Effect of electro-chemical properties of chloride salts on their diffusional parameters in symmetrical cellulose acetate membranes

Abstract

A relation was obtained between electro-chemical properties of alkali, alkali earth and aluminium chlorides (LiCl, NaCl, KCl, RbCl, CsCl, MgCl 2, CaCl 2, AlCl 3) and the distribution coefficient K and the overall diffusion coefficients D in symmetrical cellulose acetate membranes. Symmetrical cellulose acetate membranes were cast to have a wider range of water content, 15–30%. K and D were measured by the unsteady- and steady-state dialysis method. The K values increased with the increase of water content and in the increasing order of CaCl 2, MgCl 2, AlCl 3, LiCl, KCl, NaCl, RbCl and CsCl, and are in the range of 10 −2 for alkali chlorides, 10 −5-10 −4 for alkali earth chlorides, and 10 −4 for AlCl 3. D was found to increase with the increase of the solute concentration, and the extrapolated values of D to zero concentration D (0) were obtained as 0.01-0.1 μm 2/s for membranes cast from a dense dope solution and 0.1–0.4 μm 2/s for thin membranes. D (0) increases with the increase of water content in the membrane and in the order of NaCl, LiCl, MgCl 2, RbCl, CaCl 2, CsCl, KCl and AlCl 3 for dense membranes and NaCl, RbCl, CsCl, KCl and LiCl for thin membranes. D can be divided into the concentration independent diffusion coefficients in the dense part ( D d ), and in the porous ( D p ), applying a two-part (perfect or dense and imperfect or porous) model of 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 as D p,c = D d exp (γ × W W ). It is shown that the averaged D d , D d increased almost linearly with the increase of the product of valence times ionic mobility u o of the solutes at infinite dilution, and that the parameter γ increased with the increase of Stokes radius of ion r Stokes. The value of K increased slightly with the increase of water content and decreased with the increase of the Flory—Huggics parameter χ, which is calculated from the measured values of distribution coefficients and data from the literature. It was also found that the gradient of linear decrease of χ (λ cation) and of linear increase of χ (λ anion) depend on equivalent ionic conductivity of anion and cation of salt, λ an and λ cat, respectively. It seems possible to predict permeability for some specific solute through a membrane from measured diffusional values such as D (0) and K for NaCl or some standard solute.