The mechanism of water transport in membranes

Abstract Biological membranes, lipid membranes and organic polymer membranes have some chemical similarities. All can transport water and it is likely that the molecular transport processes have some common features in the three types of system. Polymer membranes, being stable and strong, can be subjected to more varied and intensive study than can either lipid or biological membranes. Furthermore, the structures, dimensions and molecular organization of polymer membranes, which are very simple compared with their biological counterparts, can be characterized in some detail. It is already a reasonable objective to analyse transport phenomena in polymer membranes in terms of the molecular processes likely to occur in media of known structure. If the level of understanding in this area could be improved it might become possible to make some confident deductions about structure in lipid and biological membranes from observations on their transport properties. This review is intended to assess the current situation regarding the interpretation of flux data in structural terms and, perhaps, to encourage further developments. Although observations on transport processes are made by observing changes in the bulk phases in contact with the membrane, it is the processes within the membrane that are under study. The mathematical formulation of membrane transport must be designed to emphasize the role of the membrane as a phase in which irreversible processes are occurring. This requires the determination of diffusion coefficients, concentrations, activity coefficients and their profiles within the membrane. The concentration and activity of water in a membrane are studied through equilibrium absorption isotherms. These vary widely in form with the chemical structure and organization in polymer membranes. Almost nothing detailed is known about sorption by lipid membranes. Most flow processes are studied between pairs of aqueous solutions that contain solutes which may also be able to permeate the membrane. The possibility of several irreversible processes occurring simultaneously in the membrane can best be handled through the formalism of non-equilibrium thermodynamics. This formulation provides also a convenient method of introducing pressure and osmotic pressure as driving forces in addition to that of simple Fickian diffusion flow. A bridge between the phenomenological coefficients of thermodynamics and molecular processes can be built only on a model system. By regarding the membrane as intrinsically homogeneous and isotropic the frictional model can be applied in which steady flow is represented by a balance between thermodynamic driving forces and frictional retarding forces among the various flowing components and the membrane. A case of particular interest arises where the solute is an isotopically labelled species of water. The number of independent frictional coefficients in the two-component system is reduced from three to two. They can be determined from measurements of the tracer diffusion coefficient and the hydrodynamic or osmotic permeability of the membrane. This has been done for two sets of polymer membranes: highly hydrated hydrogels and moderately hydrated cellulose acetates. The membranes were prepared so as to be as nearly homogeneous as possible and it was found that the frictional coefficients observed were consistent with the homogeneous model theory. By applying a form of argument which has several times been used to diagnose the existence of pores in biological and lipid membranes, it was possible to deduce from the data on the synthetic membranes that they too transport water in, sometimes quite large, pores. This conclusion would be at variance with their known structures and shows that the argument is unsound when it is used as a criterion for the presence or absence of pores. It is clear also that the argument is valid only when applied to a homogeneous system and so would not be valid if applied to a system that were truly porous but also transported water through the matrix material supporting the pores.