Appraisal of equations for neutral solute flux across porous sieving membranes.

General transport equations, based on irreversible thermodynamics applied to membranes without regard to their structures, are compared with results based upon specific membrane models. It is pointed out that the range of validity of the general linear transport equations of irreversible thermodynamics may be extremely small, and that attempts to extend the range by thermodynamic considerations have always involved subtle assumptions of a nonthermodynamic nature. Simple membrane models are used as diagnostic tools to pinpoint such assumptions, with particular reference to the often-concealed assumption of membrane homoporosity. It is shown that it is not possible to write an exact equation for solute flux across an inert porous membrane only in terms of the three customary membrane parameters sigma (reflection coefficient), Ps (permeability coefficient), and Lp (hydraulic conductivity), unless the membrane is strictly homoporous. Even in the linear range heteroporosity imposes a hidden condition of delta p greater than delta pi for such a three-parameter description to be valid. Useful results in the nonlinear regime require more detailed information on membrane structure than is contained in just three parameters.