Inverse Mass Dependence in Liquid Diffusion of Heavy Ions

Abstract

AFTER the failure of early attempts to find isotope effects in liquid diffusion it was assumed¹ that these effects would be destroyed by two mutually antagonistic mechanisms. Firstly, a first approximation gas model resulted in higher speed for the lighter isotopes; secondly, however, in liquids the lighter isotopes would meet with an increased backscatter and would have less persistence than the heavier isotopes. Thus the preference of the mass independent Stokes-Einstein equation appeared justified for a first approach to liquid diffusion coefficients. The slower diffusion of the lighter alkali and alkaline earth ions was attributed wholly to the larger diameters caused by hydration and not to mass effects. The contribution of hydration will be indeed substantial for the lighter ions such as Li and Na but this influence has never been proved quantitatively especially for the heavier ions such as Sr and Ba.