Heiner Strathmann

Current‐voltage curves of bipolar membranes

Bipolar membranes consist of a layered ion‐exchange structure composed of a cation selective membrane joined to an anion selective membrane. They are analogous to semiconductor p‐n devices as both of them present current‐voltage curves exhibiting similar rectification properties. In this article, we present some current‐voltage curves obtained for different bipolar membranes at several temperatures. The results can be interpreted in terms of a simple model for ion transport and field‐enhanced water dissociation previously developed. The mechanism responsible for water splitting is assumed to be a catalytic proton transfer reaction between the charged groups and the water at the membrane interface. The effects of temperature are taken into account by introducing an Arrhenius‐type relationship for the dependence of the forward rate constant of the reaction on temperature. Finally, comparison between theory and experiments provides reasonable values for the parameters introduced in the theoretical model. The...

Development of bipolar membranes

Abstract The large-scale industrial use of bipolar membrane for the production of acids and bases from the corresponding salts is limited by the properties of the commercially available bipolar membranes. Todays membranes are unsatisfactory in terms of their selectivity and chemical stability at high pH values. In this paper the preparation of bipolar membranes is described. They are characterized in terms of their chemical stability, their selectivity and their electrical resistance. It is shown that membranes with low electrical resistance and satisfactory stability in alkaline solutions at pH values in excess of 13 can be manufactured.

Free energy calculations of small molecules in dense amorphous polymers. Effect of the initial guess configuration in molecular dynamics studies

The excess free energy of small molecules in the amorphous polymers poly(ethylene) and poly(dimethylsiloxane) was calculated, using the test-particle-insertion method. The method was applied to polymer configurations obtained from molecular dynamics simulations with differently prepared initial guess configurations. It was found that the calculated solubility coefficients strongly depend on the quality of the initial guess configuration. Slow compression of dilute systems, during which process only the repulsive parts of the nonbonded Lennard-Jones potentials are taken into account, yields polymer melts which are better relaxed, and which offer lower solubilities for guest molecules compared with polymer melts generated at the experimental density or prepared by compressing boxes with soft-core nonbonded potentials. For the last two methods initial stresses relax by straining the internal modes (bond angles, torsion angles) of the chains

Comparison of bipolar membranes by means of chronopotentiometry

Chronopotentiometry is used as a tool to investigate the transport processes in and the energy requirements of different bipolar membranes under water splitting conditions. The bipolar membranes studied are the BP-1 from Tokuyama Corporation, Japan, the MB-3, the AQ-6 from Aqualytics, and a modified sample of the WSI bipolar membrane. In accordance with the predictions from phenomenological transport equations, the switch-on transition is faster for highly selective membranes, however, also the interface structure (roughness, effective contact area) has to be considered to interpret the results unambiguously. In general, the switch-off behaviour shows a faster relaxation for the less selective membranes. Some bipolar membranes show a remarkable phenomenon: a second charging and discharging time is observed, indicating asymmetric transport behaviour of the membrane layers. For bipolar membranes with firmly attached layers, the chronopotentiometric curves allow to separate the theoretically necessary electrical energy (the reversible contribution) from the energy lost by dissipation (the irreversible contribution). The reversible part is recorded at current switch-off after steady state operation. The ratio of the two contributions indicates that in the series of MB-3, AQ-6 and BP-1 the membranes show an increasing energy efficiency. This is also the order of increasing selectivity, as indicated by the limiting current density. For the WSI membrane, the transport processes and also the chronopotentiometric response curves are different because the layers are not permanently attached. Our results indicate that chronopotentiometric series can be used for detailed comparison of bipolar membranes.

Membrane processes for sustainable industrial growth

This article offers a general survey of membrane technology and related processes by briefly looking at market sizes, industry structure, and recent and potential future developments.

Characterization of hemodialysis membranes by inverse size exclusion chromatography

Inverse size exclusion chromatography (i-SEC) was used to characterize three different cellulosic hollow fiber hemodialysis membranes, i.e. low-flux cuprophan and hemophan and high-flux RC-HP400A. With the i-SEC technique the pore size distribution and porosity of a membrane can be determined and adsorption phenomena can be studied. The membranes showed clear differences in pore size and porosity, the high-flux RC-HP400A membrane has a larger pore size as well as a higher porosity. For all the membranes it was found that the elution curves were best described by a homoporous pore volume distribution. It appeared that the bound or non-freezing water in the membranes was at least partly accessible to solutes. The test molecules creatinine and vitamin B 12 both adsorbed to the cellulosic membranes. The adsorption behavior of creatinine was strongly dependent on the NaCl concentration present. The observations could be explained by assuming that cuprophan and RC-HP400A are negatively charged whereas hemophan is positively charged due to the modification with N,N-diethylaminoethyl ether. The net charge of the hemophan is smaller.