Negative rejection of nonionic dye in aqueous alcohol solutions during nanofiltration by hydrophobic membranes

This paper reports that nonionic dye ("Oil Red O") is negatively rejected during nanofiltration of its aqueous ethanol solution by hydrophobic membranes made of high-permeability glassy polymers. This implies that the dye concentrates in the permeate region. A probable reason for the observed negative rejection is positive adsorption of the neutral dye molecules inside the membrane pores. Comparing samples of PTMSP-and PMPbased membranes we verify the slightly modified version of the "fine porous membrane" model proposed here. Based on our own experimental data for the rejection coefficient and the normalized flux (permeability) through both polymeric membranes in dependence on alcohol concentration under different values of applied pressure, we calculate the main physico-chemical parameters of these membrane systems, i.e. the diffusion and the equilibrium distribution coefficients of "Oil Red O" molecules inside the membrane. Comparing the theoretically predicted parameters to the experiment-based rejection-flux curves, we conclude that the PTMSP-based membrane is 4-11 times more permeable for the neutral dye than the PMP-based membrane, although both membranes have very similar values of the negative rejection coefficient. The diffusion coefficient of "Oil Red O" is 1.5-8 times higher inside the PTMSP membrane compared to the PMP membrane, while the distribution coefficients for both membranes are close to each other. It might mean that the PMP membrane has a denser structure than the PTMSP membrane. Both membranes can be effectively used to purify aqueous alcohol mixtures of different dyes.