Liquid–Liquid Equilibria of Thiophene + n-Alkanes + Pyridinium-Based Ionic Liquids: Effects of Cation Alkyl Chain Length and Anion Type with NRTL Modeling

Liquid–liquid equilibrium (LLE) data are reported for a series of ternary systems composed of thiophene, linear n-alkanes (n-dodecane, n-tetradecane, and n-hexadecane), and selected pyridinium-based ionic liquids—1-butylpyridinium tetrafluoroborate ([C4Py][BF4]), 1-hexylpyridinium tetrafluoroborate ([C6Py][BF4]), and 1-butylpyridinium bis(trifluoromethylsulfonyl)imide ([C4Py][NTf2]). All measurements were carried out at 313.15 K and atmospheric pressure. The systems were investigated to clarify the combined effects of cation alkyl chain length, anion identity, and hydrocarbon molecular size on phase-equilibrium behavior and sulfur-compound partitioning. The experimental LLE data were evaluated using distribution coefficients and selectivity, revealing a pronounced and composition-dependent preference of thiophene for the ionic-liquid-rich phase. Distribution coefficients ranged from 0.94 to 2.94, while selectivity values varied between 23 and 198, depending on system composition and ionic-liquid structure. The experimental data were successfully correlated using the Non-Random Two-Liquid (NRTL) activity-coefficient model, yielding low root-mean-square deviations and providing a consistent thermodynamic description of the ternary systems. The resulting phase diagrams highlight the strongly non-ideal interactions governing the solvation of aromatic sulfur compounds in pyridinium-based ionic liquids and establish reliable thermodynamic parameters for these mixtures. Overall, this work provides high-quality equilibrium data and structure–property insights that support the thermodynamic modeling of ionic-liquid-based liquid–liquid equilibrium systems.