title: Electrolytes at interfaces: accessing the first nanometers using X-ray standing waves.
abstract: Ion-surface interactions are of high practical importance in a wide range of technological, environmental and biological problems. In particular, they ultimately control the electric double layer structure, hence the interaction between particles in aqueous solutions. Despite numerous achievements, progress in their understanding is still limited by the lack of experimental determination of the surface composition with appropriate resolution. Tackling this challenge, we have developed a method based on X-ray standing waves coupled to nano-confinement which allows the determination of ion concentrations at a solid-solution interface with a sub-nm resolution. We have investigated mixtures of KCl/CsCl and KCl/KI in 0.1 mM to 10 mM concentrations on silica surfaces and obtained quantitative information on the partition of ions between bulk and Stern layer as well as their distribution in the Stern layer. Regarding partition of potassium ions, our results are in agreement with a recent AFM study. We show that in a mixture of KCl and KI, chloride ions exhibit a higher surface propensity than iodide ions, having a higher concentration within the Stern layer and being on average closer to the surface by ≈1-2 Å, in contrast to the solution water interface. Confronting such data with molecular simulations will lead to a precise understanding of ionic distributions at aqueous interfaces.
authors.affiliation: Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramoléculaire, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif sur Yvette Cedex, France. florent.malloggi@cea.fr.
authors.forename: Soumaya
authors.initials: S
authors.lastname: Ben Jabrallah
authors.affiliation: Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramoléculaire, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif sur Yvette Cedex, France. florent.malloggi@cea.fr.
authors.forename: Florent
authors.initials: F
authors.lastname: Malloggi
authors.affiliation: Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramoléculaire, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif sur Yvette Cedex, France. florent.malloggi@cea.fr.
authors.forename: Luc
authors.initials: L
authors.lastname: Belloni
authors.affiliation: ICSM UMR 5257 - CEA/CNRS/UM/ENSCM, Site de Marcoule, Bâtiment 426 BP 17171 F-30207 Bagnols sur Cèze Cedex, France.
authors.forename: Luc
authors.initials: L
authors.lastname: Girard
authors.affiliation: Deutsches Elektronensynchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany.
authors.forename: Dmitri
authors.initials: D
authors.lastname: Novikov
authors.affiliation: Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France.
authors.forename: Cristian
authors.initials: C
authors.lastname: Mocuta
authors.affiliation: Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France.
authors.forename: Dominique
authors.initials: D
authors.lastname: Thiaudière
authors.affiliation: Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France.
authors.forename: Jean
authors.initials: J
authors.lastname: Daillant