![]() The stability constants of ternary calcium uranyl tricarbonate complexes, CaUO2(CO3)32- and Ca2UO2(CO3)3(aq), were determined in NaClO4 medium at various ionic strengths using time-resolved laser-induced luminescence spectroscopy (TRLS) - also known as time-resolved laser-induced fluorescence spectroscopy (TRLFS). The application of the consistent set of thermodynamic constants and ε values for MnUO2(CO3)3(4 2n)- (M = Mg, Ca) are examined in different geochemical contexts, where Mg over Ca concentration ratio varies to help defining the relative importance of these species. The 3 lines mm⁻¹ gratings were both used to acquire MgUO2(CO3)3²⁻ luminescence spectra, where the high-resolution 1800 lines mm⁻¹ grating detected slight spectral shifts for the principal luminescent bands relative to CanUO2(CO3)3⁽⁴⁻²ⁿ⁾⁻. The cumulative formation constant determined is log10β☁.1.3 = 26.40 ± 0.07, and the specific ion interaction coefficients ε(MgUO2(CO3)3²⁻, Na⁺) = 0.19 ± 0.11 kgw mol⁻¹ in NaClO4 and ε(MgUO2(CO3)3²⁻, Na⁺) = 0.09 ± 0.16 kgw mol⁻¹ in NaCl. The upper limits of ionic strength were chosen in order to limit luminescence quenching effects generated by high concentrations of Cl⁻ and ClO4⁻, already observed during our earlier studies on CanUO2(CO3)3⁽⁴⁻²ⁿ⁾⁻ (Shang & Reiller, Dalton Trans. ![]() The formation constants and specific ion interaction coefficients of MgUO2(CO3)32 complex were determined in 0.1 to 1 mol kgw⁻¹ NaCl and 0.1 to 2.21 mol kgw⁻¹ NaClO4 media in the framework of the specific ion interaction theory (SIT), using time-resolved laser-induced luminescence spectroscopy. As the effects of perchlorate salts on the thermodynamics of α-chymotrypsin’s activity closely resemble those of psychrophilic adaptations, it suggests that the presence of chaotropic molecules may be beneficial to life operating in low temperature environments. We have also shown that α-chymotrypsin activity appears to exhibit an altered pressure response at low temperatures while also maintaining stability at high pressures and sub-zero temperatures. By destabilising α-chymotrypsin, the perchlorate salts also cause an increasingly negative entropy of activation, which drives the reduction of enzyme activity at higher temperatures. The low temperature activation is facilitated by a reduced enthalpy of activation owing to the destabilising effects of perchlorate salts. Here we show that perchlorate salts, which are found pervasively on Mars, increase the activity of α-chymotrypsin at low temperatures. Studies of salt effects on enzyme activity have typically been conducted at standard temperatures and pressures, thus missing effects which only become apparent under non-standard conditions. 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. 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. 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. We have confirmed that the low tendency of forming contact ion pairs in concentrated solution contributes to the high hygroscopicity of the salt. 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. First, the effects of concentration in aqueous calcium perchlorate from 3 molal to 7.86 molal on ion pairing were investigated using Raman spectroscopy. ![]() 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. 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. 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.
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