Permeability of solutes through hydrated polymer membranes. Part III. Theoretical background for the selectivity of dialysis membranes

The diffusional transport of aqueous solutes through water swollen polymer membranes is examined by the concept of the free volume of the water‐polymer systems, V f,13 . The relative diffusivity (D in membrane/D in water) can be expressed by the product of the probability factor of finding holes of equal or larger cross section, q, than the cross section of solute, q2, (sieve mechanism) and the free volume factor. An analysis of experimental data indicates that within the range of solute size investigated (up to albumin) the sieve mechanism is not yet playing a role in homogeneous cellulosic membranes. The main reduction of permeability of larger solutes is caused by the free volume term which is an exponential function of the negative cross section of the solute, q2, exp (–Bq 2 /V f,13 ). In porous membranes, however, the sieve mechanism alone is responsible for the selectivity.