Distribution of electrolytes between membraneous and bulk phases, and the dielectric properties of membraneous water, studied by impedance spectroscopy measurements on dense cellulose acetate membranes

Abstract

The dielectric properties of dense cellulose acetate (CA) membranes have been determined using impedance spectroscopy. The impedance of the membranes in equilibrium with external salt solutions (10-4-1 mot dm-3) and at varying pH as a function of the frequency of the applied field was measured. From the data, and using classical Donnan theory together with the Nernst-Planck electrodiffusion equations and known dielectric theory, it is possible to find the concentration ratios of the ions between the membranes and the surrounding solution as a function of the external salt concentration, as well as their diffusion coefficients. In addition, the relative permittivities of the membrane matrix and of the solution present in the membrane alveoles, as well as the size of the latter, can be estimated. The results show (1) that there is no qualitative difference between NaCl and KCI.(2) The concentration ratios of the cations and the anions differ very much from unity at low mol dm-3) external salt concentration. The cation ratios attain values of several hundred, decreasing rapidly as the external concentration increases. ( 3) The (relative) permittivity of the solvent inside the membrane is ca. 20, indicating either that the rotational degrees of freedom of the water in the alveoles are, to a considerable extent, hindered compared with normal water, or that the temporary H-bond clusters in pore water are less extended than in bulk water, with a resulting lower effecitve dipole moment; and (4) the average radius of the alveoles is ca. 6.5 f 1.0 nm. Our results are at variance with measurements on similar membranes reported a few years ago by Wiggins and van Ryn, J. Macrornol. Sci., Part A , Chern., 1986, 23, 875, who found considerable qualitative differences in the behaviour of the two alkali-metal salts. On the other hand, the low relative permittivity estimated for the water in the membrane supports the idea of anomalous water in the membrane alveoles, however, this seems to be caused by the properties and size of the alveoles, rather than by the presence of a particular cat ion.

Electroosmosis through a Cation-Exchange Membrane: Effect of an ac Perturbation on the Electroosmotic Flow

“…However, in the past years, the use of time dependence electric fields have been shown useful for studying certain membrane aspects. ac impedance spectroscopy has been used in the study of single ion-exchange membranes as well as in the case of bipolar membranes (7)(8)(9)(10)(11)(12). In fact, consideration of the membrane-or heterogeneous media in general-from a point of view of their dielectric properties has been used from some time ago.…”

Theoretical and Experimental Study of the Voltage–Current Characteristic of an Asymmetric Cellulose Acetate Membrane

“…Here, the electroneutrality conditions Eqs. [4] and [5] hold as a good approximation, and, as will be discussed (Eq.…”

“…These interfacial zones are electrical double layers in which the electroneutrality condition is not satisfied. The positions Ϫ␦ 1 and ␦ 2 indicate the outer boundaries of these so-called space-charge regions where the electroneutrality condition may assumed to be valid again, i.e., [5] in which c is the external concentration of the salt MX, and c ␦ the electrolyte concentration in the support layer just outside the space-charge region near the selective/nonselective layer interface. The equalities in Eq.…”

“…To that purpose a suitable reference position ( x r ) is taken where it may be expected that the electroneutrality conditions [4] and [5] hold as a good approximation. The electric field at x ϭ x r is obtained from Eq.…”

“…The selective layer has weak cation exchange properties. Furthermore, the dielectric constant of water within this dense layer is significantly smaller than that in free solution (5), so that ions are dielectrically excluded from the selective layer. According to the theory of Kedem and Katchalsky (6), voltage-current behavior of the above asymmetric bilayer membrane is also still influenced by concentration polarization at the selective/nonselective layer interface, which, in turn, is a consequence of the different ion transport numbers of the two layers.…”