MEMBRANE-TRANSPORT PROPERTIES IN RELATION TO MICROSCOPIC AND MACROSCOPIC STRUCTURAL INHOMOGENEITY

Attention is drawn to the importance of studying the effect of membrane structural inhomogeneity on transport properties, not only for the purpose of exploiting such effects in membrane design but also for a proper understanding of the transport behavior of many supposedly homogeneous types of membranes. In fact, many such membranes may exhibit (a) microstructural inhomogeneity on a scale larger than can be tolerated by the relevant homogeneous membrane model, and (b) structural inhomogeneity on a macroscopic scale generated by the membrane fabrication process. The recognition of effect (a) in practice is model-dependent, and is therefore open to misinterpretation. The ionic transport properties of charged membranes illustrate this point very well. In this case, the homogeneous membrane model is conventionally supported by the invocation of counter-ion binding by the charged groups of the membrane. This paper presents evidence against this theory and then proceeds to show, with the aid of some new data, how microstructural inhomogeneity modelled in simple ways can account more plausibly and more generally for the observed behavior. Effect (b) is also easy to miss in practice; but, as shown here, it may be (i) detected in a model-independent manner by supplementing the usual steady-state by appropriate transient-state transport measurements and (ii) further characterized with the aid of only simple membrane models. © 1990.