Transport through a slab membrane governed by a concentration-dependent diffusion coefficient - Part IV. D(C)/D-0=1+(alpha C)-beta(alpha C)(2)

Using the specific functional form D(C)/D-0 = 1+(alphaC)-beta(alphaC)(2) an investigation has been made of (isothermal) transport through a slab membr ane under 'simple' boundary conditions and governed by a diffusion coeffici ent, D(C), which, with increasing concentration, at first increases, passes through a maximum value and finally decreases. The flux, integral diffusio n coefficient and concentration profile characteristic of steady-state perm eation have been evaluated; special attention has been paid to the position s of such profiles in relation to the corresponding linear distribution ass ociated with a constant diffusion coefficient. The corresponding transient-state transport has been studied within a frame work of the time-lag 'early-time' and 'roott' procedures. Expressions for t he 'adsorption' and 'desorption' time-lags are given. The concentration-dep endence of these time-lags, of the (four) integral diffusion coefficients d erived from them and of the arithmetic-mean time-lag ratios have been consi dered in some detail. The 'early-time' and 'roott' finite-difference proced ures have likewise been employed to derive four further integral diffusion coefficients, so enabling a comparison to be made of the nine integral coef ficients pertaining to established experimental techniques. Particular interest attaches to the situation for which n =beta(alphaC(0)) = 1 (where C-0 is the ingoing or upstream concentration of diffusant) resul ting in D(C-0) being symmetrical about C-0/2. Some consideration has been g iven, in general, to features of transient-state transport when governed by a symmetrical D(C). (C) 2001 Elsevier Science B.V. All rights reserved.