S. Tu, S. Lobanov, J. Bai
Oct 22, 2019
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Influential Citations
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Journal
The journal of physical chemistry. B
Abstract
As a candidate of Martian salts, calcium perchlorate (Ca(ClO4)2) has the potential to stabilize liquid water on the Martian surface due to its hygroscopicity and low freezing temperature when forming aqueous solution. These two properties of electrolytes in general have been suggested to result from the specific cation-anion-water interaction (ion pairing) that interrupts the structure of solvent water. To investigate how this concentration-dependent and temperature-dependent ion pairing process in aqueous Ca(ClO4)2 solution leads to its highly hygroscopic property and the extreme low eutectic temperature, we have conducted two sets of experiments. First, the effects of concentration in aqueous calcium perchlorate from 3 molal to 7.86 molal on ion pairing were investigated using Raman spectroscopy. Deconvolution of the Raman symmetric stretching band (ν1) of ClO4- showed the enhanced formation of solvent-shared ion pairs upon increasing salt concentration at room temperature. We have confirmed that the low tendency of forming contact ion pairs in concentrated solution contributes to the high hygroscopicity of the salt. Second, the near eutectic samples were studied as a function of temperature by both combined Differential Scanning Calorimetry-Raman Spectroscopic experiments and in situ X-ray diffraction. The number of solvent-shared ion pairs were found to increase with decreasing temperature when cooled below the temperature of maximum density of the solution, which is driven by a change in water towards an ice-like structure in the supercooled regime. The massive presence of solvent-shared ion pairs in turn limits the development of long-range order in the tetrahedral networks of water molecules, which is responsible for the extremely low eutectic point and deep supercooling effects observed in the Ca(ClO4)2-H2O system.