Ion adsorption and electrokinetic transport at interfaces
Location: CECAM-FR-GSO, Site de Marcoule, Marcoule
The field of charged solid-liquid interfaces is of particular interest in physical chemistry because it concerns many scientific fields and applications: electrochemistry, biology, geology, pollution control, formulation, etc [1,2]. Two important phenomena are essential to any description of these systems: the adsorption of ions on the surface which drives in particular the electrical properties of the double layer and the fluid transport which results from the external and internal forces of the medium. Electrokinetic phenomena , such as electroosmosis or electrophoresis [4-6], are particularly intriguing examples of a macroscopic motion originating from the interfacial charge distribution and fluid dynamics, allowing design of controlled nanofluidic devices. They are also very much used by the engineering sciences because they allow the measurement of global characteristic quantities like the zeta potential [7,8], which can be directly related to experiments . Despite their practical interest, these quantities are interpreted with effective models, which idealize the molecular reality .
Since the original works of Gouy, Chapman and Smoluchowski, the practical description of these systems relies on continuous solvent theories that are not well justified microscopically. In this context, in the last few years many molecular studies have tried to justify and improve these descriptions. Realistic simulations of interface by various methods (NEMD, molecular hydrodynamics, Brownian dynamics, etc.) have been performed and they globally provided a more realistic atomistic picture of the dynamical interfacial phenomena [11,12]. The general laws have been validated and their limits clarified. It was shown that the description could be improved by accounting for additional phenomena, such as hydrodynamic slip length , viscosity, mobility or local dielectric constant . Even if all these concepts make sense and are applicable at least for some particular systems, there is no general molecular theory of these systems [15,16]. One must still decide which ones are of general interest and must be systematically taken into account and in which case the others must be considered, in order to define a really modern theory of electrokinetic phenomena.
In parallel to these dynamic studies, which did not focus on the differences due to the nature of the ions (beyond their charge), many works on equilibrium were interested in the specific ion effects in particular at the interfaces [17,18]. Now, for both bulk and confined electrolyte solutions, we are able to predict and understand the Hofmeister effects representing the differences between ions. Both classical and ab initio free energy calculation methods  can for example predict specific adsorption constants, overcompensation of the surface charge, anomalous behavior or kosmotropic/chaotropic effects. It appears necessary to combine these modern studies on ionic specific effects with the molecular description of transport.
The goal of the workshop will be to define, in the light of these recent works, a program to establish a truly modern theory of ion adsorption and fluidic transport at interfaces. The development of new methodologies (surface charging and protonation , models of ions , solvent, and surfaces ), the link between the scales of description, and the interpretation of these results in relation to the experiments will be especially examined. The software tools to run these simulations and technical details (application of the external field, thermostating, detailed analyses of inhomogeneous interfacial properties) will be discussed. An important challenge is due to the very different time scales, especially in the adsorption phenomena, which makes multilevel atomistic modeling necessary .
Milan Předota (University of South Bohemia) - Organiser & speaker
Jean-François Dufrêche (University of Montpellier) - Organiser
Laurent Joly (Université Lyon 1) - Organiser
Remco Hartkamp (Delft University of Technology) - Organiser & speaker