The hydration structure of ions dissolved in aqueous solutions under nanoconfinement is central to understanding solubility and mass transport in many natural and industrial systems of importance. For example, the properties of nanoconfined water are vastly different compared to bulk water and this greatly impacts the hydration structure of solvated ions in nanoconfined spaces. Here, we use laboratory-based valence to core X-ray emission spectroscopy (VTC-XES) to study the evolution of hydration structure and quantify ion pairing in aqueous ZnCl2 solutions confined in the nanopores of activated carbon. We show that in nanoconfined aqueous ZnCl2 solutions the Zn-Cl contact ion pairing is greatly enhanced and approaches saturation at lower concentrations than for bulk solutions. As one example, the average coordination number (average number of Cl- ions coordinated per Zn2+ ion) in a 1 molal ZnCl2 solution increases from ~ 0.5 in the bulk to ~ 1.4 in the nanopores of activated carbon.  Our results speak to the importance of pore space morphology in geological brines, where ZnCl2 is a well-studied bulk model system and also for electrical energy storage, where our results help elucidate the coupling of nanostructure and the voltage window for Zn-ion batteries.  Our work further demonstrate the value of VTC-XES for hydration studies of metal ions.

The hydration structure of ions dissolved in aqueous solutions under nanoconfinement is central to understanding solubility and mass transport in many natural and industrial systems of importance. For example, the properties of nanoconfined water are vastly different compared to bulk water and this greatly impacts the hydration structure of solvated ions in nanoconfined spaces. Here, we use laboratory-based valence to core X-ray emission spectroscopy (VTC-XES) to study the evolution of hydration structure and quantify ion pairing in aqueous ZnCl~2~ solutions confined in the nanopores of activated carbon. We show that in nanoconfined aqueous ZnCl~2~ solutions the Zn-Cl contact ion pairing is greatly enhanced and approaches saturation at lower concentrations than for bulk solutions. As one example, the average coordination number (average number of Cl^-^ ions coordinated per Zn^2+^ ion) in a 1 molal ZnCl~2~ solution increases from ~ 0.5 in the bulk to ~ 1.4 in the nanopores of activated carbon.  Our results speak to the importance of pore space morphology in geological brines, where ZnCl~2~ is a well-studied bulk model system and also for electrical energy storage, where our results help elucidate the coupling of nanostructure and the voltage window for Zn-ion batteries.  Our work further demonstrate the value of VTC-XES for hydration studies of metal ions.

Enhanced ion pairing in aqueous solutions under nanoconfinement

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.