Deciphering the Solvent Effect for the Solvation Structure of Ca²⁺ in Polar Molecular Liquids

Abstract

Although the crystal structures for many inorganic compounds are readily available, researchers are still working hard to understand the relations between the structures and chemical properties in solutions, because that is where most of the chemical reactions take place. A huge amount of effort has been put on modeling the ion solvation structure, from the perspectives of both experiments and theories. In this study, the solvation structures of Ca 2+ ion in aqueous and alcohol solutions at different concentrations were carefully evaluated by Ca K-edge X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). Density functional theory (DFT) calculations were also performed to correlate with the experimental data, and then further extended to other similar systems. It was found that the number of coordination solvent molecules decreases with increasing Ca 2+ concentration and increasing solvent molecule sizes. From the EXAFS data, the first solvation shell of Ca 2+ split into two Ca-O distances in methanol solution and the counter ion Clmight also be within the first shell at high concentrations. For the first time, the effects of solvent with different polarities and sizes on the ion solvation environment were systematically evaluated.

Synthesis and characterization of a near-infrared persistent luminescent Cr-doped zinc gallate–calcium phosphate composite

“…The Ca K-edge XANES has several characteristic features that have been used to track down the local bonding information around Ca 2+ for various calcium orthophosphates, 51 and other Ca compounds such as CaCl 2 , a Ca 2+ -centered complex. 52,53 The subtle differences in features a, b, and c marked in Fig. 5(a) are an indicator of the hybridization of Ca electronic orbitals with the surrounding neighbors.…”

“…A reduced symmetry increases the extent of p-d state mixing, and therefore a more intense pre-edge peak is observed in this region. 52 The coordination number of Ca 2+ is usually between 6 and 8, and in most of the calcium orthophosphates, Ca 2+ is surrounded by more than 6 oxygens, leading to a distinct pre-edge. This further verifies that the Ca in CZGO-ACP is in the form of calcium phosphate.…”

In-situ/operando X-ray absorption spectroscopic investigation of the electrode/electrolyte interface on the molecular scale

“…TD-DFT calculations were completed on the optimized structures to model XANES spectra using the transitions from the F 1s to probable unoccupied orbitals [36]. All the calculations were done using the B3LYP functional [37], def2-TZVP basis set [38], SARC/J auxiliary basis set [39], with ZORA [39] in THF solutions using the PCM model [40,41].…”

Probing calcium solvation by XAS, MD and DFT calculations

“…The L-edge so XAS is more sensitive to the local chemical environment of calcium compared to that from hard X-ray K-edge measurement since it probes the excitation of 2s or 2p electrons while the K-edge measures the 1s electrons. In contrast to our earlier work probing the bulk properties, 19 the Ledge XAS measured using our specially designed in situ/operando cell enables us to apply electrochemical potentials while XAS intensities being collected, making in situ/operando observation of the electrode/electrolyte interface possible, which dictates the battery performance. It benets from the surface sensitive nature of the total electron yield (TEY) mode in the so X-ray L-edge XAS of calcium, which limits the probing depth within 1-2 nm.…”

“…17 A few years later, a combination of ab initio MD simulations and neutron scattering experiments were employed in the characterization of ion hydration and pairing in aqueous calcium chloride and formate/acetate solutions by Martinet et al 18 In a separate study by Guo et al, the solvent effect for the solvation structure of Ca 2+ in polar molecular liquids were studied systematically using calcium K-edge X-ray absorption spectroscopy (XAS). 19 In that work the liquid samples are sealed in mylar lms and the bulk properties are being probed, due to the penetration depth of uorescence is on the order of hundreds of nanometers. The calcium L-edge so XAS was reported to study different types of solid calcium compounds, such as CaCl 2 -$2H 2 O, CaF 2 , CaCO 3 , etc., 20 but with very limited reporting on using calcium L-edge so XAS to study calcium solvation environment in solutions.…”