Work

Non-fluorinated phosphonium-sulfonate ionic liquids as candidates for thermally demanding applications: thermal stability, viscosity and liquid-state temperature ranges

Year: 2026

Type: article

Abstract: Room temperature and thermally stable ionic liquids (ILs) are of considerable interest as fluids for high-temperature applications. In this work, a series of non-fluorinated phosphonium-sulfonate ILs was synthesized and systematically characterized to establish structure-property relationships and evaluate their potential as thermally robust fluids. Thermal stability, density and viscosity were evaluated to determine the influence of cation and anion structure. Under dynamic conditions, all ILs showed encouraging thermal stability, with T5% values of up to 406 °C. Isothermal measurements revealed that four of the seven studied ILs exhibited weight losses below 2.6 wt% after 12 h at 300 °C. It has been found that cation and anion strongly influence thermal stability; in the tributyltetradecylphosphonium ([(C4)3C14P]) cation-based ILs, the presence of an aromatic ring in the anion enhanced stability, highlighting the importance of anion structure. In dodecylbenzenesulfonate ([DBS])-based ILs, the cation bearing the longest alkyl chains exhibited the lowest weight loss under isothermal conditions. Density and viscosity were also strongly dependent on IL structure and temperature. Density decreased by increasing temperature and alkyl chain length, whereas viscosity decreased greatly with temperature and showed the opposite trend with alkyl chain length. The anion was the main factor controlling viscosity, with [DBS]-based ILs exhibiting the highest values, while the cation effect was less systematic. Overall, the integration of melting behaviour, isothermal thermal stability, density, viscosity, and viscosity-temperature modelling establishes a structure-guided thermophysical dataset for non-fluorinated phosphonium-sulfonate ILs and defines liquid-phase operating ranges extending up to 300 °C for high-temperature working-fluid applications.

Source: RSC Advances

DOI: 10.1039/d6ra08417f