Investigation of the Ionic Hydration in Aqueous Salt Solutions by Soft X-ray Emission Spectroscopy

Abstract

Understanding the molecular structure of the hydration shells and their impact on the hydrogen bond (HB) network of water in aqueous salt solutions is a fundamentally important and technically relevant question. In the present work, such hydration effects were studied for a series of representative salt solutions (NaCl, KCl, CaCl2, MgCl2, and KBr) by soft X-ray emission spectroscopy (XES) and resonant inelastic soft X-ray scattering (RIXS). The oxygen K-edge XES spectra could be described with three components, attributed to initial state HB configurations in pure water, water molecules that have undergone an ultrafast dissociation initiated by the X-ray excitation, and water molecules in contact with salt ions. The behavior of the individual components, as well as the spectral shape of the latter component, has been analyzed in detail. In view of the role of ions in such effects as protein denaturation (i.e., the Hofmeister series), we discuss the ion-specific nature of the hydration shells and find that the results point to a predominant role of anions as compared to cations. Furthermore, we observe a concentration-dependent suppression of ultrafast dissociation in all salt solutions, associated with a significant distortion of intact HB configurations of water molecules facilitating such a dissociation.

Ion Selectivity of Water Molecules in Subnanoporous Liquid‐Crystalline Water‐Treatment Membranes: A Structural Study of Hydrogen Bonding

“…[18][19][20] For aqueous salt solutions, the XES profile of water is further modulated due to a change in the hydrogen-bonded structures of water molecules hydrating the solutes. [21,22] This is demonstrated at the bottom of Figure 2 b and is discussed below in detail.…”

“…The 1b 1 ′′ peak between the 1b 1 ′ and the gas‐phase 1b 1 peaks can be associated with highly distorted hydrogen‐bonded H 2 O molecules [18–20] . For aqueous salt solutions, the XES profile of water is further modulated due to a change in the hydrogen‐bonded structures of water molecules hydrating the solutes [21, 22] . This is demonstrated at the bottom of Figure 2 b and is discussed below in detail.…”

Elemental-sensitive Detection of the Chemistry in Batteries through Soft X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering

“…Analysis based on soft X-ray spectroscopy could be carried out for different types of phase transformations and SOCs, with both surface and bulk sensitivities, and under in-situ/operando and ex-situ conditions. We also note that, although examples are not shown in this technical report, sXAS and RIXS results of low-Z elements, e.g., C, N, O, also provide critical information on the critical chemical states in battery compounds, as demonstrated in many previous publications 12,13,14,30,31 .…”

“…However, due to the extremely high chemical-state selectivity of RIXS, RIXS is potentially a much more sensitive probe of the chemical state evolution in battery materials with inherent elemental sensitivity. Recent sXES and RIXS reports by Jeyachandran et al, have showcased the high sensitivity of RIXS to specific chemical configurations in the ion-solvation systems beyond sXAS 30,31 . With the recent rapid developments of high-efficiency RIXS systems 32,33,34 , RIXS has quickly shifted from a fundamental physics tool to a powerful technique for battery research, and occasionally becomes the tool-of-choice for specific studies of both the cation and anion evolution in battery compounds.…”

Impact of n-Butylammonium Bromide on the Chemical and Electronic Structure of Double-Cation Perovskite Thin Films

“…40 The dissociation process is very sensitive to the chemical surrounding of the respective functional group, as was observed for, e.g., liquid water, 51 ammonia, 52,53 and salt solutions. 54,55 Thus, we assign the differences to a different bonding environment of the BA molecule in the BABr/ perovskite sample, compared to that in the BABr reference. Nevertheless, the BABr signature suggests that the molecule remains intact, and hence, we interpret this result such that BABr is incorporated into a 2D/3D perovskite structure as opposed to forming a separate BABr secondary phase.…”