Wolfgang Voigt

Structural changes of cellulose dissolved in molten salt hydrates

The behavior of cellulose in molten salt hydrates, e.g., LiClO 4 *3H 2 O, ZnCl 2 *4H 2 O, LiCl*5H 2 O and mixtures of thiocyanates was investigated. Melts, which give rise to a swelling or dissolution of the cellulose were the focus of interest. The melt composition and species, the water amount and the structure of the coordination sphere of the cation of the molten salt hydrate as well as the acidity influence the solubility of the cellulose in molten salt systems. The interactions between cellulose and species of the melts were determined by solution state 13 C NMR measurements at temperatures between 65 °C and 140 °C. Each regenerated cellulose showed other but specific properties which were characterized by wide angle X-ray Scattering (WAXS), solid-state NMR ( 13 C CP/MAS NMR), molecular weight distribution, surface-area determinations and scanning electron microscopy (SEM).

The behaviour of cellulose in hydrated melts of the composition LiXċn H2O (X=I−, NO3−, CH3COO−, ClO4−)

The dissolution behaviour of cellulose in low temperature molten salts was investigated. Depending on the chosen anions in the melt, cellulose shows different reaction behaviour in different Li+‐containing melts. Dissolution of the polymer was observed in molten LiClO4ċ3H2O and molten LiIċ2H2O. In the hydrated melts of LiCH3COOċ2H2O and LiNO3ċ3H2O a fine distribution of cellulose was stated. Cellulose can be regenerated by cooling the melt and removing the salt by dissolution in water.The structure of the recrystallized product is determined by the used low temperature molten salt.

Solid–liquid equilibria in mixtures of molten salt hydrates for the design of heat storage materials

Enthalpy of melting can be used to store heat in a simple way for time periods of hours and days. Knowledge of the solid –liquid equilibria represents the most important presumption for systematic evaluations of the suitability of hydrated salt mixtures. In this paper, two approaches for predicting solid–liquid equilibria in ternary or higher component systems are discussed using the limited amount of thermodynamic data available for such systems. One method is based on the modified Brunauer–Emmett–Teller (BET) model as formulated by Ally and Braunstein. In cases of a strong tendency toward complex formation of salt components, the BET model is no longer applicable. Reaction chain models have been used to treat such systems. Thereby, the reaction chain represents a method to correlate step-wise hydration or complexation enthalpies and entropies and, thus, reduce the number of adjustable parameters. Results are discussed for systems containing MgCl2, CaCl2, ZnCl2, and alkali metal chlorides.

The measurement of sulfate mineral solubilities in the Na-K-Ca-Cl-SO4-H2O system at temperatures of 100, 150 and 200°C

Abstract At T > 100°C development of thermodynamic models suffers from missing experimental data, particularly for solubilities of sulfate minerals in mixed solutions. Solubilities in Na+-K+-Ca2+-Cl−-SO42−/H2O subsystems were investigated at 150, 200°C and at selected compositions at 100°C. The apparatus used to examine solid-liquid phase equilibria under hydrothermal conditions has been described. In the system NaCl-CaSO4-H2O the missing anhydrite (CaSO4) solubilities at high NaCl concentrations up to halite saturation have been determined. In the system Na2SO4-CaSO4-H2O the observed glauberite (Na2SO4 · CaSO4) solubility is higher than that predicted by the high temperature model of Greenberg and Moller (1989) , especially at 200°C. At high salt concentrations, solubilities of both anhydrite and glauberite increase with increasing temperature. Stability fields of the minerals syngenite (K2SO4 · CaSO4 · H2O) and goergeyite (K2SO4 · 5 CaSO4 · H2O) were determined, and a new phase was found at 200°C in the K2SO4-CaSO4-H2O system. Chemical and single crystal structure analysis give the formula K2SO4 · CaSO4. The structure is isostructural with palmierite (K2SO4 · PbSO4). The glaserite (“3 K2SO4 · Na2SO4”) appears as solid solution in the system Na2SO4-K2SO4-H2O. Its solubility and stoichiometry was determined as a function of solution composition.

Chemistry of salts in aqueous solutions: Applications, experiments, and theory

Salts comprise a very large and important group of chemical compounds. Natural occurrence of salts and industrial processes of their recovery, conversion, purification, and use depend on solubility phenomena and their chemistry in aqueous solutions, mostly in multi-ion systems. Modeling of these processes as well as developing new ones requires knowledge of the properties of the aqueous salt solutions in extended T-p-x ranges including a growing number of components in solutions (CO2, SO2, lithium salts, salts of rare earth metals, actinides, etc.). At least for the thermodynamic properties, the general accepted methodology is to use thermodynamic databases of aqueous species and solids in combination with an appropriate ion-interaction model to perform equilibrium calculations for species distributions in solution and phase equilibria. The situation in respect to available thermodynamic models and data for their parameterization is discussed at selected examples. Thereby, the importance of accurate experimental determinations of phase equilibrium data for derivation of model parameters is emphasized. Furthermore, it is concluded that experimental investigations should follow a chemical systematic. Simple physical models or quantum chemical calculations cannot predict unknown quantities in the databases with sufficient accuracy. Finally, solubility changes in salt-water systems at enhanced temperatures are considered. Systems, which can be considered as molten hydrates, display interesting phase behavior and chemical reactivity as protic ionic liquids. The latter can be exploited in chemical syntheses to substitute mixtures of concentrated acids like HNO3/H2SO4 by simple salts like ferric nitrate. For an understanding of the chemical and phase behavior of water-salt systems in terms of structure–property relations, a renaissance of chemical guided basic investigations of such systems would be worthwhile.

Solubility and Crystallization in the System MgCl2–MgSO4–H2O at 50 and 75°C

The system MgCl2–MgSO4–H2O has been investigated experimentally and modeled thermodynamically according to the Pitzer method at 50 and 75°C. It was found that, even when seemingly all requirements for reaching the stable thermodynamic equilibrium are fulfilled, the crystallization of higher hydrates as metastable phases is possible, and cannot be avoided in each crystallization field of a stable lower hydrate of magnesium sulfate. Crystallization of MgSO4 · x H2O (x = 1, 4, 6) and MgCl2 · 6 H2O at 50°C and of MgSO4 · H2O and MgCl2 · 6 H2O at 75°C as stable phases has been observed. Three metastable crystallization fields of MgSO4 · x H2O (x = 4, 6, 7) have been detected at 50°C and two of MgSO4 · x H2O (x = 4, 6) at 75°C. The results obtained and the contradictions existing in the literature with respect to the solubility and the crystallizing solid phases are discussed in terms of the crystal structures.

CALCULATION OF SALT ACTIVITIES IN MOLTEN SALT HYDRATES APPLYING THE MODIFIED BET EQUATION. I: BINARY SYSTEMS

SummaryOn the basis of the modified BET model according to Stokes and Robinson, an equation for the calculation of salt activities in molten salt hydrates has been derived. The equation is used to describe successfully the liquidus curves of the hydrates of MgCl2, Mg(NO3)2 and CaCl2 inT-x diagrams. A promissing feature of the model is the small number of adjustable parameters and its extrapolative power.ZusammenfassungAuf der Grundlage des modifizierten BET-Modells nach Stokes und Robinson wurde eine Gleichung zur Berechnung der Salzaktivitäten in geschmolzenen Salzhydraten abgeleitet. Mit der Gleichung wurden die Liquiduslinien der Hydrate von MgCl2, Mg(NO3)2 und CaCl2 imT-x-Diagramm beschrieben. Besondere Charakteristika des Modells sind die kleine Anzahl adjustierbarer Parameter und die Möglichkeiten zu Extrapolationen.

Solubility equilibria in multicomponent oceanic salt systems from t = 0 to 200 °C. Model parameterization and databases* ,†

In this paper the application of Pitzers equations for modeling of solubility equilibria in the hexary oceanic salt system Na+, K+, Mg2+, Ca2+ // Cl, SO42H2 O and its subsystems over large temperature intervals is discussed. Lack of ternary solution data considerably restricts the use of the equations as an evaluation tool for the quality of solubility data. Models optimized for the available data in two temperature intervals have been derived. Since Pitzers model gives no guidance for temperature dependence, the number of parameters is dominated by temperature coefficients. Thus, for extended temperature intervals other approaches like the Cohen-Adad formalism can be advantageous in solubility data evaluation.

Quality assurance in thermodynamic databases for performance assessment studies in waste disposal

Performance assessment studies in underground disposal of radioactive or toxic waste need to consider all reactive interactions between waste and its surroundings. Thermodynamic equilibrium and reaction path calculations represent an important tool for this purpose. The reliability of the results depends first of all on the quality of the thermodynamic database used for the calculations. Several quality criteria of thermodynamic databases are discussed in connection with the characteristics of current database projects [Nuclear Energy Agency Thermochemical Database (NEA-TDB), Yucca Mountain database, Dortmund Databank (DDB), Common Thermodynamic Database (CTD), FreeGS, and Thermodynamic Reference Database (THEREDA)] including the situation for molten salts. The future role of the IUPAC standard for thermophysical and thermochemical data storage is emphasized.

Isopiestic measurements at high temperatures: I. Aqueous solutions of LiCl, CsCl, and CaCl2 at 155°C

Isopiestic measurements have been made for LiCl (aq) and CsCl (aq) at a temperature of 155°C. Equilibrium molalities ranged up to 21 mol-kg−1. MgCl2(aq) was chosen as the reference electrolyte. The apparatus used for the isopiestic experiments is an enhanced version of that developed by Grjotheim and co-workers. To test its precision osmotic coefficients of CaCl2 (aq) have also been determined and compared with previously reported vapor pressure measurements at high concentrations. The results show a very good coincidence. The data can be described by the ion interaction model of Pitzer. The resulting set of parameters allows a fit of the experimental osmotic coefficients with a standard error of 0.0078 and 0.0114 for LiCl(aq) and CsCl (aq), respectively. The osmotic coefficients of LiCl are consistent with data at lower molalities, but there are discrepancies for the CsCl solutions.