Influence of molecular size,polarity and charge on the retention of organic molecules by nanofiltration

Because of the growing interest in nanofiltration for industrial use, a better insight in the retention mechanisms in nanofiltration is needed, which will make it possible to understand membrane performances for specific applications. In this paper, the retention of a series of organic molecules by four nanofiltration membranes was studied. The membranes that were used are NF70 (Dow/FilmTec), NTR 7450 (Nitto-Denko), UTC-20 (Toray Ind.) and Zirfon^® (VITO). In order to correlate the retention with the size of the molecule, which is the main factor that determines the retention, use was made of different parameters for the molecular size: molecular weight, the Stokes diameter, the equivalent molar diameter, and a diameter obtained with energy minimisation calculations. For each size parameter, the correlation with retention in nanofiltration experiments was calculated. For the Zirfon^® membrane, retentions were too low to obtain a good correlation. For the three other membranes, a good correlation with retention was found for each of the size parameters. Two other factors were found to have an influence on retention of organic molecules: the polarity of the molecule, and the charge of the molecule. The importance of these factors depends on the molecules as well as on the type of membrane.