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Title: Enhanced Formation of Solvent-Shared Ion Pairs in Aqueous Calcium Perchlorate Solution toward Saturated Concentration or Deep Supercooling Temperature and Its Effects on the Water Structure

Journal Article · · Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry
ORCiD logo [1];  [2];  [3];  [1];  [1];  [1];  [4];  [4]
  1. Stony Brook Univ., Stony Brook, NY (United States)
  2. Stony Brook Univ., Stony Brook, NY (United States); GFZ German Research Center for Geosciences, Potsdam (Germany)
  3. Brookhaven National Lab. (BNL), Upton, NY (United States)
  4. Stony Brook Univ., Stony Brook, NY (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)
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As a candidate of Martian salts, calcium perchlorate [Ca(ClO4)2] has the potential to stabilize liquid water on the Martian surface because of 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 high hygroscopic property and the extreme low eutectic temperature, we have conducted two sets of experiments. First, the effects of concentration on aqueous calcium perchlorate from 3 to 7.86 m 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 was found to increase with decreasing temperature when cooled below the temperature of maximum density of the solution, driven by a change in water toward an ice-like structure in the supercooled regime. The massive presence of solvent-shared ion pairs in turn limits the development of the 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.

Research Organization:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
SC0012704
OSTI ID:
1576226
Report Number(s):
BNL-212364-2019-JAAM
Journal Information:
Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry, Vol. 123, Issue 45; ISSN 1520-6106
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 8 works
Citation information provided by
Web of Science

Figures / Tables (7)

Figure 1(p. 6)figure Figure 1
Figure 2(p. 6)figure Figure 2
Figure 3(p. 7)figure Figure 3
Figure 4(p. 8)figure Figure 4
Figure 5(p. 9)figure Figure 5
Table 1(p. 9)table Table 1
Figure 6(p. 13)figure Figure 6

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