Characteristics of water transport in relation to microscopic structure in Nafion membranes

Abstract

The interactions among Nafion membranes, water molecules and organic aliphatic ammonium cations which have both hydrophilic and hydrophobic parts have been studied using the streaming-potential method and the impedance-measurement method. The results showed that, for membranes equilibrated with an aliphatic ammonium chloride solution, the water-transference coefficients of the membranes increased linearly with increasing molar volume of the cations, independently of their hydrophobicity, indicating that the water molecules are carried along with the hydrophobic cations predominantly by the pumping effect of the cations. By comparison with the results for hydrophilic alkali-metal cations, a model of water transport accompanying cation transport inside the Nafion membranes was proposed. The transported water molecules are attributed to the hydrated water molecules of the transferred cations, which are almost the same as in free solutions, and to the water molecules hydrodynamically ‘pumped’ by the cations, which are proportional to the hydrated cation size, owing to the cation's volume exclusion effect in the narrow channel inside the membrane. The results also revealed that the incorporation of such aliphatic cations caused a decrease in water content of the membrane. Furthermore, the water-transference coefficients of most of the cations probed were found to be larger than, or at least close to, the water content of the corresponding membranes. It seems that even these hydrophobic organic cations move preferentially through hydrophilic domains in the Nafion membranes as hydrophilic inorganic cations do, and only some of the exchange sites in the membrane take part in the ionic conduction and the rest of the exchange sites are not active and cannot contribute to the ionic conductance of the membrane when the membrane is equilibrated with these hydrophobic organic cations. The effective diameter of the channel connecting ion–water clusters in the membrane was estimated to be ca. 0.67–0.83 nm.