THE USE OF MOMENT THEORY TO INTERPRET DIFFUSION AND SIEVING MEASUREMENTS IN TERMS OF THE PORE-SIZE DISTRIBUTION IN HETEROPOROUS MEMBRANES

Moment theory has been applied to model porous membranes to show that one can place reasonable bounds on the cumulative pore size distributi on, the hindered diffusivity or the reflection coefficient of large so lutes in a heteroporous membrane by measuring the diffusive permeabili ty to a small solute, the hydraulic permeability and one or two additi onal transport characteristics. These additional measurements involve either the flux of a small solute at Pe similar to 1, the hindered dif fusivity of a large solute or the reflection coefficient of a large so lute at Pe>>1. Membrane heteroporosity is incorporated in the predicte d bounds without requiring one to make any a priori assumptions about the nature of the pore size distribution. in this paper, the results f rom calculations performed with different model membranes containing l og-normal pore size distributions are reported. A comparison of the re sults obtained with three different membranes shows that one can disti nguish between membranes with the same average pore size but different pore size distributions by measuring either the hindered diffusion co efficient or the reflection coefficient of two different sized solutes . A comparison of the bounds on D and the bounds on sigma predicted fr om different types of transport measurements shows that, under certain conditions, one can place tighter bounds on one transport characteris tic by measuring a different one.