LINEAR-APPROXIMATIONS FOR THE DESCRIPTION OF SOLUTE FLUX THROUGH PERMSELECTIVE MEMBRANES

Solute flux in the combined diffusive and convective transport through a homogeneous permselective membrane is a non-linear function of volu me flux. A set of linear approximations for this function, which are b ased on the application of the weighted mean value of boundary solute concentrations for the description of the mean intramembrane concentra tion, is reviewed. The choice of a particular approximation depends on the range of the Peclet number involved in the investigated problem. Furthermore, it is shown that the original Kedem-Katchalsky formalism, which used a logarithmic mean for the mean intramembrane solute conce ntration, is a particular case of the general non-linear formalism for zero net solute flux with non-vanishing diffusive and convective flux es. Thus, the Kedem-Katchalsky logarithmic mean concentration may be u sed as a linear approximation for the case of diffusive transport in t he opposite direction to convective transport. Linear approximations o f solute flux equations are also useful for simple phenomenological de scriptions of complex membrane systems, which otherwise would need sop histicated mathematical models.