Isotopic mass dependence of metal cation diffusion coefficients in liquid water

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

“…We divided each simulation into two 8-ns blocks for data analysis. The total energy varied by less than 1% during each 8-ns simulation, indicating excellent energy conservation and justifying our decision to not couple the simulation box to an external thermostat (15,17).…”

“…Each simulation was carried out for 16 ns (with a 1-fs time step) in the NVE ensemble and was preceded by 202 ps of equilibration at the desired temperature. Due to the absence of a counter ion and the application of periodic boundary conditions, these simulations approximate conditions of infinite dilution (15)(16)(17)(30)(31)(32)(33)(34)(35). We chose to model all alkali and alkaline earth metal cations that have more than one naturally occurring stable isotope (Li + , K + , Rb + , Ca 2+ , Sr 2+ , Ba 2+ ), with the exception of Mg 2+ , for which preliminary calculations showed too few water-exchange events to accurately calculate k wex via direct simulation.…”

“…Growth of calcite from seawater-like aqueous solutions is not likely to be diffusion-limited for Ca, so the isotopic effects are inferred to be due to kinetic effects occurring at the solid/fluid interface (11,13). Diffusion in liquid water can result in kinetic isotopic fractionation (14)(15)(16)(17), but in the case of Ca isotopes, the available data suggest that the largest possible light-isotope enrichment from diffusion-limited growth is ∼0.4‰ in δ 44/40 Ca, which is too small to explain the observations (17).…”

“…MD simulations are routinely used to investigate the k wex values of alkali and alkaline earth metals in liquid water (30)(31)(32)(33)(34)(35) and these simulations readily allow for the determination of the mass dependence of k wex (15,17). Here, we use MD simulations in conjunction with reaction rate theory calculations to determine the isotopic mass dependence of k wex for several common cations at three different temperatures, and we show that this dependence could account for the observed Ca isotopic effects attending calcite growth from aqueous solution.…”

Molecular dynamics simulations of the electrical double layer on smectite surfaces contacting concentrated mixed electrolyte (NaCl–CaCl2) solutions

“…In the present study, we used the extended simple point charge (SPC/E) water model [84], the Smith-Dang solute-water interaction potentials for Na + and Cl - [85], the Åqvist solute-water potential for Ca 2+ [86], and the CLAYFF model for smectite [87] (Table 1). These interatomic potentials are known to predict accurately the molecular structure [88], static dielectric constant [89], and self-diffusion coefficient [85] in bulk liquid water; the solvation structure [85], diffusion coefficients [90], and isotopic mass dependence of the diffusion coefficients [91] of ionic solutes in liquid water; NaCl ion pairing and solubility in ambient water [92]; and the structure and diffusion coefficients of water and solutes in Namontmorillonite interlayer nanopores [54]. We also verified that the interatomic potentials in Table 1 quite accurately predict the observed mass densities of NaCl/CaCl 2 brines at 298 K [93] (Fig.…”