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Water-Nafion phase equilibria and proton conductivities were measured in two ways. First, Nafion was in contact with saturated water vapor. Second, Nafion was in contact with liquid water at the same temperature. At 29 degrees C, for preboiled, vapor-equilibrated Nafion exposed to water with an activity = 1 and air pressures ranging from 0 to 0.96 bar, the water content was lambda = 23 +/- 1 mol H(2)O/mol SO3-. For the preboiled, liquid-equilibrated membrane, lambda = 24 +/- 2. At 100% relative humidity (RH), the water content of preboiled Nafion decreased as the temperature rose from 30 to 80 degrees C but did not recover its initial water content when the temperature returned to 30 degrees C. The water content of predried Nafion at 1 atm and 30 degrees C was lambda = 13.7 +/- 0.2 when vapor-equilibrated and lambda = 13.1 +/- 0.5 when liquid-equilibrated. A Nafion membrane originally boiled in water had much higher liquid- and 100% RH vapor-equilibrated proton conductivities than the same membrane originally dried at 110 degrees C with a RH less than 2%. The liquid-equilibrated and 100% RH vapor-equilibrated membrane conductivities were the same when the membrane had the same thermal history. The conductivity data was fit to a model, and the water content was determined at different temperatures. The predried membrane water content increased with temperature, and the preboiled membrane's water content changed slightly with temperature. Both water sorption and proton-conductivity data do not exhibit Schroeder's paradox. These studies and previous results suggest that Schroeder's paradox is resolved when attention is given to the thermal history of the absorbing polymer.

The hydrolysis and carbonate complexation of the alkaline earth cations Ca 2+ and Sr 2+ were investigated by using a combined experimental and modeling approach. The modeling approach uses both macroscopic thermodynamic models and molecular modeling at the density functional theory (DFT) level. The molecular modeling calculations identify possible speciation schemes in the thermodynamic modeling and provide molecular level insight into the macroscopically observed thermodynamic measurements. In order to develop accurate thermodynamic models valid to high electrolyte concentration and to test for the possible existence of species suggested by the molecular models, experimental measurements were made on the solubility of Ca(OH) 2 , Sr(OH) 2 ·8H 2 O, and carbonate compounds extending to high base molality (≈5 mol kg −1 ), or carbonate molality (≈2 mol kg −1 ). A thermodynamic model is developed that satisfactorily explains the macroscopic aqueous thermodynamic data and correlates with the molecular modeling results. The first published values of the equilibrium constants for the formation of Sr(CO 3 ) 2− 2 (aq) and Ca(CO 3 ) 2− 2 (aq), and of the solubility product of Sr(OH) 2 ·8H 2 O are also provided. In certain cases, specifically when the DFT calculations suggest that the hydroxyl groups are more closely associated with the first hydration layer of water molecules than directly with the central alkaline earth cation, the thermodynamic relations for these cation–hydroxyl interactions are described by means of Pitzer ion-interaction parameters, rather than by the explicit introduction of an aqueous hydrolysis species.