Infrared investigations of sulfonated ionomer membranes. I. Water–alcohol compositions and counterions effects

Abstract FT‐IR spectra of Nafion 117, Raipore 1010, and PESS membranes loaded with different alkaline counterions and equilibrated with different liquid media were recorded by means of the attenuated total reflectance technique. The antisymmetric (vas) and especially the symmetric stretching vibrations (vs) of the sulfonate groups are affected by the presence of solvents in its local chemical environment. In the dry membrane the smaller the bare counterion, the higher the frequency of the symmetric vibration and the stronger the sulfonate‐counterion interactions in the contacting ion pair. After equilibration with pure water, ethanol, and 2‐propanol, the vs vibration shifts to a constant value corresponding to the solvated sulfonic grouping in the case of Nafion‐Li+ membrane. In the cases of PESS‐Li+ and Raipore‐Li+ membrane only water can cause a similar shift. The greater affinity of the Nafion membrane to alcohols can be attributed to the stronger acidity of the corresponding sulfonic acid. In water‐alcohol mixtures the variation of vs with water content indicates that the ion pairs in the PESS‐Li+ membrane are progressively dissociated and solvated by increasing amounts of water in 2‐propanol. Complete dissociation occurs at about 9 wt % water. Smaller shifts are observed with bigger cations, due to the weaker sulfonate‐cation interactions.