Water States in Perfluorosulfonic Acid Membranes Using Differential Scanning Calorimetry

The water states in perfluorosulfonic acid membranes (Nafion®) were evaluated using low temperature differential scanning calorimetry (DSC) on both vapor and liquid penetrants. At low sorption levels, water sorbed in Nafion existed in the nonfreezable bound state until a critical value was reached. The critical, nonfreezable water content corresponded to 4.8 water molecules per sulfonate group. Beyond that critical value, additional sorbed water was partitioned between freezable and nonfreezable bound states. The freezable water was in intermediate or freezable bound water state with subzero fussion temperatures. The observed water fusion enthalpy for every additional gram of sorbed water was less than that of pure water. The partition coefficient (K) between the nonfreezable bound state and the freezable state water was estimated as 0.755. The critical nonfreezable water content (W c ) and the partition coefficient for vapor permeate in Nafion were similar to those of liquid water penetrant. These findings allow one to estimate the bound and freezable water distribution in Nafion. The K value, in conjunction with the critical water content (W c ), can be used as a quantitative indicator to characterize water states in ionomers. This model may serve as the basis to account for water transport, ionic conductivity, and proton transfer changes in various solid electrolyte membranes.