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Atomistic details about the hydration of ions in aqueous solutions are still debated due to the disordered and statistical nature of the hydration process. However, many processes from biology, physical chemistry to materials sciences rely on the complex interplay between solute and solvent. Oxygen K-edge X-ray excitation spectra provide a sensitive probe of the local atomic and electronic surrounding of the excited sites. We used ab initio molecular dynamics simulations together with extensive spectrum calculations to relate the features found in experimental oxygen K-edge spectra of a concentration series of aqueous NaCl with the induced structural changes upon solvation of the salt and distill the spectral fingerprints of the first hydration shells around the Na+- and Cl--ions. By this combined experimental and theoretical approach, we find the strongest spectral changes to indeed result from the first hydration shells of both ions and relate the observed shift of spectral weight from the post- to the main-edge to the origin of the post-edge as a shape resonance.

International audience; The understanding of the microstructure of associated liquids promoted by hydrogen-bonding andconstrained by steric hindrance is highly relevant in chemistry physics, biology and for many aspects ofdaily life. In this study we use a combination of X-ray diffraction, dielectric spectroscopy and moleculardynamics simulations to reveal temperature induced changes in the microstructure of different octanolisomers, i.e., linear 1-octanol and branched 2-, 3- and 4-octanol. In all octanols, the hydroxyl groupsform the bases of chain-, cyclic- or loop-like bonded structures that are separated by outwardlydirected alkyl chains. This clustering is analyzed through the scattering pre-peaks observed from X-rayscattering and simulations. The charge ordering which pilots OH aggregation can be linked to thestrength of the Debye process observed in dielectric spectroscopy. Interestingly, all methods used hereconverge to the same interpretation: as one moves from 1-octanol to the branched octanols, thecluster structure evolves from loose large aggregates to a larger number of smaller, tighter aggregates.All alcohols exhibit a peculiar temperature dependence of both the pre-peak and Debye process, whichcan be understood as a change in microstructure promoted by chain association with increased chainlength possibly assisted by ring-opening effects. All these results tend to support the intuitive picture ofthe entropic constraint provided by branching through the alkyl tails and highlight its capital entropicrole in supramolecular assembly.

We present an X-ray Compton scattering study on aqueous trimethylamine N-oxide (TMAO) and guanidine hydrochloride solutions (GdnHCl) as a function of temperature. Independent from the concentration of the solvent, Compton profiles almost resemble results for liquid water as a function of temperature. However, the number of hydrogen bonds per water molecule extracted from the Compton profiles suggests a decrease of hydrogen bonds with rising temperature for all studied samples, and the differences between water and the solutions are weak. Nevertheless, the data indicate a reduced bond weakening with rising TMAO concentration up to 5 M of 7.2% compared to 8% for pure water. In contrast, the addition of GdnHCl appears to behave differently for concentrations up to 3.1 M with a weaker impact on the temperature response of the hydrogen bond structure.