Temperature effects on transport of water, charged, and uncharged solutes across polymeric thin film composite nanofiltration membranes: An investigation into pore-transport mechanisms and electrokinetic properties

The overall goal of this dissertation is to rigorously and systematically quantify temperature effects on the morphology and electrokinetic properties of thin film composite nanofiltration membranes in order to obtain insights on their permselectivity.; Specifically, crossflow filtration experiments were performed to measure transport of water, hydrophilic neutral organic solutes spanning a range of molecular sizes, and a variety of electrolytes across two commercial nanofiltration membranes in the temperature range 5--41°C. Non-viscous contributions to activation energies of pure water permeation across these polymeric membranes were calculated to be 3.9 and 6.4 kJ/mol. Analysis of solute rejection using a phenomenological transport model revealed that the sizes of pores in the active layer of the thin film composite membranes followed a lognormal distribution at any given temperature. Consistent with the definition of a nanofiltration membrane, the vast majority of pores (>99%) were smaller than 2 nm for both membranes. However, evidence for a small fraction of larger aggregate pores (∼16 nm) was also obtained for one membrane. Additionally, increasing temperature increased mean pore radii while simultaneously reducing the pore number density suggesting changes in the structure and morphology of the polymer matrix comprising the membrane active layer. Consistent with the free volume theory of activated gas transport, activation energies of neutral solute permeability increased 10 fold from (∼5--55 kJ/mol) when the Stokes radius approximately doubled from 0.2 nm (ethanol) to 0.37 nm (dextrose) indicating their hindered diffusion.; Next, irreversible thermodynamics, extended Nernst Planck formulation and theory of rate processes were employed to interpret rejections of three 1-1, two 2-1 and one 2-2 electrolyte. Analysis of electrolyte rejection using irreversible thermodynamics revealed increasing trends for reflection coefficient and permeability with temperature. Further, these trends were quantitatively related to changes in electromigrative and convective contributions using the extended Nernst Planck formulation. Activation energies of electrolyte permeation were found to be primarily determined by the Donnan potential at the interface i.e. for low potential, permeation of electrolytes were weak functions of temperature.; These results provide the first rigorous and quantitative insights on the influence of feed water temperature (seasonal effects) on membrane selectivity.