Oxygen and Hydrogen Isotopic Preference in Hydration Spheres of Magnesium and Calcium Ions

Abstract

Molecular orbital calculations were performed to estimate the 18 O/ 16 O and D/H isotopic reduced partition function ratios (rpfrs) of water molecules around magnesium and calcium ions. As model for water molecules in the ith hydration sphere of the cation in aqueous solutions containing that cation, we considered water molecules in the ith hydration sphere that were surrounded by water molecules in the (i+1)th hydration sphere in clusters, M 2+ (H 2 O) n (M = Mg or Ca; n up to 100). The calculations indicated that the decreasing order of the 18 O preference over 16 O in the primary hydration sphere is Mg 2+ > Ca 2+ > bulk water. That is, water molecules in the primary hydration spheres of the Mg 2+ and Ca 2+ ions are expected to be enriched in the heavier isotope of oxygen relative to water molecules in bulk, and the degree of the enrichment is larger for the Mg 2+ ion than for the Ca 2+ ion. No such preference was observed for hydrogen isotopes in any hydration sphere or for oxygen isotopes in the secondary and outer hydration spheres.

Water-in-Salt Electrolyte-Based Extended Voltage Range, Safe, and Long-Cycle-Life Aqueous Calcium-Ion Cells

“…Thus, this suggests that the insufficient number of water molecules at ultrahigh concentration is responsible for the decrease in the hydration number of Ca ions. An earlier report suggested that the hydration number of the Ca ions in 1 m aqueous electrolyte is 9. ,, However, the hydration number at the ultrahigh concentration is much smaller than that in the dilute aqueous electrolyte …”

“…An earlier report suggested that the hydration number of the Ca ions in 1 m aqueous electrolyte is 9. 15,25,26 However, the hydration number at the ultrahigh concentration is much smaller than that in the dilute aqueous electrolyte. 26 Furthermore, the peak at 737 cm −1 experienced a blue shift with the elevated concentration (Figure 3h).…”

“…32 It is demonstrated that the Ca ion has a hydration number of up to 9 for the first solvation shell with a hydration radius of 3.09 Å. 25,26 However, the hydration number of the Ca ion is smaller in the ultrahigh-concentration electrolyte due to the insufficient presence of water molecules in the electrolyte. 15 Eventually, the smaller hydration number and radius of the Ca ion in the ultrahigh-concentration electrolyte affect the electrochemical reactions.…”

A Novel Strategy to Improve the Electrochemical Performance of a Prussian Blue Analogue Electrode for Calcium-Ion Batteries

“…In previous studies on aqueous electrolytes, it has been reported that calcium cations can have a hydration number of up to nine for the first shell if there are sufficient water molecules in the electrolyte, although there is some variation in this value depending on the analysis method. [29][30][31] Based on this, the discharge/charge tests were carried out in Ca-WP electrolytes with H 2 O/Ca 2+ molar ratios of more than or less than nine to clarify the effect of decreasing the hydration number of calcium ions.…”