Ion-Solvation-Induced Molecular Reorganization in Liquid Water Probed by Resonant Inelastic Soft X-ray Scattering

Abstract

The molecular structure of liquid water is susceptible to changes upon admixture of salts due to ionic solvation, which provides the basis of many chemical and biochemical processes. Here we demonstrate how the local electronic structure of aqueous potassium chloride (KCl) solutions can be studied by resonant inelastic soft X-ray scattering (RIXS) to monitor the effects of the ion solvation on the hydrogen-bond (HB) network of liquid water. Significant changes in the oxygen K-edge emission spectra are observed with increasing KCl concentration. These changes can be attributed to modifications in the proton dynamics, caused by a specific coordination structure around the salt ions. Analysis of the spectator decay spectra reveals a spectral signature that could be characteristic of this structure.

Ionic Solutions Probed by Resonant Inelastic X-ray Scattering

“…Since changes seem to be dependent on the concentration, it indicates a local impact of Na + . This finding is in agreement with recent results [5,16,33,34]. It is intriguing that the oxygens from CH 3 COO − ions don't contribute to the XE signal significantly see Figure 3.…”

“…For the 1 and 2 M spectra, a residue is introduced as a difference between the measured data and the two component fit. It has been proposed that the residue can be interpreted as being the signal from the water molecules in the first solvation shell of the salt ion [16]. Yet, the residues are not significant, which could be due to insufficient signal from water molecules in the first solvation shell.…”

“…Moreover, photons offer bulk information as the penetration length is much higher compared to electrons. Overall X-ray spectroscopy is an ideal tool to study the local electronic structure [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23].…”

Soft X-ray spectroscopy of nanoparticles by velocity map imaging

“…It was observed that the shape of the O K edge NEXAFS spectrum of water changes with the addition of salts, with changes progressing as the salt concentration increases. 49,50 Similar changes are also observed in vibrational sum-frequency spectroscopy. 51 These changes are interpreted as iodine anions (which show a propensity for the interface) 52 weakening the water bonding coordination network near the surface region.…”

Cationic and Anionic Impact on the Electronic Structure of Liquid Water

“…X-ray spectroscopy allows measuring directly the electronic properties of a system and provides direct information about the local electronic structure around a specific element in a bulk chemical environment. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] Previous soft X-ray absorption studies on aqueous solutions reported strong distortions of the pre-, main-and post edge of the X-ray absorption spectra on the oxygen K-edge in the vicinity of the ions. 7,30,31,33 These changes have been assigned to weakening and strengthening of the hydrogen bonds in the solvation shell around the cations.…”