Structure of aqueous MgSO4 solution: Dilute to concentrated (original) (raw)
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Quantum chemical analysis of the structures of MgSO4 hydrates
2012
Magnesium sulfate salts can form hydrated compounds with up to seven degree of hydration with an energy exchange of the order of 2.8GJ/m3 [1]. In addition, this salt is abundant in nature and thus this material is a potential candidate for storing energy in seasonal heat storage systems. One of the main issues in using this material for seasonal heat storage system is its slow kinetics and low extent of water take-up under normal atmospheric conditions [2]. In addition, the salt undergoes considerable changes in its crystalline structure during hydration and dehydration, and often they encounter the formation of cracks and pores in the crystal structure [3]. This significantly affects the efficiency of the salt in storing energy and also reusability of the material. A molecular level investigation is necessary to understand the process of hydration and dehydration in detail. Presence of an extensive network of hydrogen bonds in MgSO4.7H2O crystal is identified by Allan Zalkin et al ...
The Journal of Chemical Physics, 2007
We studied the solvation structures of the divalent metal cations Mg 2+ and Ca 2+ in ambient water by applying a Car-Parrinello-based constrained molecular dynamics method. By employing the metal-water oxygen coordination number as a reaction coordinate, we could identify distinct aqua complexes characterized by structural variations of the first coordination shell. In particular, our estimated free-energy profile clearly shows that the global minimum for Mg 2+ is represented by a rather stable sixfold coordination in the octahedral arrangement, in agreement with experiments. Conversely, for Ca 2+ the free-energy curve shows several shallow local minima, suggesting that the hydration structure of Ca 2+ is highly variable. Implications for water exchange reactions are also discussed.
QM/MM molecular dynamics simulations of the hydration of Mg (II) and Zn (II) ions
The hydration of Mg 2+ and Zn 2+ is examined using molecular dynamics simulations using three computational approaches of increasing complexity: the CHARMM non-polarizable force field based on the TIP3P water model, the Drude polarizable force field based on the SWM4-NDP water model, and a combined QM/MM approach in which the inner coordination sphere is represented using a high quality density functional theory (DFT) model (PBE/def2-TZVPP) and the remainder of the bulk water solvent is represented using the polarizable SWM4-NDP water model. The characteristic structural distribution functions (radial, angular, and tilt) are compared, which show very good agreement between the polarizable force field and QM/MM approaches. They predict an average Mg-O distance of 2.11Å and an Zn-O distance of 2.13Å, in good agreement with the available experimental neutron scattering and EXAFS data, while the Mg-O distances calculated using the non-polarizable force field are 0.1Å too short. Mg 2+ (aq) and Zn 2+ (aq) both have a coordination number of 6 and have a remarkably similar octahedral coordination mode, despite the chemical differences between these ions. Thermodynamic integration was used to calculate the relative hydration free energies of these ions (∆∆G hydr ). The non-polarizable model is in error by 60 kcal mol −1 and incorrectly predicts that Mg 2+ has the more negative hydration energy. The Drude polarizable model predicts a ∆∆G hydr of only -13.2 kcal mol −1 , an improvement over the results of the non-polarizable force field, but still signficantly different than the experimental value of -30.1 kcal mol −1 . The combined QM/MM approach performs much better, predicting a ∆∆G hydr of -34.8 kcal mol −1 in excellent agreement with experiment. These calculations support the experimental observation that Zn 2+ has more favourable solvation free energy than Mg 2+ despite having a very similar solvation structure.
The structure of the magnesium hydroxide sulfate hydrate MgSO4.1/3Mg(OH)2.1/3H2O
Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry
The magnesium hydroxide sulfate hydrate MgSO4.~Mg(OH)2.~H20 crystallizes in the tetragonal space group I4~/amd with cell dimensions al-a 2-5.242 (1), c-12.995 (3) A, Z-4. The structure was solved by Patterson methods and refined to R = 0.041 using 160 single-crystal X-ray diffraction intensities measured with a four-circle diffractometer. Mg 2+ is octahedrally coordinated by the O atoms of four different SO 2-and two OHor H20. The structure consists of straight chains of face-sharing oxygen octahedra, two-thirds of which have Mg 2+ at their centers. This phase is considered to be stoichiometric; a partially ordered atomic arrangement is proposed to account for the integral coefficients in the formula. The structure is related to those of the sulfate monohydrates of divalent Mg, Mn, Fe, Co, Ni and Zn.
Crystal Growth & Design, 2010
The structural and dynamical properties of the alkaline earth metal ions Mg 2þ , Ca 2þ , and Sr 2þ and their carbonate and bicarbonate complexes in aqueous solution are examined through first principles molecular dynamics simulations based on the density functional theory. Calculations were conducted in explicit heavy water molecules and at the average temperature of 400 K, conditions which are necessary to obtain a liquid-like water structure and diffusion time-scales when using gradient corrected density functionals. According to these simulations, the magnesium ion undergoes a significant contraction of its coordination sphere in the Mg(H)CO 3 (þ) aqueous complex, whereas calcium and strontium increase their average first shell coordination number when coordinated to HCO 3 or CO 3 2-
The Journal of Physical Chemistry B, 2005
The ultrasonic velocities, densities, viscosities, and electrical conductivities of aqueous solutions of magnesium nitrate and magnesium acetate have been measured from dilute to saturation concentrations at 0 ≤ t/°C ≤ 50. The temperature derivative of the isentropic compressibility, κ s , became zero at 2.28 mol kg −1 and 2.90 mol kg −1 for Mg(OAc) 2 and Mg(NO 3 ) 2 solutions, respectively, at 25 o C. The total hydration numbers of the dissolved ions were estimated to be, respectively, 24.3 and 19.2 at these concentrations. Differences in κ s for various M 2+ salts, using the present and literature data, correlated with reported M 2+ -OH 2 bond lengths and to a lesser extent with cationic charge density (ionic radius). The influence of anions on κ s appears to follow the Hofmeister series and also correlates approximately with the anionic charge density.
Journal of Solution Chemistry, 2008
Isopiestic vapor-pressure measurements were made for {yMgCl2+(1−y)MgSO4}(aq) solutions with MgCl2 ionic strength fractions of y=(0,0.1997,0.3989,0.5992,0.8008, and 1) at the temperature 298.15 K, using KCl(aq) as the reference standard. These measurements for the mixtures cover the ionic strength range I=0.9794 to 9.4318 mol⋅kg−1. In addition, isopiestic measurements were made with NaCl(aq) as reference standard for mixtures of {xNa2SO4+(1−x)MgSO4}(aq) with the molality fraction x=0.5000 that correspond to solutions of the evaporite mineral bloedite (astrakanite), Na2Mg(SO4)2⋅4H2O(cr). The total molalities, m T=m(Na2SO4)+m(MgSO4), range from m T=1.4479 to 4.4312 mol⋅kg−1 (I=5.0677 to 15.509 mol⋅kg−1), where the uppermost concentration is the highest oversaturation molality that could be achieved by isothermal evaporation of the solvent at 298.15 K. The parameters of an extended ion-interaction (Pitzer) model for MgCl2(aq) at 298.15 K, which were required for an analysis of the {yMgC...