title: Diverging effects of isotopic fractionation upon molecular diffusion of noble gases in water: mechanistic insights through ab initio molecular dynamics simulations.
abstract: Atmospheric noble gases are routinely used as natural tracers to analyze gas transfer processes in aquatic systems. Their isotopic ratios can be employed to discriminate between different physical transport mechanisms by comparison to the unfractionated atmospheric isotope composition. In many applications of aquatic systems molecular diffusion was thought to cause a mass dependent fractionation of noble gases and their isotopes according to the square root ratio of their masses. However, recent experiments focusing on isotopic fractionation within a single element challenged this broadly accepted assumption. The determined fractionation factors of Ne, Ar, Kr and Xe isotopes revealed that only Ar follows the prediction of the so-called square root relation, whereas within the Ne, Kr and Xe elements no mass-dependence was found. The reason for this unexpected divergence of Ar is not yet understood. The aim of our computational exercise is to establish the molecular-resolved mechanisms behind molecular diffusion of noble gases in water. We make the hypothesis that weak intermolecular interactions are relevant for the dynamical properties of noble gases dissolved in water. Therefore, we used ab initio molecular dynamics to explicitly account for the electronic degrees of freedom. Depending on the size and polarizability of the hydrophobic particles such as noble gases, their motion in dense and polar liquids like water is subject to different diffusive regimes: the inter-cavity hopping mechanism of small particles (He, Ne) breaks down if a critical particle size achieved. For the case of large particles (Kr, Xe), the motion through the water solvent is governed by mass-independent viscous friction leading to hydrodynamical diffusion. Finally, Ar falls in between the two diffusive regimes, where particle dispersion is propagated at the molecular collision time scale of the surrounding water molecules.
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Water Resources and Drinking Water, 8600 Duebendorf, Switzerland.
authors.forename: Halua
authors.identifier: http://orcid.org/0000-0003-4029-1407
authors.initials: H
authors.lastname: Pinto de Magalhães
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Water Resources and Drinking Water, 8600 Duebendorf, Switzerland.
authors.forename: Matthias S
authors.initials: MS
authors.lastname: Brennwald
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Water Resources and Drinking Water, 8600 Duebendorf, Switzerland and ETH Zurich, Institute of Biogeochemistry and Pollution Dynamics, 8092 Zurich, Switzerland and ETH Zurich, Institute of Geochemistry and Petrology, 8092 Zurich, Switzerland.
authors.forename: Rolf
authors.initials: R
authors.lastname: Kipfer
mesh_terms: D005591:Chemical Fractionation; D004058:Diffusion; D007554:Isotopes / Q000737:chemistry; D008956:Models, Chemical; D056004:Molecular Dynamics Simulation; D005741:Noble Gases / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D007554:Isotopes; D005741:Noble Gases; D014867:Water