Solute hindrance in non-porous membranes: An ATR-FTIR study

The hindered transport theory (HTT) is widely used to analyze the size-dependence of transport in membranes such as reverse osmosis and nanofiltration, however, serious concerns have been raised recently about its suitability for molecular solutes. To address this concern, the diffusivities of dye molecules (189 to 961 Da) in a crosslinked poly(vinyl alcohol) film were measured using ATR-FTIR spectroscopy. Possible errors due to liquid layers on both sides of the film were analyzed and appeared to have a minor effect on the results. The values of diffusion coefficient were found to be in the range 3.3 × 10^{− 14} to 4.4 × 10^{− 13} m²/s. Based on the pore size of 2.24 nm estimated from hydraulic permeability, these values were about 3 orders of magnitude lower than the predictions by HTT. This result strongly suggests that HTT may not be a suitable model for the transport of small molecules in membranes such as RO and NF, pointing to the need for more adequate models.