TY - JOUR
T1 - Isotopic mass dependence of metal cation diffusion coefficients in liquid water
AU - Bourg, Ian Charles
AU - Richter, Frank M.
AU - Christensen, John N.
AU - Sposito, Garrison
N1 - Funding Information:
The research reported in this paper was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract Nos. DE-AC02-05CH11231 (ICB/JNC/GS) and DE-FG02-01ER15254 (ICB/FMR). This research used resources of the National Energy Research Scientific Computing Center, which is supported by the office of sciences of the U.S. Department of Energy under contract No. DE-AC02-05CH11231. This research also was supported by the National Science Foundation through TeraGrid resources provided by the San Diego Supercomputer Center. The authors are grateful to Professor R.E. Zeebe and two anonymous reviewers for helpful comments on the manuscript.
PY - 2010/4/15
Y1 - 2010/4/15
N2 - Isotope distributions in natural systems can be highly sensitive to the mass (m) dependence of solute diffusion coefficients (D) in liquid water. Isotope geochemistry studies routinely have assumed that this mass dependence either is negligible (as predicted by hydrodynamic theories) or follows a kinetic-theory-like inverse square-root relationship (D ∝ m-0.5). However, our recent experimental results and molecular dynamics (MD) simulations showed that the mass dependence of D is intermediate between hydrodynamic and kinetic theory predictions (D ∝ m-β with 0 ≤ β < 0.2 for Li+, Cl-, Mg2+, and the noble gases). In this paper, we present new MD simulations and experimental results for Na+, K+, Cs+, and Ca2+ that confirm the generality of the inverse power-law relation D ∝ m-β. Our new findings allow us to develop a general description of the influence of solute valence and radius on the mass dependence of D for monatomic solutes in liquid water. This mass dependence decreases with solute radius and with the magnitude of solute valence. Molecular-scale analysis of our MD simulation results reveals that these trends derive from the exponent β being smallest for those solutes whose motions are most strongly coupled to solvent hydrodynamic modes.
AB - Isotope distributions in natural systems can be highly sensitive to the mass (m) dependence of solute diffusion coefficients (D) in liquid water. Isotope geochemistry studies routinely have assumed that this mass dependence either is negligible (as predicted by hydrodynamic theories) or follows a kinetic-theory-like inverse square-root relationship (D ∝ m-0.5). However, our recent experimental results and molecular dynamics (MD) simulations showed that the mass dependence of D is intermediate between hydrodynamic and kinetic theory predictions (D ∝ m-β with 0 ≤ β < 0.2 for Li+, Cl-, Mg2+, and the noble gases). In this paper, we present new MD simulations and experimental results for Na+, K+, Cs+, and Ca2+ that confirm the generality of the inverse power-law relation D ∝ m-β. Our new findings allow us to develop a general description of the influence of solute valence and radius on the mass dependence of D for monatomic solutes in liquid water. This mass dependence decreases with solute radius and with the magnitude of solute valence. Molecular-scale analysis of our MD simulation results reveals that these trends derive from the exponent β being smallest for those solutes whose motions are most strongly coupled to solvent hydrodynamic modes.
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U2 - 10.1016/j.gca.2010.01.024
DO - 10.1016/j.gca.2010.01.024
M3 - Article
AN - SCOPUS:77649271966
SN - 0016-7037
VL - 74
SP - 2249
EP - 2256
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 8
ER -