Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

Molecular dynamics simulations show that the desolvation rates of isotopes of Li⁺, K⁺, Rb⁺, Ca²⁺, Sr ²⁺, 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, k_wex, for water molecules in the first hydration shell follows an inverse power-law mass dependence (k_wex ∝ m^-γ), where the coefficient γ 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 γ 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.