Frictional interpretation of thermodynamic transport parameters for porous nanofiltration membranes

Phenomenological coefficients arising from the application of irreversible thermodynamics to the passage of water and dilute solutions of alcohols, sugars, NaCl and NaClO₄ across two commercially available nanofiltration membranes are physically interpreted using the frictional model proposed by Spiegler, Kedem and Katchalsky. The effects of temperature in the range 5–41°C and NaCl concentration in the range 1–50 meq l⁻¹ were also quantified. Pure water permeability, solute reflection coefficient and solute diffusive permeability are linked to solute–water, solute–membrane and water–membrane frictional interactions within the nanofilters’ polymeric network. Changes in intra-membrane frictional coefficients with feed water temperature and concentration are related to variations in nanofilter morphological and charge characteristics. As may be expected, water–membrane friction coefficients were several orders of magnitude smaller than solute–membrane friction coefficients demonstrating that the semi-permeable nanofilters hindered solute passage to a substantially greater degree than water. Analogous to viscosity, all frictional coefficients decreased with temperature. Greater steric hindrances faced by larger solutes to passage across nanofilters are manifested as increasing activation energies of solute–membrane interactions and solute–water interactions. Hydrodynamic theories of hindered transport are shown to closely follow trends in frictional coefficients with increasing solute size. Antagonistic effects of changes in electrostatic and steric interactions with temperature reduced activation energies of electrolyte–membrane frictional interactions.