Sorption in Proton-Exchange Membranes An Explanation of Schroeder’s Paradox

A physicochemical model is proposed to describe sorption in proton-exchange membranes (PEMs), which can predict the complete isotherm as well as provide a plausible explanation for the long-unresolved phenomenon termed Schroeder's paradox, namely, the difference between the amounts sorbed from a liquid solvent vs. from its saturated vapor. The solvent uptake is governed by the swelling pressure caused within the membrane as a result of stretching of the polymer chains upon solvent uptake, Π M , as well as a surface pressure, Π σ , due to the curved vapor-liquid interface of pore liquid. Further, the solvent molecules in the membrane are divided into those that are chemically, or strongly, bound to the acid sites, λ C i, and others that are free to physically equilibrate between the fluid and the membrane phases, λ F i. The model predicts the isotherm over the whole range of humidities satisfactorily and also provides a rational explanation for the Schroeder's paradox.

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