Diffusional characteristics of sodium chloride in symmetrical cellulose acetate membranes measured by an unsteady-state dialysis method

Abstract

Unsteady-state dialysis and steady-state reverse osmosis (RO) experiments for aqueous NaCl solutions were carried out with cellulose acetate (CA) symmetrical membranes. The distribution and overall diffusion coefficients of NaCl, K and D were obtained from the unsteady-state dialysis experimental data, taking the effect of osmosis into consideration. D was found to increase with the increase of the solute concentration. The concentration dependency of D can be explained by applying a two-part (perfect or dense and imperfect or porous) model of the membrane. The concentration independent diffusion coefficients of NaCl in the dense part D d and in the porous D p are obtained from the assumption of the two-part model for the membrane. D d does not depend on the water content in the membrane W w , but on dope composition. Contrary to D d , D p increases with the increase of W w and can be correlated only with W w and D p ( W w =0) is found almost equal to the averaged D d for each type of membrane: D p,c = D d exp ( γ × W w ). It seems from these facts that the structure of membrane becomes tight with the decrease of the ratio of formamide to CA and loose with the decrease the concentration of CA in dope solutions. D c(c = 0) at the zero concentration can be calculated as follows: D c (0) ≈ W w D p,c + (1−W w) D d . Solute permeability ω obtained by RO experiments agrees well with ω c = [K · D c (0)] (ℓ · β) , those calculated from D c (0) and distribution coefficient K.