Conductivity and association of imidazolium and pyridinium based ionic liquids in methanol

A detailed investigation of the dilute solutions of several imidazolium-based ([BMIM][Br], [EMIM][BF4], [BMIM][BF4], [HexMIM][BF4], [BMIM][Tf]) and pyridinium-based ([BMP][BF4]) ionic liquids, and two tetraalkylammonium salts (Bu4NBr and Bu4NBPh4) in methanol was carried out between 278.15 and 328.15 K. The limiting molar and ionic association constants were derived using the Lee-Wheaton equation. The limiting molar conductivity for room-temperature ionic liquids with common anion ([BF4]−) is found to obey the Stokes' law: Λ0 increases as the cation size decreases. Ionic association in all studied systems does not show definite correlation with cation structure, while it strongly depends on anion size and structure. Ion association constants are discussed and the role of non-Coulombic forces is demonstrated with the help of short-range square-mound potential. Performed data analysis indicates the formation of contact ion pair of similar structure in studied solutions of ionic liquids, with anion coordinated by imidazole or pyridine ring. The formation of H-bonding between bromide-anion and hydroxy-group of methanol was observed.