文章基本信息

标题：Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems
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作者：Amy E. Hofmann ; Ian C. Bourg ; Donald J. DePaolo 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2012
卷号：109
期号：46
页码：18689-18694
DOI：10.1073/pnas.1208184109
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Molecular dynamics simulations show that the desolvation rates of isotopes of Li+, K+, Rb+, Ca2+, Sr2+, and Ba2+ may have a relatively strong dependence on the metal cation mass. This inference is based on the observation that the exchange rate constant, kwex, for water molecules in the first hydration shell follows an inverse power-law mass dependence (kwex {propto} m-{gamma}), where the coefficient {gamma} is 0.05 {+/-} 0.01 on average for all cations studied. Simulated water-exchange rates increase with temperature and decrease with increasing isotopic mass for each element. The magnitude of the water-exchange rate is different for simulations run using different water models [i.e., extended simple point charge (SPC/E) vs. four-site transferrable intermolecular potential (TIP4P)]; however, the value of the mass exponent {gamma} is the same. Reaction rate theory calculations predict mass exponents consistent with those determined via molecular dynamics simulations. The simulation-derived mass dependences imply that solids precipitating from aqueous solution under kinetically controlled conditions should be enriched in the light isotopes of the metal cations relative to the solutions, consistent with measured isotopic signatures in natural materials and laboratory experiments. Desolvation effects are large enough that they may be a primary determinant of the observed isotopic fractionation during precipitation.
关键词：aqueous geochemistry ; kinetic isotope effect ; ligand exchange