The solid–liquid equilibrium of the binary system H2O–DMSO and the influence of a salt (NaCl, KCl) on the thermodynamic behavior: Correlations using a revised LIQUAC model

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Abstract

The solid–liquid equilibrium of the binary system water (H2O)–dimethyl sulfoxide (DMSO) is investigated and the results are compared to experimental values to ensure that the applied measurement procedure provides reliable results. After that, the freezing point depression of water is measured in the presence of different amounts of salt. Furthermore solubility measurements are carried out and compared to published experimental data to show that the apparatus used is also capable to provide reliable results for salt solubilities. Afterwards the ternary system is investigated using this measurement procedure to determine the solubilities of the salt in the mixed solvent system as well as the freezing point depression of the mixed solvents after the addition of salt. Two salts are investigated, sodium chloride (NaCl) and potassium chloride (KCl). Most of the measured ternary data seems to be reported for the first time. A correlation of the experimental results is also carried out using a revised version of the electrolyte model LIQUAC.

Highlights

► A method for the measurement of salt solubilities and freezing points is presented. ► The solid–liquid equilibrium of DMSO–H2O + salt (NaCl, KCl) is measured. ► A revised LIQUAC model is presented. Experimental data are correlated with the revised LIQUAC model. ► Results of the model are given for different electrolyte properties.

Introduction

In industrial chemical processes salts play a decisive role when regarding waste water treatment, crystallization and desalination processes. This is why for process design the salt effect for these processes has to be described reliably. The model development to describe the influence of salts on the phase equilibrium behavior was first carried out for highly diluted solutions by Debye and Hückel [1], who developed a theory based on statistical thermodynamics. This model is the only model of this kind not using adjustable parameters but it can only be applied for highly dilute solutions up to an ionic strength of about 0.01 mol/kg. However there are several semi-empirical expansions of this model, for example, the expansions of Bromley [2] and Pitzer [3] to extend the application to higher ionic strengths. Another type of extension to this model is achieved when combining these models with gE models like NRTL [4], UNIQUAC [5] and UNIFAC [6]. The model which is used in this article is the LIQUAC-model developed by Li et al. [7] and expanded by Kiepe et al. [8]. This model allows describing the mean activity coefficient of the salts, the osmotic coefficient, the vapor–liquid and the solid–liquid equilibrium of aqueous electrolyte solutions. Recently, Huang et al. [9] proposed a set of equations to calculate the solubility of salts also in mixed solvent systems. This procedure was later used in an article of Li et al. [10] to correlate parameters for the LIQUAC model for mixed solvent systems to give an adequate description of the solubilities of these systems. In this article the parameters of the LIQUAC model are revised when necessary and a general model is presented which in the case of molecular groups is identical to the LIFAC model [11].

Section snippets

Measurement of the solid–liquid equilibrium H2O–DMSO–salt

The measurements concerning the freezing point depression and the salt solubilities of the ternary system are carried out in the same apparatus but using different measurement procedures. The apparatus is shown in Fig. 1.

The LIQUAC model

In this article the LIQUAC model is chosen for the description of the solid–liquid equilibrium data. This model was originally developed by Li et al. [7] to represent the mean activity coefficients of salts, osmotic coefficients and the vapor–liquid equilibrium behavior of electrolyte systems. A few modifications of this model can be found in the literature [12], [13] which make the model applicable for the description of salt solubilities. Some years later, a modified version of this model has

Results and discussion

The van der Waals volumes and surface areas have been calculated for the cations sodium and potassium and the chloride ion. Furthermore the required parameters for the revised model have already been fitted for aqueous systems and have also been used to represent the ternary system containing DMSO. The required van der Waals properties for all ions of the ternary system can be found in Table 5. An example for the calculation of the van der Waals properties of the sodium–cation is presented in

Conclusion

In this article measurements concerning the SLE of the ternary system H2O–DMSO–NaCl (KCl) have been carried out. Not only salt solubilities in these two systems but also freezing point depressions are reported. Data concerning these systems seems to be reported for the first time. Furthermore it has been checked, how a slightly revised LIQUAC model can reproduce the experimental data.

From the results presented in this paper it can be recognized, that the slightly revised LIQUAC/LIFAC-model is

Acknowledgements

The authors would like to thank the Deutsche Forschungsgemeinschaft (DFG) for their ongoing financial support on the actual research project. Furthermore, we would like to thank DDBST GmbH for providing the latest version of the Dortmund Data Bank for the data regression of the needed parameters.

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