Liquid Ion‐Exchange Membranes (LIEMs) in Electrodialysis Processes

Abstract

In a series of preliminary experiments it was shown that LlEMs can be substituted for solid membranes in the conventional electrodialysis process. A specific mass transfer was demonstrated, causing either an increase or a decrease in the concentration of aqueous salt solutions, depending on the experimental set-up. The liquid membranes tested were shown to be iori-permselecnve.Research in the field of liquid ion-exchanging membranes started as early as the beginning of the century [I], but it was not until 1953 that Bonhoeffer [2] showed the similarity between solid and liquid ion exchanging membranes. Systems in which an organic phase containing a liquid ion exchanger was in contact with aqueous solutions were studied mainly for the purpose of extracting certain ionic species into the organic phase [3] but also for measuring bi-ionic potentials [1], [4], [7], [9]. In 1964, Sollner [4] was able to show that a xylene solution of a commercial liquid ion exchanger could act as an ion-permselective membrane when separating two aqueous solutions of different constitutions.Tha aim of the present work is to study the properties of LIEMs, in the hope of shedding light upon the process taking place during electrodialysis in water desalination. Exchangers and solvents were therefore chosen so as to produce LIEMs approximating the functioning of solid ion exchange membranes, particular regard being paid to water impermeability, electrical conductance and ion permselectivity. Thus, the cationic exchanger tested (sodium-dinonylnaphthalenesulfonate, Na-DNNS), has negligible water solubility and is highly soluble in organic solvents. The anionic exchanger (trilaurylamine hydrochloride, TLA. HCI) has similar properties and, furthermore, can easily be purified. Nitrobenzene was chosen as solvent because of its high dielectric constant, which permitted the preparation of LIEMs with fair electrical conductivities, combined with a relatively low water solubility. Other workers had used TLA. HCI [5], Na-DNNS [6], or even a solution of Na-DNNS in nitrobenzene [7] for similar reasons. For the transfer experiments, an electrodialysis cell was built according to the following general scheme: