On the effect of the porosities of microporous membranes on their ionic selectivities☆
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Abstract:

AbstractIn order to help clarify the correlation between the structure and the transport parameters in the microporous membranes mentioned in this paper, the effect of porosity upon ionic selectivity is examined for Nucleopore®, Millipore® and Pall® membranes. These membranes separate two NaCl solutions of concentrations c1 (constant for all the experiment,) and c2 (fixed for each experiment, c2 >c1). The concentrations are small enough to take the place of the activities in the transport equations. The temperature is 298.0 ± 0.1 K and the stirring speed used to homogenize the solutions is 50 rpm.The ionic selectivity is characterized, in the steady state, by an experimental parameter K, which is obtained from measurements of the membrane potential as a function of the relative saline concentration nc = c2/c1 (with c1 being constant). This parameter K makes it possible to obtain the relative ionic permeability np = P-/P+ and the transport numbers. The relation between K, np and t+ or t- is obtained by integrating the Nernst-Planck equations and making use of the Goldman hypothesis of constant electric field. The porosity θ, expressed in percents of the empty volume over the total volume of the membrane, is measured by means of a pycnometric method which is explained in this paper. From the values of K and θ, obtained for the membranes and experimental conditions of these experiments, it can be concluded that ionic selectivity increases linearly with porosity.

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