Permeation Characteristics of Electrolytes and Neutral Solutes through Titania Nanofiltration Membranes at High Temperatures

Nanoporous titania membranes with controlled pore sizes ranging from 0.7 to 2.5 nm, which had molecular weight cutoffs (MWCO) ranging from 500 to 2000, were successfully prepared by sol−gel processing, and the transport characteristics were evaluated across a temperature range of 30−80 °C. With increasing temperature, the permeate flux increased 2- to 3-fold, depending on the pore size. The water permeation mechanism was found to be different from viscous flow and was explained by the state of the water (free water/bound water/nonfreezing water) inside confined pores. The rejection of neutral solutes such as raffinose, the separation mechanism of which is molecular sieving (steric hindrance), decreased with temperature whereas that of electrolytes (MgCl2 and NaCl), the separation mechanism of which is the charge effect (Donnan exclusion), was approximately constant. The temperature dependence of neutral and electrolyte solutes was analyzed using the Spiegler−Kedem equation by combining the Arrhenius equations for diffusivity and viscosity, which we obtained ΔE m, the activation energy of diffusion, after eliminating the effect of viscosity. For large ΔE m, which corresponds to the rejection of neutral solutes on the basis of molecular sieving, rejection decreased with temperature but remained unchanged for small ΔE m, which corresponds to the rejection of electrolytes based on the charge effect.