Transport properties of Nafion®/cyclodextrin membranes

The transport properties of Nafion(R)/cyclodextrin membranes were studied for simple gases and water. Cyclodextrins (CD) in ionic form have been introduced as counter-ions in the ionic phase of Nafion(R) by solution exchange. The CD and Nafion(R) transport parameters were at first determined using sorption methods. The cyclodextrin gas and water solubility coefficients are higher than the Nafion(R) ones but the diffusion coefficients are lower. For the Nafion(R)/CD composite properties, the gas permeability and diffusion decrease with the CD content showing that the free volume of the cyclodextrin molecules is not accessible to the diffusant molecule. Moreover the detailed analysis of the permeation and sorption data shows that gases do not diffuse through the Nafion(R) ionic zone. A similar decrease as a function of the cyclodextrin content is obtained on the water diffusion and permeability whereas it is known that water diffuses through the ionic phase. This behaviour has been explained by the fact that cyclodextrins reduce the water swelling capacity of this ionic phase and modify its reference transport properties. A small angle neutron scattering (SANS) microstructural analysis of the composite membrane confirms this phenomenon. (C) 2001 Elsevier Science B.V. All rights reserved.