A realistic approach for determining the pore size distribution of nanofiltration membranes

High flux nanofiltration (NF) membranes with tunable molecular weight cut-off (MWCO) are desired for energy-efficient separation processes in many industries. The separation mechanism of these membranes involves size sieving and charge-based Donnan exclusion. The precise control over the pore size of NF membranes would produce better solute–solute selectivity. Thus it is essential to know the pore size and its distribution to ideally custom-design them. In this article, the advantages and limitations of various mathematical models of pore size estimation are discussed. The probability density function (PDF) model, which is a simple and efficient model for estimating pore size distribution (PSD) based on the rejection of electroneutral solutes, is discussed in detail, along with the flaws in its usage. The model has been randomly used in the past, which makes it difficult to compare the results obtained in different studies. We have introduced some critical points to be contemplated for the standardization of the model.