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 was 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 increase 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 membranes from a thin one. 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 membranes from dense and NaCl, RbCl, CsCl, KCl and LiCl for membranes from thin. 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 = D̄ind exp (γ × W w ). It is shown that the averaged D d , D̄ind increases almost linearly with the increase of the product of valence times ionic mobility u° of the solutes at infinite dilution, and that the parameter γ increases with the increase of Stokes radius of ion r Stokes. The value of K increases slightly with the increase of water content and decreases with the increase of the Flory—Huggins parameter χ. The Flory—Huggins parameter χ is calculated from the measured values of distribution coefficients and data offered from literatures. It was found that the gradient of linear decrease of χ (λ cat) and of linear increase of χ (λ an) depends 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.