Coordination variation of hydrated Cu2+/Br1− ions traversing the interfacial water in mesopores

Abstract

Resolution of the atomistic and electronic details about the coordination structure variation of hydrated ions in the interfacial water is still a tough challenge, which is, however, essentially important for the understanding of ion adsorption, permeation and other similar processes in aqueous solutions. Here we report the tracing of coordination structure variation for hydrated Cu2+/Br1- ions traversing the interfacial water in Vycor mesopores (ϕ = 7.6 nm) by employing both X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies. By controlled desorption/adsorption of water, the filling fraction of the mesopores, thus the water layer thickness, can be adjusted, which in turn effects the variation of coordination structure of the ions therein. It is found that both Cu2+ and Br1- ions prefer staying exclusively in the core water, and in this circumstance no ion pairs have been detected in the solution of concentrations up to 1.0 M. Following capillary decondensation occurring at a filling fraction of ∼35% which corresponds to a water layer of about three monolayers, Br1- ions begin immediately to reconstruct their first coordination shell, characterized by ionic dehydration, shrinkage of ion-water bond length, and formation of ion pairs. In contrast, Cu2+ ions can retain a bulk-like coordination structure till being driven to bond directly to the pore surface when the filling fraction is below 20%. At the final stage of dehydration via thermal vacuum treatment at 110°C, Cu2+ ions can be completely reduced to the Cu1+ state, and recover at room temperature only when the filling fraction is above 14%. These results may be inspirable for the investigation of similar problems concerning hydrated ions in water solution under different confining conditions.

Quantification of Enhanced Ion-Pairing in Nanoconfined Electrolytes Using Valence-to-Core X-ray Emission Spectroscopy

“…In addition, the latter study found the extent of cation desolvation to be inversely proportional to the size of the pores. (34) Similarly, classical molecular dynamics simulation on aqueous NaCl (37) and X-ray absorption spectroscopy experiment on aqueous CuBr2 (35) agree about the bulk-like cation-hydration structure in larger pores and the decrease in hydration and enhancement in ion pairing in smaller pores. Moreover, Nelson et al, (36) used Zn K-edge EXAFS to study aqueous Zn(NO3)2, and concluded that Zn 2+ adsorbs in a tetrahedral coordination in nanoporous silica with pores <10 nm whereas the Zn 2+ ions exists instead as a mixture of tetrahedrally and octahedrally coordinated surface complexes for pore size >10 nm.…”

“…It has been proposed that water forms a dense ordered layer on the solid -aqueous solution interface and the ordering can extend up to 6 -7 Å from the wall after which it becomes more diffuse as in the bulk case. (35,37) Such an ordering of water near the interface decreases its dielectric constant and favors ion pairing from an entropic perspective. Further, for NPC with a very large surface area and a very high surface to volume ratio, a large fraction of water molecules in the solution would be consumed in the interfacial layer effectively increasing the concentration of the remaining core solution.…”

“…License: CC BY-NC-ND 4.0 in aqueous solutions are an emergent issue, as are efforts to isolate solvent interfacial effects from confinement effects on solute species coordination. (19,(33)(34)(35)(36) Having admitted the difficulty of the challenge, we summarize here prior work on confinement effects on ion association. In general, the below studies qualitatively agree on the trend of cation dehydration and ion pairing enhancement under confinement.…”

“…(22) The question then arises about how to best add to the above literature with complementary (or even initial) characterization of the cation local environment in nanoconfined solutions. While EXAFS is a natural choice for this question, (35,36) it comes with its own difficulties through an undesired sensitivity to the second coordination shell which complicates quantification of ion pairing. (38)(39)(40) In the present study, we use valence to core X-ray emission spectroscopy (VTC-XES) to quantify the effects of nanoconfinement by nanoporous carbon on the population of Zn 2+ complexes in aqueous ZnCl2 solution.…”

Effects of water on the structure and transport properties of room temperature ionic liquids and concentrated electrolyte solutions*

“…[96] The chemistry of EDL, the type of ion adsorbed on the surface of electrodes, and especially the component and the structure of solvation sheath of ions in EDL region determine the formation, chemistry, and structure of SEI. [96][97][98][99][100][101] In 2004, Abe and coworkers first studied the activation energy for the transfer of desolvated Li + at SEI between graphite and electrolyte solutions, i.e., 1 mol/dm 3 LiCF 3 SO 3 dissolved in 1,2-dimethoxyethane (DME) and DMSO, respectively. [102] The measured activation energies ranging from 53 kJ/mol to 59 kJ/mol are about twice as much as that of the solvated Li + transfer in a reference system without stable SEI film, i.e., graphite/LiClO 4 dissolved in mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (Fig.…”