DIFFUSION OF HYDROGEN AND DEUTERIUM ATOMS IN WATER

Diffusion coefficients for hydrogen and deuterium atoms in aqueous ele ctrolyte solutions are measured using the laser photoemission from met al into solution. Corrected for the solution ionic strength, the coeff icients found are as follows: D-H = (7.7+/-1.0) x 10(-5) cm(2) s(-1) a nd D-D = (3.4+/-0.6) x 10(9) cm(2) s(-1) at 25 degrees C in H2O and D2 O, respectively. The activation energy is equal to 12+/-1.5 kT mol(-1) . On the basis of comparing these diffusion coefficients with those fo r muonium, H-2, and the noble gases, a conclusion is drawn that, simil ar to the helium atom, the hydrogen and deuterium atoms generate aroun d themselves clathrate structures of type I (the channel diameter of a bout 0.54 nm), whereas atoms and molecules with the larger van der Waa ls radius are localized in cavities of type II, with larger diameters. The neon atom and hydrogen molecule occupy an intermediate position. The anomalously high values of the diffusion coefficients (greater tha n or equal to 7 cm(2) s(-1)), which are typical for light impurities, are attributable to jumps between neighboring cavities of type I, caus ed by thermal fluctuations. The considerable isotopic effect, D-H(H2O) /D-D(D2O), is assigned to the participation of isotope-sensitive libra tion motion of water in the thermoactivated reorganization of the cavi ty.