Thermodynamics of water sorption by perfluorosulphonate (Nafion-117) and polystyrene–divinylbenzene sulphonate (Dowex 50W) ion-exchange resins at 298 ± 1 K

Abstract

Water sorption as a function of activity (a_H2O) by H⁺-, Li⁺-, N(CH₃)₄⁺- and Cs⁺-exchanged forms of a perfluorosulphonate ion exchanger (Nafion-117) and by 4 and 8% crosslinked Dowex 50 W resins has been investigated in order to derive sorption isotherms using the isopiestic technique. All exchangers show the sequence of water sorption as H⁺ > Li⁺ > N(CH₃)₄⁺ > Cs⁺. However, the H⁺ and Li⁺ forms of Nafion-117 absorb significantly larger amounts of water, while the Cs⁺ form sorbs much less water at all water activities compared to the corresponding ionic forms of Dowex 50W × 8, the N(CH₃)⁺₄ isotherms of the two exchangers being quite close to each other. Sorption of water by the 4 and 8% crosslinked Dowex 50W resins is found to be the same at low a_H2O( < 0.55), but the 4% crosslinked exchanger sorbs more water in the higher a_H2O region. The overall decrease in free energy on water sorption (–ΔG_sw) computed from the isotherms follows the sequence Cs⁺-117 < Cs⁺-× 8 < N(CH₃)⁺₄-117 < N(CH₃)⁺₄-× 8 < Cs⁺-× 4 < N(CH₃)⁺₄-× 4 < Li⁺-× 8 < Li⁺-× 4 < H⁺-× 8 < H⁺-× 4 < Li⁺-117 < H⁺-117. Following different procedures for Nafion and Dowex resins, cationic hydration numbers (n₊) evaluated reveal greater hydration of H⁺ and Li⁺ in Nafion compared to that in Dowex resins. Cs⁺ in Dowex resins is reasonably hydrated, but in Nafion a solvent-shared ion pair of the type SO^–₃(H₂O)Cs⁺ appears to form. Swelling pressures (π) computed for the two types of exchangers follow the order Nafion-117) > Dowex 50W × 8 > Dowex 50W × 4 (any ionic form). The results are discussed in relation to the relative degrees of hydration of the (respective) ions, including the sulphonate anion, and the differences between the two types of ion exchangers.