Despite their great importance in low-temperature geochemistry, self-diffusion coefficients of noble gas isotopes in liquid water (D) have been measured only for the major isotopes of helium, neon, krypton and xenon. Data on the self-diffusion coefficients of minor noble gas isotopes are essentially non-existent and so typically are estimated by a kinetic theory model in which D varies as the inverse square root of the isotopic mass (m): D proportional to m-0.5. To examine the validity of the kinetic theory model, we performed molecular dynamics (MD) simulations of the diffusion of noble gases in ambient liquid water with an accurate set of noble gas-water interaction potentials. Our simulation results agree with available experimental data on the solvation structure and self-diffusion coefficients of the major noble gas isotopes in liquid water and reveal for the first time that the isotopic mass-dependence of all noble gas self-diffusion coefficients has the power-law form D proportional to m-beta with 0 < beta < 0.2. Thus our results call into serious question the widespread assumption that the square root model can be applied to estimate the kinetic fractionation of noble gas isotopes caused by diffusion in ambient liquid water.

Noble gas isotopes are used extensively as tracers in hydrologic and paleoclimatic studies. These applications require knowledge of the isotopic mass (m) dependence of noble gas diffusion coefficients in water (D), which has not been measured but is estimated using experimental D-values for the major isotopes along with an untested relationship from kinetic theory, D prop m-0.5. We applied molecular dynamics methods to determine the mass dependence of D for four noble gases at 298 K, finding that D prop m-beta with beta < 0.2, which refutes the kinetic theory model underlying all current applications.