The two-phase model of structure microheterogeneity revisited by the study of the CMS cation exchange membrane

The chronopotentiometric study of the CMS membrane in contact with 0.05 M NaCl shows a chronopotentiometric response at over-limiting currents typical of homogeneous membranes. Based on the two-phase model, the fraction of the interstitial phase in presence of NaCl was determined from the conductivity of the CMS membrane conditioned in NaCl solution; the interstitial volume fraction value was 13%. In addition, we have determined the sorbed amount of NaCl in the CMS membrane in contact with NaCl solution at different concentrations by the many-stage desorption method. The NaCl sorbed molality in the overall membrane varied linearly as a function of the molality of the NaCl external solution. It is noted that the majority of NaCl sorbed remains in the interstitial phase. The equilibrium partition between the external solution and the interstitial solution was estimated to be 0.89. It is shown in this work that the consideration of an equilibrium partition of the electrolyte between the external solution and the interstitial phase strengthens the two phase model of Zabolotsky and Nikonenko.

As seen in Fig. 4, the NaCl sorbed molality in the overall membrane changes linearly as a function of the external NaCl molality with the straight-line slope being equal to 0.47. Otherwise, our results showed that the majority of NaCl sorbed remains in the interstitial phase. One can deduce that the NaCl sorbed molality in the interstitial phase is a linear relationship with the external NaCl molality. To determine the equilibrium partition of the electrolyte between the external solution and the interstitial phase, the water fraction sorbed in the interstitial phase must be known. Actually, it can be estimated by following relation: By means of chronopotentiometric and current Á voltage characteristic studies, we have shown that the CMS membrane acts as a typically homogeneous membrane with respect to the transport of NaCl even the presence of modification polyelectrolyte layer at the surface. On the microstructural scale, the CMS membrane contains the gel phase and the interstitial phase according to the two-phase model proposed by Zabolostky and Nikonenko. The distribution of NaCl sorbed in each phase in the membrane was discussed in detail. Majority of NaCl sorbed remains in the interstitial phase. The determination of the amount of sorbed NaCl in the interstitial phase confirms the existence of partition equilibrium between the interstitial phase and the external solution that can be ¯ ¯ simply expressed by m ¼ K ms. Such partition equilibrium can be considered within the microheterogeneous model in order to represent the internal structure of the electromembranes.