Impact of CarboxylateAnion Chain Length and Temperatureon Physicochemical Properties of Butyrolactam-Based Protic Ionic Liquids

Lactam cation-based protic ionic liquids (PILs) have promising applications in fuel cells, thermal transfer fluids, and SO₂ absorption. A key challenge in their practical use is understanding the factors influencing their intermolecular interactions and physicochemical properties. Five butyrolactam-based PILs were synthesized with a common cation ([BT]⁺) and varying alkyl chain lengths of the anion ([RCOO]⁻, where R = H, CH₃, C₃H₇, C₅H₁₁, and C₁₁H₂₃). The influence of anion chain length and temperature dependency of density (ρ), viscosity (η), ionic conductivity (σ), and refractive index (<i>n</i>_D) of these PILs were measured at 0.1 MPa pressure and temperature ranging from 293.15 to 333.15 K. Entropy (<i>S</i>), lattice potential energy (<i>U</i>_POT), free volume (<i>V</i>_f), and molar refraction (<i>R</i>_m) were derived based on the measured data of density and refractive index. The phase transition temperature was determined, with no signs of glass transition being observed. The optical properties of PILs were examined by using ultraviolet–visible (UV–vis) and fluorescence spectroscopies. Despite being nonaromatic in nature, these ionic liquids exhibited intrinsic fluorescence emission characteristics similar to those observed in viscous liquids like PEG 200 and PEG 400. These PILs, with hydrogen bonding ability and low viscosity, are expected to enhance gas solubility and improve absorption rates. This will help to improve the efficiency of acid gas capture.