agreement was found between them. Some application examples were given based on the predicted phase diagrams.

A reaction model was developed to describe the thermodynamic properties of aqueous electrolyte solutions. In the model various types of association (ion–solvent, ion–ion) are incorporated using as much as possible structure information. In the framework of the model, an electrolyte aqueous solution is treated as a mixture of charged or neutral associated species consisting of cations, anions and the solvent water, among the species the short range interactions are assumed to be equal. The abundance of each species is determined by its specific Gibbs energy of formation related to the assumed master species. The total Gibbs energy of mixing consists of a long range electrostatic term and short range interaction terms, the latter are sum of Gibbs energy of all species. Based on the total Gibbs energy of mixing, activity expressions for each species were derived. The Gibbs energy of formation of each associated species is correlated by its stepwise formation Gibbs energies, thus reducing the number of necessary adjustable parameters. At the example systems CuCl2–MCln–H2O (M = Li, Ca, Mg) model parameters were determined by fitting experimental data of water activities and solubilities on the basis of ion associates in agreement with available structure information. Component activities, solubility isotherms and species abundances were calculated and compared with experimental results. This facilitates an understanding of structure–property relationships in the titled systems.Highlights► A reaction model was developed for strongly associated and hydrated systems. ► Hydration reaction chain was extended to represent ionic association reaction. ► Properties of the ternary systems were predicted by binary parameters only. ► The properties include water activity, solubility and species distribution. ► Both the predicted and experimental results are in excellent agreement.

Water activity in the ternary system LiCl–SrCl2–H2O and its sub-binary systems has been elaborately measured by the isopiestic method. The measured water activities were used to justify the reliability of solubility isotherms reported in literature by correlating them with two thermodynamic models, that is, the extended Pitzer model and the Pitzer–Simonson–Clegg model. It was found that the extended Pitzer model cannot correlate consistently the water activities measured and either set of the solubility isotherms reported in literature for this concerned system, no matter how its parameters were adjusted. However, the Pitzer–Simonson–Clegg model can correlate consistently our measured water activities and the solubility isotherms reported by the literature (Kydynov et al. Issled. Obl. Khim. Tekhnol. Miner. Solei Okislov1965, 146–150), which should be more reliable than solubility data reported in other references.

The phase diagram of the quaternary system KNO3–LiNO3–Mg(NO3)2–H2O was simulated over the temperature range from 273 to 333 K by a Pitzer–Simonson–Clegg model, whose parameters were determined by fitting to experimental solubility and water activity data of the binary systems KNO3–H2O, LiNO3–H2O, Mg(NO3)2–H2O and the ternary systems KNO3–LiNO3–H2O, KNO3–Mg(NO3)2–H2O, LiNO3–Mg(NO3)2–H2O. Three new eutectic points with melting points within room temperatures were found in these systems, which could be used as candidates for storing heat from low temperature heat resources. Exothermal and endothermal behaviors of the predicted phase change materials were measured. Results show that the predicted phase change materials possess excellent heat storage and release ability.Highlights► We simulated and predicted the phase diagram of the system KNO3–LiNO3–Mg(NO3)2–H2O. ► We found three new eutectic compositions at room temperatures (288–298) K. ► Predicted eutectic materials showed excellent heat -storing and -releasing ability.

Solubility isotherms of the ternary system (NH4Cl+CaCl2+H2O) were elaborately determined at T=T= (273.15 and 298.15) K by using the isothermal method. In the equilibrium phase diagram, there are two solubility branches corresponding to the solid phases CaCl2⋅6H2O and NH4Cl. Invariant point compositions are 36.32 wt% CaCl2 and 3.4 wt% NH4Cl at 273.15 K, and 45.86 wt% CaCl2 and 5.22 wt% NH4Cl at 298.15 K. A Pitzer–Simonson–Clegg thermodynamic model was applied to represent the thermodynamic properties of this ternary system and to construct a partial phase diagram of the ternary system at temperatures between (273.15 and 323.15) K. It was found in the predicted solubility phase diagram that the double salt 2NH4Cl⋅CaCl2⋅3H2O, found by other authors at (323.1 and 348.1) K, will disappear at temperatures below 298.15 K. Besides, it was found that there are two peritectic points in the ternary system with peritectic temperatures at 299.65 K and 298.15 K, and the former peritectic point falls just on the line between the composition points of NH4Cl and CaCl2⋅6H2O. According to phase rule, a solution made of this point will begin to crystallize at 299.65 K and end at 298 K and therefore can be acted as a “pseudo eutectic” phase change material (PCM). A heat storing and releasing experiment of 50 grams of the PCM was carried out, obtaining a satisfying result.Highlights► We measure solubility isotherms of the system NH4Cl–CaCl2–H2O at 273.15 and 298.15 K. ► We simulated phase diagram of this system from 273 to 323 K by a thermodynamic model. ► A pseudo ‘eutectic’ phase change material is found at about 299 K.

Solubility isotherms of the ternary system (LiCl + NH4Cl + H2O) were elaborately determined at T = (273.15, 298.15, and 323.15) K by an isothermal method. In the equilibrium phase diagram, there are two solubility branches at 273.15 K, corresponding to the solid phase LiCl·2H2O and NH4Cl. The invariant point composition at 273.15 K is w = 0.401 for LiCl, w = 0.024 for NH4Cl, and w = 0.575 for H2O. However, there are three solubility branches at (298.15 and 323.15) K, corresponding to the solid phase LiCl·H2O, NH4Cl, and a new found solid solution phase (NH4Cl)x(LiCl·H2O)1−x. A Pitzer−Simonson−Clegg thermodynamic model was selected to represent the thermodynamic properties of this system. Thermodynamic consistence between our solubility data and water activity from other research groups shows that these concerned experimental data are reliable.

In this work, structures and thermodynamic properties of [CuCl3]− and [CuCl4]2− hydrates in aqueous solution were investigated using density functional theory and ab initio methods. Contact ion pair (CIP) and solvent-shared ion pair (SSIP) structures were both taken into account. Our calculations suggest that [CuCl3(H2O)n]− clusters might favor a four-coordinated CIP structure with a water molecule coordinating with the copper atom in the equatorial position for n = 3 and 4 in aqueous solution, whereas the four-coordinated SSIP structure with one chloride atom dissociated becomes more stable as n increases to 5. For the [CuCl4]2− cluster, the four-coordinated tetrahedron structure is more stable than the square-planar one, whereas for [CuCl4(H2O)n]2− (n ≥ 1) clusters, it seems that four-coordinated SSIP structures are slightly more favorable than CIP structures. Our calculations suggest that Cu2+ perhaps prefers a coordination number of 4 in CuCl2 aqueous solution with high Cl− concentrations. In addition, natural bond orbital (NBO) calculations suggest that there is obvious charge transfer (CT) between copper and chloride atoms in [CuClx]2−x (x = 1−4) clusters. However, compared with that in the [CuCl2]0 cluster, the CT between the copper and chloride atoms in [CuCl3]− and [CuCl4]2− clusters becomes negligible as the number of attached redundant Cl− ions increases. This implies that the coordination ability of Cl− is greatly weakened for [CuCl3]− and [CuCl4]2− clusters. Electronic absorption spectra of these different hydrates were obtained using long-range-corrected time-dependent density functional theory. The calculated electronic transition bands of the four-coordinated CIP conformer of [CuCl3(H2O)n]− for n = 3 and 4 are coincident with the absorption of [CuCl3]−(aq) species (∼284 and 384 nm) resolved from UV spectra obtained in CuCl2 (ca. 10−4 mol·kg−1) + LiCl (>10 mol·kg−1) solutions, whereas the calculated bands of [CuCl3(H2O)n]− in their most stable configurations are not when n = 0 − 2 or n > 4, which means that the species [CuCl3]−(aq) exists in those CuCl2 aqueous solutions in which the water activity is neither too low nor too high. The calculated bands of [CuCl4(H2O)n]2− clusters correspond to the absorption spectra (∼270 and 370 nm) derived from UV measurements only when n = 0, which suggests that [CuCl4]2−(aq) species probably exist in environments in which the water activity is quite low.

In this work, structures, and properties of Cu2+ and CuCl+ hydrates in the gas and aqueous phases have been investigated using the B3LYP method. Contact ion pair (CIP) and solvent-shared ion pair (SSIP) were both taken into account for CuCl+ hydrates. Our calculations show that [Cu(H2O)n]2+ clusters favor a very open four-coordinated structure for n = 5−12 in the gas phase, while a five-coordinated conformer is favored for n ≥ 8 in the aqueous phase. An approximate complete solvation shell of Cu2+ in the aqueous phase needs more than 12 water molecules, while that of CuCl+ in the aqueous phase needs only about eight water molecules. For [CuCl(H2O)n]+ clusters, the most stable structure is a four-coordinated CIP conformer for n = 4−7 in the gas phase and a five-coordinated CIP conformer for n = 8−10 in the aqueous phase. However, the five-coordinated CIP/h conformer (CIP conformer that the axial chloride atom tends to dissociate) of [CuCl(H2O)n]+ clusters becomes more favorable as n increases to 11. As the hydration process proceeds, the charges on the copper atom of [Cu(H2O)n]2+ clusters decrease, while those of [CuCl(H2O)n]+ clusters increase (probably due to the dissociation of Cl−). The d−d electron transition and partial charge transition band around 160 nm of the five-coordinated conformer of [Cu(H2O)n]2+ clusters and those bands (∼170 and ∼160 nm) of SSIP or five-coordinated CIP/h conformers of [CuCl(H2O)n]+ clusters are coincident with the absorption of [Cu]2+(aq) species (∼180 nm) resolved from the spectra obtained in trace CuCl2 (ca. 10−5 mol·kg−1) + LiCl (0−18 mol·kg−1) aqueous solution, while those of five-coordinated CIP conformers of [CuCl(H2O)n]+ clusters (n = 8 and 9) around 261 and 247 nm correspond to the absorption of [CuCl]+(aq) species (∼250 nm). Our calculated electronic spectra indicate that the typical peak of copper(II)−chloride complexes changes from 180 to 250 nm, and 275 nm, as the process of Cl− coordination. For [Cu]2+(aq), [CuCl]+(aq), and [CuCl2]0(aq) species, the central Cu(II) atom prefers five-coordination.

Water activities in the ternary LiNO3+KNO3+H2O system and its sub-binary systems have been measured by the isopiestic method at 273.1 and 298.1 K. The measured results were treated by a Pitzer-Simonson-Clegg thermodynamic model, from which the predicted solubility isotherms were compared with the experimental results. Based on this comparison, the reliability of the measured results was discussed. The measured results help in predicting the phase diagram of the ternary system, as well as other multi-component systems based on the ternary system.

In this work, the hydrates of copper dichloride in gas and aqueous phase have been investigated using the B3LYP method. Low-lying conformers of CuCl2(H2O)n clusters for n = 1−10 were obtained by an extensive conformation search. Contact ion pair (CIP) and solvent-shared ion pair (SSIP) with one dissociated chloride atom (SSIP/s) and SSIP with two dissociated chloride atoms (SSIP/d) all were considered. Our calculations present such a trend that a four-fold CIP conformer is more favorable for CuCl2(H2O)n cluster (n ≤ 7) and four-fold SSIP/s for n = 8−10 in the gas phase, while in aqueous solution, more stable structures are five-fold SSIP/s conformer for n = 7−9 and four-fold CIP conformer for n = 2−6. Hydrogen bond (HB) plays an important role in the CuCl2 solvation, especially HBs formed between the first and second solvation shell water molecules. Electronic absorption spectra of CuCl2(H2O)n clusters were obtained using long-range-corrected time-dependent density functional theory. The calculated electronic absorption peak around 270 nm of CIP conformers is coincident with the absorption of [CuCl2]0aq species resolved from the spectra obtained in solutions of trace CuCl2 (ca. 10−5 mol/kg) + LiCl (0−18 m), while those of SSIP/s (∼250 nm) and SSIP/d (∼180 nm) conformers probably correspond to the absorption spectra of [CuCl]+aq and [Cu]2+aq species, respectively. Natural bond orbital charge population analyses show that charge transfer (CT) between a central copper(II) atom and ligands (Cl and H2O) increases as the hydrated cluster expands, especially CT from Cu2+ to the first solvation shell, which enhances the strength of HBs. Such CT becomes more apparent for SSIP structure with the dissociation of chloride ion. OH stretching vibration frequencies of proton donor type water in CuCl2(H2O)n clusters are obviously red-shifted in comparison to those of water clusters, due to CT between the central atom Cu and ligands. SSIP conformers have apparent IR absorption peaks of OH stretching vibration at ∼3000 cm−1 for the effect of half-dissociated chloride atoms.

The solubility of the binary system (LiNO3 + H2O) from T = 273.15 K to T = 333.15 K and solubility isotherms of the ternary system (LiCl + LiNO3 + H2O) were elaborately measured at T = 273.15 K and T = 323.15 K. These solubility data, as well as water activities in the binary systems from the literature, were treated by an empirically modified BET model. The isotherms of the ternary system (LiCl + LiNO3 + H2O) were reproduced and a complete phase diagram of the ternary system in the temperature range from 273.15 K to 323.15 K predicted. It is shown that the solubility data for the binary system (LiNO3 + H2O) measured in this work are slightly different from the literature data. Simulated results showed that the saturated salt solution of (2.8LiCl + LiNO3) is in equilibrium with the stable solid phase LiNO3(s) over the temperature range from 283.15 K to 323.15 K, other than the solid phases LiNO3 · 3H2O(s) and LiClH2O(s) as reported by Iyoki et al. [S. Iwasaki, Y. Kuriyama. T. Uemura, J. Chem. Eng. Data 38 (1993) 396–398].

Solubility isotherms of the ternary system (LiCl + CaCl2 + H2O) were elaborately determined at T = (283.15 and 323.15) K. Several thermodynamic models were applied to represent the thermodynamic properties of this system. By comparing the predicted and experimental water activities in the ternary system, an empirical modified BET model was selected to represent the thermodynamic properties of this system. The solubility data determined in this work at T = (283.15 and 323.15) K, as well as those from the literature at other temperatures, were used for the model parameterization. A complete phase diagram of the ternary system was predicted over the temperature range from (273.15 to 323.15) K. Subsequently, the Gibbs free energy of formation of the solid phases CaCl2 · 4 H2O(s), CaCl2 · 2 H2O(s), LiCl · 2H2O(s), and LiCl · CaCl2 · 5H2O(s) was estimated and compared with the literature data.

Solubility data of CaCl2·nH2O (n = 2, 4, 6) were predicted and evaluated by a Stokes and Robinson's adjusted BET-model using the vapor pressures of saturated solution as criteria. Prior to the prediction, the BET model was parameterized with the most recently reported experimental osmotic coefficients. The comparison with other models showed that, despite of fewer model parameters, the BET model could represent the activities in the system CaCl2 + H2O as good as the Pitzer model in a large temperature range from 298.15 to 523.15 K. Meanwhile, experimental vapor pressure data of the saturated CaCl2 solution were critically evaluated and some of them are selected as criteria for solubility prediction. In principle, the predicted solubility has higher accuracy than an average set of various experimental data and therefore is recommended for use in relevant cases.