Probing ion-specific effects on aqueous acetate solutions: Ion pairing versus water structure modifications.

Tristan Petit,Gerrit Conrad,Kathrin M Lange,Kenji Yamamoto,Marcus Dantz,Emad F Aziz,Tim Brandenburg,Edlira Suljoti,Kai F Hodeck,Christoph Schwanke

doi:10.1063/1.4884600

Abstract

The effect of monovalent cations (Li+, K+, NH4+, Na+) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet. We show ion- and counterion dependent effects on the emission spectra of the oxygen K-edge, which we attribute to modifications of the hydrogen bond network of water. For acetates, ion pairing with carboxylates was also probed selectively by XAS and RIXS. We correlate our experimental results to speciation data and to the salting-out properties of the cations.

Highlights

Since the pioneering experiments of Hofmeister on precipitation of egg white proteins,1 the capability of ions to salt-in or salt-out proteins in solution has been thoroughly investigated.2–4the origin of the so-called “Hofmeister effect” still remains under discussion and different salting mechanisms have been proposed.3 More it was suggested that modification of the hydrogen bond structure of water molecules induced by ions is responsible for changes in macromolecule solubility.5 According to this picture, ions were classified into “chaotropes” and “kosmotropes” depending on their salting abilities.2,6,7 later studies based on pump-probe spectroscopy,8 pressure perturbation calorimetry,9 or X-ray absorption spectroscopy (XAS)10 demonstrated that ions have a minimal impact on the water structure beyond their first hydration shell
The effect of monovalent cations (Liþ, Kþ, NH4þ, Naþ) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet
Later studies based on pump-probe spectroscopy,8 pressure perturbation calorimetry,9 or X-ray absorption spectroscopy (XAS)10 demonstrated that ions have a minimal impact on the water structure beyond their first hydration shell

Summary

Introduction

Since the pioneering experiments of Hofmeister on precipitation of egg white proteins, the capability of ions to salt-in or salt-out proteins in solution has been thoroughly investigated.2–4the origin of the so-called “Hofmeister effect” still remains under discussion and different salting mechanisms have been proposed. More it was suggested that modification of the hydrogen bond structure of water molecules induced by ions is responsible for changes in macromolecule solubility. According to this picture, ions were classified into “chaotropes” and “kosmotropes” depending on their salting abilities. later studies based on pump-probe spectroscopy, pressure perturbation calorimetry, or X-ray absorption spectroscopy (XAS) demonstrated that ions have a minimal impact on the water structure beyond their first hydration shell. It was suggested that modification of the hydrogen bond structure of water molecules induced by ions is responsible for changes in macromolecule solubility.. It was suggested that modification of the hydrogen bond structure of water molecules induced by ions is responsible for changes in macromolecule solubility.5 According to this picture, ions were classified into “chaotropes” and “kosmotropes” depending on their salting abilities.. Later studies based on pump-probe spectroscopy, pressure perturbation calorimetry, or X-ray absorption spectroscopy (XAS) demonstrated that ions have a minimal impact on the water structure beyond their first hydration shell. Such conclusions were later questioned by neutron diffraction results.

Methods

Results

Conclusion