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

Unsteady-state dialysis and steady-state reverse osmosis (RO) experiments f or aqueous NaCl solutions were carried out with cellulose acetate (CA) symm etrical membranes. The distribution and overall diffusion coefficients of N aCl, K and D were obtained from the unsteady-state dialysis experimental da ta, taking the effect of osmosis into consideration. D was found to increas e with the increase of the solute concentration. The concentration dependen cy of D can be explained by applying: a two-part (perfect or dense and impe rfect or porous) model of the membrane, The concentration independent diffu sion coefficients of NaCl in the dense part D-d and in the porous D-p are o btained from the assumption of the two-part model for the membrane. D-d doe s not depend on the water content in the membrane W-w, but on dope composit ion. Contrary to D-d D-p increases with the increase of W-w and can be corr elated only with W-w and D-p(W-w = 0) is found almost equal to the averaged (D) over bar(d) for each type of membrane: D-pc = (D) over bar(d) exp (gam ma x 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 zer o concentration can be calculated as follows: D-c((0)) approximate to W-w D -pc + (1 - W-w) (D) over bar(d). Solute permeability w obtained by RO exper iments agrees well with w(c) [K . D-c((0))]/(l . beta); those calculated fr om D-c((0)) and distribution coefficient K.