Thermodynamic considerations on the stability of water in Nafion

This work entails modeling the thermodynamic forces contributing to the total free energy of a Nafion membrane to find how liquid water equilibrates and agglomerates inside the membrane. Since the sulfonate acid sites attract water to dissociate, there is a mixture of water and ions within the Nafion membrane. This mixture contributes an entropic term to the free energy of the system which decreases with increasing water content. The hydrophobic Nation backbone yields a contact angle greater than 90' when in contact with water. This curvature in the surface of the water induces a capillary pressure which can be very high if the size of the agglomeration is small. As the membrane takes on water, polymer chains of the Nation are stretched and straightened reducing their configurational entropy. Minimizing the free energy under the influence of all these forces at varying pressures and temperatures gives insight into the nature of liquid or vapor filled pores throughout a Nation membrane. Specifically it indicates a critical pore size in which liquid or vapor is the favorable state. (c) 2007 Elsevier B.V. All rights reserved.