DISTRIBUTION OF ELECTROLYTES BETWEEN MEMBRANOUS AND BULK PHASES, AND THE DIELECTRIC-PROPERTIES OF MEMBRANOUS WATER, STUDIED BY IMPEDANCE SPECTROSCOPY MEASUREMENTS ON DENSE CELLULOSE-ACETATE MEMBRANES
Iw. Plesner et al., DISTRIBUTION OF ELECTROLYTES BETWEEN MEMBRANOUS AND BULK PHASES, AND THE DIELECTRIC-PROPERTIES OF MEMBRANOUS WATER, STUDIED BY IMPEDANCE SPECTROSCOPY MEASUREMENTS ON DENSE CELLULOSE-ACETATE MEMBRANES, Journal of the Chemical Society. Faraday transactions, 90(16), 1994, pp. 2381-2393
Citations number
22
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
The dielectric properties of dense cellulose acetate (CA) membranes ha
ve been determined using impedance spectroscopy. The impedance of the
membranes in equilibrium with external salt solutions (10(-4)-1 mol dm
-3) and at varying pH as a function of the frequency of the applied fi
eld was measured. From the data, and using classical Donnan theory tog
ether with the Nernst-Planck electrodiffusion equations and known diel
ectric 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 coeffic
ients. 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 KCl. (2) The concentrati
on ratios of the cations and the anions differ very much from unity at
low (10(-4) mol dm-3) external salt concentration. The cation ratios
attain values of several hundred, decreasing rapidly as the external c
oncentration 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 +/- 1.0 nm. Our results are at variance wi
th measurements on similar membranes reported a few years ago by Wiggi
ns and van Ryn, J. Macromol. Sci., Part A, Chem., 1986, 23, 875, who f
ound considerable qualitative differences in the behaviour of the two
alkali-metal salts. On the other hand, the low relative permittivity e
stimated for the water in the membrane supports the idea of anomalous
water in the membrane alveoles, however, this seems to be caused by th
e properties and size of the alveoles, rather than by the presence of
a particular cation.