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 correlated using the Non‑Random Two‑Liquid (NRTL) activity‑coefficient model, noting limitations for certain systems, yielding low average root‑mean‑square deviations between experimental and calculated compositions of 0.2239. 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.