Adsorption of Water Vapor by Poly(styrene) Sulfonic Acid Sodium Salt—Isothermal and Isobaric Adsorption Equilibria

Abstract

Air-conditioning and dehumidifying systems by sorption on solids are of great interest, particularly in humid climates, because they allow a reduction of thermal loads, which minimizes denergy consumption to maintain the indoor setpoint temperature and the use of HFC. Recent studies have shown that hydrophilic polymers, such as sulfonic polymers, could have a very high performance in water adsorption from air. The aim of this study was to characterize the water vapor adsorption properties of a completely sulfonated and mono-sulfonated poly(styrene) sulfonic acid sodium salt (PSSASS) and to elucidate the mechanism of adsorption on these materials. Adsorption isotherms have been determined by thermal gravimetric analysis (TGA) between 298 K and 317 K for pressures ranging from 0.01 to 4.5 kPa. They have type II of the IUPAC classification and a small hysteresis loop was observed between adsorption and desorption processes only for the mono-sulfonated sample. Water content was up to 80 w% at 80% relative humidity. Adsorption isotherms have been well fitted with the Frenkel-Halsey-Hill model(so-called FHH model). Adsorption-desorption isobars have been determined by TGA under 3.7 kPa in the temperature range of 298 K-373 K. They have shown that these polymers could be completely regenerated by heating at 313 K under humidified air. No degradation of the adsorption properties has been observed after several regenerations. Adsorption enthalpies and entropies have been deduced from the Clapeyron equation and from differential scanning calorimetric (DSC) measurements. A good agreement was found. Our results suggested that the adsorption of water on these polymers occurs in two steps: formation of water aggregates on sodium cations of sulfonated groups and then capillary condensation with a structural modification of the polymer, which decreased as sulfonation degree increased.