Sorption kinetics and time-lag theory. Part 1.—Constant diffusion coefficient

Abstract

The time-lag concept has been applied to sorption kinetics so as to provide an alternative procedure for obtaining diffusion coefficients D, in systems where D is a constant. From the graph of fractional approach to equilibrium, η(t), [0 ⩽η(t) < 1] against t^ν(ν > 0), the quantity I [graphic omitted] η(t) d(t^ν)(obtained as the area beneath the curve) may be found as a function of t^ν. For large t the curve of I against t^ν approaches a linear asymptote which intersects the axis of t^ν to give a time-lag, L_ν, where L_ν= [graphic omitted] [1 –η(t)]d(t^ν).

Explicit equations for L_ν have been obtained and investigated for various geometries of the diffusion media. Those for ν=½ and ν= 1 in particular often lead to simple relations from which D can be evaluated. Both constant pressure and constant volume sorption kinetics have been considered, in the latter instance when Henry's law governs the sorption isotherm.