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

Abstract

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 distribution coefficients in dense CA-membranes agree roughly with calculated values. We also focus on new results for the binding of divalent cations to the glucuronic acid groups of the CA-chains, which may temporarily transform the membranes from weak cationic exchange membranes to weak anionic exchange membranes. The divalent cations may be washed out, but the rate of dissociation is very low. There seems to be two relaxations, the slower being of the order of weeks, the faster being of the order of days. The dissociation of Ba ++ was followed at 25°C and at 35°C and at different external concentrations of NaCl. The slow relaxation seems connected with the Coulomb interaction between the COO − groups and the Ba ++ ions, whereas the fast relaxation is probably reflecting dissociation from physical dipole-ion bonds.