Self-diffusion in liquid ammonia and deuteroammonia

Coefficients of self-diffusion have been measured by the spin-echo technique for neat NH3 and ND3 from 200 to 298°K. The data are well represented by D=(4.5± 0.2)× 10−3exp [−(2.07± 0.02)/RT] for NH3 and by D=(6.0± 0.3)× 10−3exp [−(2.31± 0.02)/RT] for ND3. The difference in activation energy for self-diffusion between NH3 and ND3 is shown to be consistent with the viscosity measurements of Hutchison and O'Reilly. These results are interpreted by means of a quasilattice model for self-diffusion in the liquid state. Values of the work required to create a molecular vacancy are evaluated from liquid density, isothermal compressibility and the scaled particle theory. The quasilattice model yields pre-exponential factors for self-diffusion that are in reasonable agreement with experiment.