Electromotive force and impedance studies of cellulose acetate membranes: Evidence for two binding sites for divalent cations and for an alveolar structure of the skin layer

• Published in: Desalination 1991, Vol.80 (2), p.293-327
• Main Authors: Sørensen, Torben Smith, Jensen, Jørgen Birger, Malmgren-Hansen, Bjørn
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal gels. Colloidal sols; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry
• ISSN: 0011-9164; 1873-4464
• DOI: 10.1016/0011-9164(91)85164-P

The electromotive force (EMF) has been measured for a great number of concentration cells of the type: Ag | AgCl ⋎ ⪢variable⪡ solution | Cellulose Acetate Membrane | ⪢fixed⪡ solution | AgCl | Ag. The solutions were aqueous solutions of mixed electrolytes at atmospheric pressure and mostly at 25°C (in some few cases 35°C ). The ions considered were the cations H +, Li +, Na +, K +, Mg ++, Ca ++, Ba ++ and the anions Cl − and F − (Cl − was always present). The ⪢fixed⪡ solution was the same in each series of experiments, but may be varied from one series to the next. In the ⪢variable⪡ solution, at least one electrolyte concentration was varied during the series of EMF measurements. The variable electrolyte concentration was varied over almost 4 decades (from 10 −4 M to 0.6−1.0 M). The 2.5-Cellulose Acetate (CA) membranes were mostly dense membranes cast by ourselves. A few were asymmetic membranes. The skin layer in asymmetric membranes is assumed to have properties similar to dense membranes. The EMF measurements were interpreted by means of a Donnan-Nernst-Planck (Toorell-Meyer-Sievers) model, which functions quite well due to the low fixed charge in the membrane. The membrane diffusion potential is calculated by the Henderson method and in some cases by solving transcendental equations according to Planck, Pleijel and Schlögl. There is no great difference between the membrane potentials calculated by the two methods, but the ion profiles and the actual rates of electrodiffusion may be found by the latter method. Earlier results are recapitulated, especially the evidence for an alveolar structure found by interpreting the membrane capacitance increase with salt concentration - found by means of impedance measurements - in the light of a combined Trukhan-Brüggemann theory. Alveoles in dense membranes or in the skin layer of asymmetric membranes seem to have a mean radius corresponding to ca. 70 Å. The dielectric constant in the alveoles is ca. 30 in contrast to a dielectric constant of ca. 16 in the ⪢lamellar phase⪡ inbetween the alveoles. The dielectric constant of ca. 30 in the alveolar phase is also supported by a simple dielectric calculation of the Nernst distribution of mono- and divalent ions between external water and the alveolar solution. Corrections for activity coefficients only seems important above 0.5 M. The Onsager-Samaras dielectric repulsion from the lamellar wall is of some significance only at low salt concentrations. Measured Nernst dist